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  #131  
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Default Darwin’s argument againsts Lamarck’s and Paley’s concepts

Darwin’s argument againsts Lamarck’s and Paley’s concepts

We remember the name of Charles Darwin each time we talk about the theory of evolution. Very few of us know that that Darwin didn’t discover this theory himself but collected a lot of evidence supporting this theory and also developed a mechanism, which described the process of evolution. In reality, the theory of evolution originates from the ancient Greece and was studied and developed by many scholars during the human history. Darwin’s theory of natural selection is a part of more general evolution theory. Important part of his work consist of the arguments, he gives against the prominent theories of the past. In my paper I will center on the arguments Darwin adduced against the Paley’s and Lamarck’s theories.

When studying at Cambridge, Darwin discovered Paley’s theologian works. They had a deep influence on him but finally he came to reject the providential explanation of life origin developed by Paley. Darwin studied and outgrew Paley’s theory he admired during the first years of his study. Paley, one of the most famous British theologians explained the creation of immutable species by God’s will. His argument was profound and powerful, but his theory was prejudiced by Darwin’s theory. In his works, Darwin called Paley’s theory unconvincing and arguable. In his Autobiography he wrote “The old argument of design in nature, as given by Paley, which formerly seemed to me so conclusive, fails, now that the law of natural selection has been discovered. We can no longer argue that, for instance, the beautiful hinge of a bivalve shell must have been made by an intelligent being. like the hinge of a door by man” (Darwin, p. 87). Many scientists, who share Darwin’s evolutionary theory call his arguments against Paley persuasive and considered that Darwin defeated Paley’s arguments of divine origin. Neal Gillespie, faithful follower of Darwin’s teaching in his book “Charles Darwin and The Problem of Creation” underlines the meaning of Darwin’s argument against Paley’s theory. “It has been generally agreed (then and since) that Darwin’s doctrine of natural selection effectively demolished William Paley’s classical design argument for the existence of God.” (Gillespie, 83). He also stated, that despite some sympathy to Paley’s argument Darwin expressed in the beginning of his research, “In the final analysis, Darwin found God’s relation to the world inexplicable; and a positive science, one that shut God out completely, was the only science that achieved intellectual coherence and moral acceptability.” (Gillespie, 83).

Arguing Paley’s theory of creation, Darwin underlines separation between science and religion. He stated that their correlation wasn’t possible and couldn’t be used as a mean to explain the origin of spices. Darwin loudly argues Paley’s special creation doctrine contrasting it to his own doctrine of natural selection. Darwin is convincing in his arguments and gives unshakable proofs supporting his theory. He builds his argument in such a way that all those who study it could become sure that the element of design in natures possesses the dominant role. Proving this argument naturally leads to the conclusion that the origin of nature is changeable and mutable, which proves Darwin’s’ thesis. Very soon after its appearance, Darwin’s theory of origin of spices replaced Paley’s natural theology and found a lot of admires all over the world. Paley’s arguments could be popular in the beginning of the 19th century when no worthy alternative could be found to contrast to his theory of special creation. Biology was presented as a fragmentary science during that time. It couldn’t provide any unifying concepts, which would be able to contradict the theological explanations. The number of scientist, who entered the scene in the middle of the 19th century have changed the functions and the face of this science forever. The idea of evolution by natural selection gave a worthy alternative to the theological perspective and very soon became dominant not only for scientists, but also for wide public. A lot of people believed Darwin’s theory about naturally driven evolutionary process.
Another important essence of Darwin’s works consists of his argument against Lamarck’s theory of the inheritance of acquired characteristics. Lamarck developed a theory of the mutability of species through the use and the action of environment. He stated that spices change over time, climbing the ladder of life. He regarded the evolution as a progress from more simple forms to more complex ones. Darwin never totally rejected Lamarck’s theory. He found it incomplete, though. Darwin agreed Lamarck’s ideas of progress and evolutions of the living creatures. He also agreed that the purpose of these changes could be the better adapting to the environment. Darwin took Lamarck’s idea of the development of organisms from simpler to more complicated forms and created his own theory of evolution in contrast to Lamarck’s doctrine of growth of adapted complexity. Darwin argued Lamarck’s reasons of spices mutability conditioned by the need to meet the changes of the surrounding.

In his On the Origin of Species by Means of Natural Selection Darwin gave convincing arguments against Lamarck’s theory. “It is very difficult to decide how far changed conditions, such as of climate, food, etc., have acted in a definite manner. There is reason to believe that in the course of time the effects have been greater than can be proved by clear evidence.” (Darwin, The Origin of Species, p. 139). Darwin didn’t agree with Lamarck’s that heritable adaptation was a result of natural influence on the organism. He also argued Lamarck’s ideas of predetermined character of evolution. Though Lamarck’s theory was a constructive one, he couldn’t completely get read of providential tendencies of Paley. So he states that everything good acquired by the organism during its life is represented in their descendants through reproduction. This concept got the name the inheritance of acquired characters and we mostly remember Lamarck for this theory.

“All that nature has caused individuals to gain or lose by the influence of the circumstances to which their race has been exposed for a long time, and, consequently, by the influence of a predominant use or disuse of an organ or part, is conserved through generations in the new individuals descending from them, provided that these acquired changes are common to the two sexes or to those which have produced these new individuals”. (Lamarck, p. 166). Darwin disapproves Lamrack’s theory. He proved that not all the changes are adopted by the descendants. Darwin stated that adaptation characteristics change naturally, without any influence from above. Putting forward his arguments, he made the last step away from the wise creator, putting all the responsibilities to nature. He disagrees Lamarck’s theory that heritable adaptations result the way surrounding influences the organism. He saw these changes as accidental and purposeless variations. He stressed on the meaning of evolution, in contrast to creationist approach. Darwin didn’t agree with Lamarck’s theory that changes once acquired by the individual are passed to all generations. He stated that genes are not changed but the most appropriate individuals are selected by nature to survive and reproduce and this gradually changes the characteristics of the whole spice. In order to support his thesis, Darwin gives examples, facts of natural history, contrasts and comparisons. Darwin’s language is unique and very exact at the same time. Sometimes he uses satire and irony to make his writings more lively and interesting. He creates picturesque combinations and bright images to make the readers understand his argument. For each argument he uses a lot of proofs and evidence in order to support his thesis. Darwin did a big job collecting data, making studies and researches and preparing arguments in order to create his own concept of evolutionary theory. He skillfully used good experience of the other scientists who researched in this area and found their counterparts and proved them with his strong arguments.


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  #132  
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Default Lamarckism

Lamarckism (or Lamarckian evolution) is the once widely accepted idea that an organism can pass on characteristics that it acquired during its lifetime to its offspring (also known as based on heritability of acquired characteristics or "soft inheritance"). It is named for the French biologist Jean-Baptiste Lamarck (1744–1829), who incorporated the action of soft inheritance into his evolutionary theories and is often incorrectly cited as the founder of soft inheritance. It proposed that individual efforts during the lifetime of the organisms were the main mechanism driving species to adaptation, as they supposedly would acquire adaptive changes and pass them on to offspring.

After publication of Charles Darwin's theory of natural selection, the importance of individual efforts in the generation of adaptation was considerably diminished. Later, Mendelian genetics supplanted the notion of inheritance of acquired traits, eventually leading to the development of the modern evolutionary synthesis, and the general abandonment of the Lamarckian theory of evolution in biology. In a wider context, soft inheritance is of use when examining the evolution of cultures and ideas, and is related to the theory of Memetics.
While enormously popular during the early 19th century as an explanation for the complexity observed in living systems, the relevance of soft inheritance within the scientific community dwindled following the theories of August Weismann and the formation of the modern evolutionary synthesis.
History

Between 1794 and 1796 Erasmus Darwin wrote Zoönomia suggesting "that all warm-blooded animals have arisen from one living filament... with the power of acquiring new parts" in response to stimuli, with each round of "improvements" being inherited by successive generations. Subsequently Jean-Baptiste Lamarck repeated in his Philosophie Zoologique of 1809 the folk wisdom that characteristics which were "needed" were acquired (or diminished) during the lifetime of an organism then passed on to the offspring. He incorporated this mechanism into his thoughts on evolution, seeing it as resulting in the adaptation of life to local environments.
Lamarck founded a school of French Transformationism which included Étienne Geoffroy Saint-Hilaire, and which corresponded with a radical British school of comparative anatomy based at the University of Edinburgh which included the surgeon Robert Knox and the anatomist Robert Edmund Grant. Professor Robert Jameson wrote an anonymous paper in 1826 praising "Mr. Lamarck" for explaining how the higher animals had "evolved" from the "simplest worms" – this was the first use of the word "evolved" in a modern sense. As a young student, Charles Darwin was tutored by Grant, and worked with him on marine creatures.
The Vestiges of the Natural History of Creation, authored by Robert Chambers and published anonymously in England in 1844, proposed a theory modelled after Lamarckism, causing political controversy for its radicalism and unorthodoxy, but exciting popular interest and paving the way for Darwin.
Darwin's Origin of Species proposed natural selection as the main mechanism for development of species, but did not rule out a variant of Lamarckism as a supplementary mechanism. Darwin called his Lamarckian hypothesis Pangenesis, and explained it in the final chapter of his book Variation in Plants and Animals under Domestication, after describing numerous examples to demonstrate what he considered to be the inheritance of acquired characteristics. Pangenesis, which he emphasised was a hypothesis, was based on the idea that somatic cells would, in response to environmental stimulation (use and disuse), throw off 'gemmules' which travelled around the body (though not necessarily in the bloodstream). These pangenes were microscopic particles that supposedly contained information about the characteristics of their parent cell, and Darwin believed that they eventually accumulated in the germ cells where they could pass on to the next generation the newly acquired characteristics of the parents. Darwin's half-cousin, Francis Galton carried out experiments on rabbits, with Darwin's cooperation, in which he transfused the blood of one variety of rabbit into another variety in the expectation that its offspring would show some characteristics of the first. They did not, and Galton declared that he had disproved Darwin's hypothesis of Pangenesis, but Darwin objected, in a letter to Nature that he had done nothing of the sort, since he had never mentioned blood in his writings. He pointed out that he regarded pangenesis as occurring in Protozoa and plants, which have no blood. With the development of the modern synthesis of the theory of evolution and a lack of evidence for either a mechanism or even the heritability of acquired characteristics, Lamarckism largely fell from favor.
In the 1920s, experiments by Paul Kammerer on amphibians, particularly the midwife toad, appeared to find evidence supporting Lamarckism, but his specimens with supposedly-acquired black foot-pads were found to have been tampered with. In The Case of the Midwife Toad Arthur Koestler surmised that the specimens had been faked by a Nazi sympathiser to discredit Kammerer for his political views.
A form of "Lamarckism" was revived in the Soviet Union of the 1930s when Trofim Lysenko promoted Lysenkoism which suited the ideological opposition of Joseph Stalin to Genetics. This ideologically driven research influenced Soviet agricultural policy which in turn was later blamed for crop failures.
Since 1988 certain scientists have produced work proposing that Lamarckism could apply to single celled organisms. The lack of evidence for Lamarckian acquisition in higher order animals is still clung to in certain branches of psychology, as, for example, in the Jungian racial memory.
Neo-Lamarckism is a theory of inheritance based on a modification and extension of Lamarckism, essentially maintaining the principle that genetic changes can be influenced and directed by environmental factors.
Lamarck's theory

The identification of "Lamarckism" with the inheritance of acquired characteristics is regarded by some as an artifact of the subsequent history of evolutionary thought, repeated in textbooks without analysis. Stephen Jay Gould wrote that late 19th century evolutionists "re-read Lamarck, cast aside the guts of it ... and elevated one aspect of the mechanics - inheritance of acquired characters - to a central focus it never had for Lamarck himself. He argued that "the restriction of "Lamarckism" to this relatively small and non-distinctive corner of Lamarck's thought must be labelled as more than a misnomer, and truly a discredit to the memory of a man and his much more comprehensive system. Gould advocated defining "Lamarckism" more broadly, in line with Lamarck's overall evolutionary theory.
Lamarck incorporated two ideas into his theory of evolution, in his day considered to be generally true:
  1. Use and disuse – Individuals lose characteristics they do not require (or use) and develop characteristics that are useful.
  2. Inheritance of acquired traits – Individuals inherit the traits of their ancestors.
Examples of what is traditionally called "Lamarckism" would include:
  • Giraffes stretching their necks to reach leaves high in trees (especially Acacias), strengthen and gradually lengthen their necks. These giraffes have offspring with slightly longer necks (also known as "soft inheritance").
  • A blacksmith, through his work, strengthens the muscles in his arms. His sons will have similar muscular development when they mature.
With this in mind, Lamarck has been credited in some textbooks and popular culture with developing two laws:
  1. In every animal which has not passed the limit of its development, a more frequent and continuous use of any organ gradually strengthens, develops and enlarges that organ, and gives it a power proportional to the length of time it has been so used; while the permanent disuse of any organ imperceptibly weakens and deteriorates it, and progressively diminishes its functional capacity, until it finally disappears.
  2. All the acquisitions or losses wrought by nature on individuals, through the influence of the environment in which their race has long been placed, and hence through the influence of the predominant use or permanent disuse of any organ; all these are preserved by reproduction to the new individuals which arise, provided that the acquired modifications are common to both sexes, or at least to the individuals which produce the young.
In essence, a change in the environment brings about change in "needs" (besoins), resulting in change in behavior, bringing change in organ usage and development, bringing change in form over time — and thus the gradual transmutation of the species.
However, as scientist historians such as Michael Ghiselin and Stephen Jay Gould have pointed out, none of these views were original to Lamarck. On the contrary, Lamarck's contribution was a systematic theoretical framework for understanding evolution. He saw evolution as comprising two processes;
  1. Le pouvoir de la vie (a complexifying force) - in which the natural, alchemical movements of fluids would etch out organs from tissues, leading to ever more complex construction regardless of the organ's use or disuse. This would drive organisms from simple to complex forms.
  2. L'influence des circonstances (an adaptive force) - in which the use and disuse of characters led organisms to become more adapted to their environment. This would take organisms sideways off the path from simple to complex, specialising them for their environment.
Current views on "Lamarckism"

The argument that instinct in animals is evidence for hereditary knowledge is generally regarded within science as false. Current views suggest that behaviours are more probably passed on through a mechanism called the Baldwin effect. While such a theory might explain the observed diversity of species and the first law is generally true, the main argument against Lamarckism is that experiments simply do not support the second law — purely "acquired traits" do not appear in any meaningful sense to be inherited. For example, a human child must learn how to catch a ball even though his or her parents learned the same feat when they were children. Lamarck’s theories gained initial acceptance because the mechanisms of inheritance were not elucidated until later in the 19th Century, after Lamarck's death.
Several historians have argued that Lamarck's name is linked somewhat unfairly to the theory that has come to bear his name, and that Lamarck deserves credit for being an influential early proponent of the concept of biological evolution, far more than for the mechanism of evolution, in which he simply followed the accepted wisdom of his time. Lamarck died 30 years before the first publication of Charles Darwin's Origin of Species. As science historian Stephen Jay Gould has noted, if Lamarck had been aware of Darwin's proposed mechanism of natural selection, there is no reason to assume he would not have accepted it as a more likely alternative to his "own" mechanism. Note also that Darwin, like Lamarck, lacked a plausible alternative mechanism of inheritance - the particulate nature of inheritance was only to be observed by Gregor Mendel somewhat later, published in 1866. Its importance, although Darwin cited Mendel's paper, was not recognised until the Modern evolutionary synthesis in the early 1900s. An important point in its favour at the time was that Lamarck's theory contained a mechanism describing how variation is maintained, which Darwin’s own theory lacked.
Neo-Lamarckism

Unlike neo-Darwinism, the term neo-Lamarckism refers more to a loose grouping of largely heterodox theories and mechanisms that emerged after Lamarck's time, than to any coherent body of theoretical work.
In the 1920s, Harvard University researcher William McDougall studied the abilities of rats to correctly solve mazes. He found that children of rats that had learned the maze were able to run it faster. The first rats would get it wrong 165 times before being able to run it perfectly each time, but after a few generations it was down to 20. McDougall attributed this to some sort of Lamarckian evolutionary process.
McDougall's results were later shown to be incorrect and caused by poor experimental controls by Oscar Werner Tiegs and Wilfred Eade Agar.
At around the same time, Ivan Pavlov, who was also a Lamarckist, claimed to have observed a similar phenomena in animals being subject to conditioned reflex experiments. He claimed that with each generation, the animals became easier to condition. Neither McDougall or Pavlov suggested a mechanism to explain their observations.
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Default Lamarckism

Lamarckism

Chevalier de Lamarck was a French naturalist and invertebrate zoologist who lived from 1844-1829. He is best known for a theory of evolution developed in his book, Philosophie zoologique, published in 1809. This theory, known today as Lamarckism, is based on the socalled "inheritance of acquired traits," meaning that characteristics that an organism may develop during its lifetime are heritable, and can be passed on to its progeny.

The anatomical, biochemical, and behavioral characteristics that an individual organism displays as its develops through life is known as its phenotype. However, the phenotype that an individual actually develops is somewhat conditional, and is based on two key factors: (1) the fixed genetic potential of the org anism (or its genotype; this refers to the specific qualities of its genetic material, or DNA [deoxyribonucleic acid]); and (2) the environmental conditions which an organism experiences as it grows. For example, an individual plant (with a particular, fixed genotype) that is well supplied with nutrients, moisture, and light throughout its life will grow larger and will produce more seeds than if that same plant did not experience such beneficial conditions. Conditional developmental possibilities as these are now known to be due to differing expressions of the genetic potenti al of the individual (biologists refer to the variable expression of the genome of an organism, as influenced by environmental conditions encountered during its development, as "phenotypic plasticity."


However, at the time of Lamarck and other biologists of the late eighteenth and nineteenth centuries the mechanisms of inheritance were not known (this includes Charles Darwin and Alfred Russel Wallace, the co-discoverers of the theory of evolution by natural selection, first published in 1859). These scientists thought that the developmental contingencies of individual organisms (which they called "acquired traits") were not initially fixed genetically, but that they could somehow become incorporated into the genetic make-up of individuals, and thereby be passed along to their offspring, so that evolution could occur. For example, if the ancestors of giraffes has to stretch vigorously to reach their food of tree foliage high in the canopy, this physical act might somehow have caused the individual animals to develop somewhat longer necks. This "acquired" trait somehow became fixed in the genetic complement of those individuals, to be passed on to their offspring, who then also had longer necks. Eventually, this presumed mechanism of evolution could have resulted in the appearance of the modern, extremely long-necked giraffe.

Modern biologists, however, have a good understanding of the biochemical nature of inheritance. They know that phenotypic plasticity is only a reflection of the variable, but strongly fixed genetic potential that exists in all individuals. Therefore, the idea of the inheritance of acquired traits is no longer influential in evolutionary s cience. Instead, biologists believe that evolution largely proceeds through the differential survival and reproduction of individuals whose genetic complement favors these characters in particular environments, compared with other, "less-fit" individuals of their population. If the phenotypic advantages of the "more-fit" individuals are due to genetically fixed traits, they will be passed on to their offspring. This results in genetic change at the population level, which is the definition of evolution. This is, essentially, the theory of evolution by natural selection, first proposed by Darwin and Wallace in 1859.



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Default Lamarckism

Lamarckism

Lamarckism
The theory of evolution as put forth by French biologist Lamarck has come to be known as Lamarckism. This theory has two salient features:

Use and Disuse of Parts

According to Lamarck, continuous use of a part results in it being well-developed and disuse of a part over a long period of time will result in its degeneration. For example, giraffes were forced to extend their necks and stretch their legs to reach higher vegetation over a period of time. This resulted in every generation having a little longer neck and legs than the previous one. Webbed feet in aquatic birds are thought to have developed due to constant spreading of toes and the stretching of the skin between. Successive generations of fish living in water pools deep inside caves in Europe have over hundreds of years lost their eyesight with eyelids becoming permanently sealed.

Inheritance of Acquired Characters
According to Lamarck, the characters that an organism acquired due to a change in their environment such as long neck, webbed feet, sealed eyes, etc. were passed on to the next generation. In this way, evolution from simpler to complex forms took place.
However, this theory was not widely accepted as it is known that acquired characters are only phenotypic changes and not genotypic. Thus, while the cases of giraffe, aquatic birds and blind fish do show that evolution has occurred, Lamarckism does not provide a satisfactory answer to the mystery of evolution.
Theory of Continuity of Germplasm
This was proposed by Weismann who did not agree with Lamarck's theory of inheritance of acquired characters. To prove his point, he cut off the tails of many successive generations of mice. This resulted in forced disuse of the tail. According to the theory of use and disuse, the tails should have become progressively shorter. However, this did not happen.
According to Weismann, the changes affected only the somatic (vegetative) cells and did not affect the germ cells or the gametes. Only the changes that affect the germ cells and the germplasm (the collection of genes) will be inherited by successive generations.



Darwinism
Charles Darwin and Alfred Russel Wallace proposed that nature has its own ways of selecting the best from the available species for continuation of life. Darwin became famous for his Theory of evolution by natural selection, the mechanism of which works as follows:
Individuals of a species produce more offspring than necessary to replace themselves.
This result in competition and struggle for existence among the individuals. Within the species, there is variation that results in minor differences between the individuals.
Thus in the struggle for existence only the ones with the variations best adapted to their environment survive.
In this manner nature ensures the 'survival of the fittest'.

Neo-Darwinism
The theory put forth by Darwin and Wallace gained wide acceptance. However, in the light of modern evidences, it was slightly modified and called Neo-Darwinism. In Neo-Darwinism, organic evolution is by natural selection of inherited characters. It utilizes evidences from various fields such as genetics, palaeontology, molecular biology, ecology and ethology (study of behaviour).


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Default Origin Of Life on Earth

Earth is said to have been formed about4.8 billion years ago.
· Earliest earth was a hot revolving ball of gas.
· The formation of first cells in planet earth is explained by the theory of chemical evolution proposed by Oparin and Haldane, independently.
· According to this theory, the reducing atmosphere of primitive earth helped in the formation of simple inorganic compounds followed by simple organic compounds. Then complex organic compounds and subsequently their interaction leading to the formation of self duplicating nucleic acids.
· The nucleic acids and other macro molecules became surrounded by membranes to form the protocells.
· The first forms of life were probably prokaryotic chemo-autotrophs.
· Anaerobic chemotrophs probably appeared subsequently followed by evolution of chlorophyll containing anaerobic photo-autotrophes.
· The first aerobic photo-autotrophs (cyanobacteria) are said to have appeared about 3.5 billion years ago.
· Life is said to have originated in water, because of its unique properties.
· The origin of life was followed by organic evolution with the appearance of well adapted newer form of life from the pre-existing simple forms of life.

Theories of origin of life

Several attempts have been made from time to time to explain the origin of life on earth. As a result, there are several theories which offer their own explanation on the possible mechanism of origin of life. Following are some of them:

Theory of Special Creation

According to this theory, all the different forms of life that occur today on planet earth, have been created by God, the almighty. This idea is found in the ancient scriptures of almost every religion. According to Hindu mythology, Lord Brahma, the God of Creation, created the living world in accordance to his wish. According to the Christian belief, God created this universe, plants, animals and human beings in about six natural days. The Sikh mythology says that all forms of life including human beings came into being with a single word of God. Special creation theory believes that the things have not undergone any significant change since their creation.
The theory of Special Creation was purely a religious concept, acceptable only on the basis of faith. It has no scientific basis.

Theory of Spontaneous Generation

This theory assumed that living organisms could arise suddenly and spontaneously from any kind of non-living matter. One of the firm believers in spontaneous generation was Aristotle, the Greek philosopher (384-322 BC). He believed that dead leaves falling from a tree into a pond would transform into fishes and those falling on soil would transform into worms and insects. He also held that some insects develop from morning dew and rotting manure. Egyptians believed that mud of the Nile river could spontaneously give rise to many forms of life. The idea of spontaneous generation was popular almost till seventeenth century. Many scientists like Descartes, Galileo and Helmont supported this idea. In fact, Von Helmont went to the extent stating that he had prepared a 'soup' from which he could spontaneously generate rats! The 'soup' consisted of a dirty cloth soaked in water with a handful of wheat grains. Helmont stated that if human sweat is added as an 'active principle' to this, in just 17 days, it could generate rats!
The theory of Spontaneous Generation was disproved in the course of time due to the experiment conducted by Fransisco Redi, (1665), Spallanzani (1765) and later by Louis Pasteur (1864) in his famous Swan neck experiment. This theory was disapproved, as scientists gave definite proof that life comes from pre-existing life.

Theory of Catastrophism

It is simply a modification of the theory of Special Creation. It states that there have been several creations of life by God, a catastrophe resulting from some kind of geological disturbance. According to this theory, since each catastrophe completely destroyed the existing life, each new creation consisted of life form different from that of previous ones.

A French scientist Georges Cuvier (1769-1832) and Orbigney (1802 to 1837) were the main supporters of this theory.
Cosmozoic Theory (Theory of Panspermia)
According to this theory, life has reached this planet Earth from other heavenly bodies such as meteorites, in the form of highly resistance spores of some organisms. This idea was proposed by Richter in 1865 and supported

Theory of Chemical Evolution

According to this theory,
· Spontaneous generation of life, under the present environmental conditions is not possible.
· Earth's surface and atmosphere during the first billion years of existence, were radically different from that of today's conditions.
· The primitive earth's atmosphere was a reducing type of atmosphere and not oxidising type.
· The first life arose from a collection of chemical substances through a progressive series of chemical reactions.
· Solar radiation, heat radiated by earth and lighting must have been the chief energy source for these chemical reactions.

Steps involved in Origin of life


The earth when it was formed about 4.8 billion years ago, was a hot revolving ball of gas consisting of atoms of various elements. Heavy elements such as iron and nickel were found in the center while comparatively lighter ones like those of aluminium and silicon formed the middle layer. The lightest elements like hydrogen, oxygen and carbon were found in the outermost layer. Due to the extremely high temperature, the atoms of these elements could not combine to form molecules.
As the earth started cooling gradually, the atoms started combining with one another to form molecules.

Formation of Inorganic Molecules and Compounds

With a considerable decrease in the earth's temperature over thousands of years, the atoms of different elements came together at random and formed inorganic molecules. Since the lighter elements (hydrogen, oxygen, carbon and nitrogen) were the most abundant in the outermost layer, their atoms reacted with each other to form the first inorganic molecules. Thus, the earliest molecules formed were those of hydrogen (H2), nitrogen (N2), ammonia (NH3), methane (CH4), carbon dioxide (CO2) and water vapour (H2O). All the atoms of oxygen probably combined with those of hydrogen and carbon to form water vapour and carbon dioxide. Hence, the lack of free molecular oxygen was responsible for the reducing type of atmosphere that existed on the primitive earth. The energy required for the configuration of these molecules must have come from the ultraviolet radiation in the sunlight.

Formation of Simple Organic Compounds

As the earth cooled further, the primitive inorganic molecules interacted and combined with one another to form simple organic compounds. Simple sugars, fatty acids, glycerol, amino acids and nitrogen bases (purines and pyrimidines) were probably the simple organic compounds that resulted from the interactions of the inorganic molecules.
Water vapour present in the primitive atmosphere formed the clouds, which then resulted in rainfall continuously for several centuries. This rain water filled the hollows and basins of the earth's crust to form the oceans. Water in these oceans contained ammonia and methane. These compounds reacted among themselves to form the primitive organic compounds, which had carbon-carbon linkages. Thus, ocean water provided the basis for formation of organic compounds.

Formation of Complex Organic Compounds

The smaller and simpler organic compounds that were formed initially in the earth, gradually started combining among themselves to form complex organic compounds. Simple sugars combined among themselves to form complex polysaccharides such as starch and cellulose. Fatty acids and glycerol molecules combined to form lipids. Amino acids combined among themselves to form polypeptides and proteins. Purines and pyrimidines combined with simple sugars and phosphates to form nucleotides, which in turn combined to form nucleic acids. Heat of the sun probably provided the energy required for the formation of complex organic compounds.
Haldane suggested that due to the accumulation of complex organic molecules, the sea ultimately became a sort of 'hot, dilute soup' where in, the molecules collided, reacted and aggregated to form more complex molecules.

Formation of Molecular Aggregates

It is suggested that the large organic molecules formed abiotically in the primitive earth came together spontaneously and due to intermolecular attraction, formed large colloidal aggregates called Coacervates. An envelope of water molecules formed around each such aggregate due to the hydrophilic nature of some of these compounds. A membrane of fatty acids protected and enclosed these molecules, increasing the chances of chemical reactions. Gradually, breakdown and building up reactions started for which the energy required was provided by the breakdown reactions. The coacervates selectively absorbed proteins and other materials from the ocean resulting in their active growth. The coacervates not only started growing rapidly but also started multiplying.

Formation of First Cells (Protobionts)

The coacervates were in a state of dynamic equilibrium, constantly taking in new materials from the oceans and releasing degraded materials. Thus, they had all the basic properties of life such as metabolism, growth and reproduction. However, they lacked the complexity of molecular organization, catalytic proteins (enzymes) and precise control of nucleic acids. Later, the nucleic acids are said to have taken control of coacervate and the process of replication became precise in the due course of time. With the nucleic acids being established as the genetic material, the coacervates got transformed into the primitive living systems which have been called as protobionts or eobionts.
Some of the proteins in protobionts are said to have developed the ability to catalyse chemical reactions, thereby functioning as the first enzymes. The formation of enzymes greatly enhanced the rate of synthesis of various molecules in the protobionts.

In the course of time, the protobionts became enclosed by a protein lipid membrane, allowing the accumulation of some molecules and the exclusion of others. This property improved the ability of protobionts to survive and compete with others. With the processes of metabolism, growth and reproduction becoming regular, precise and regulated, the first cells or organisms were formed. The term progenote has been suggested by Carl Woese to describe the first cell which served as the ancestor of all the forms of life existing today.
The first forms of life developed among the organic molecules, in the oxygen free atmosphere. Hence, they presumably obtained energy by the fermentation of organic compounds. They were heterotrophs, requiring ready-made organic compounds as food.

Chemoheterotrophs

They were prokaryotic like bacteria. They were anaerobes. They must have been dependent on the organic molecules present in the broth for body building and obtaining energy.

Chemoautotrophs

They were unable to synthesize organic molecules from inorganic raw materials, with the help of chemical energy obtained by the degradation of chemical compounds present in the sea.

Photoautotrophic

The next step was to development of pigment molecules chlorophyll. It would absorb solar energy and convert it into chemical energy. This process is termed as photosynthesis. The earliest formed organisms were photoautotrophic bacteria. They were anaerobic and did not produce O2 as byproduct during photosynthesis, because they did not use water as a reagent.

Aerobic Photoautotrophs

They evolved 3300 to 3500 million years ago. They were like present day cyanobacteria and could release O2 into the atmosphere because they used water as the reagent. Thus, the whole reducing atmosphere changed to an oxidising atmosphere.

Autotrophs are said to have arisen much later in the primitive earth due to a mutation in the primitive heterotrophs. The appearance of autotrophs, particularly photo autotrophs changed the situation. The appearance of photosynthetic organisms resulted in the release of free molecular oxygen into the atmosphere gradually transforming it into an oxidizing type from the existing reducing type.















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Default Ontogeny recapitulates phylogeny

Ontogeny recapitulates phylogeny

The theory that the stages in an organism's embryonic development and differentiation correspond to the stages of evolutionary development characteristic of the species. Also called Haeckel's law, recapitulation theory.

Biogenetic law, in biology, a law stating that the earlier stages of embryos of species advanced in the evolutionary process, such as humans, resemble the embryos of ancestral species, such as fish. The law refers only to embryonic development and not to adult stages; as development proceeds, the embryos of different species become more and more dissimilar. An early form of the law was devised by the 19th-century Estonian zoologist K. E. von Baer, who observed that embryos resemble the embryos, but not the adults, of other species. A later, but incorrect, theory of the 19th-century German zoologist Ernst Heinrich Haeckel states that the embryonic development (ontogeny) of an animal recapitulates the evolutionary development of the animal's ancestors (phylogeny).

Ernst Haeckel, German scientist, a contemporary and supporter of Charles Darwin, used this phrase to describe his embryological observation, which says basically that the development of the individual retraces the evolutionary steps of the species --- from its conception to its birth (or hatching, as the case may be), every animal passes through the evolutionary phases identical to the general process of evolution, from one-celled animals to advanced life-forms, over eons of time. In other words, every animal embryo "evolves" from a microscopic mass of cells to a fish, then to an amphibian, then to a reptile, and so on.

In terms of the human brain:

* At 25 days - the embryonic brain resembles the brain of a worm
* At 40 days - the embryonic brain resembles the brain of a vertebrate (fish)
* At 100 days - the embryonic brain resembles a mammalian brain
* At 5 months - the embryonic brain resembles the brains of other primates
The statement "ontogeny recapitulates phylogeny" is credited to Ernst Haeckel, and was the credo and motivation for much embrological research in the 19th and early 20th century. Despite the fact that this is still taught to high school and university students, it has been thoroughly disproven and discredited. That is not to say, however, that the essential idea that an organism's evolutionary history is not reflected in its embryological development. In fact, the modern credo is instead "phylogeny recapitulates ontogeny", the reverse of Haeckel's idea and in fact the hypothesis originally put forth by Karl Ernst von Baer, Haeckel's predecessor.
The latter statement means, and what modern biologists believe, that the embryonic stages of an organism or species should resemble the embryonic (and not adult) stages of its ancestors. In other words, the embryonic development of any species is constrained to resemble (closely) the embryonic development of its ancestors. In other words, by examining the ontogeny of an individual species, we can infer certain aspects of their evolutionary history.

Consider the two following cases as evidence supporting the modern hypothesis. First, snakes and legless lizards develop 'leg buds' as embryos, only to have them re-absorbed prior to hatching. This same pattern is observed in whales and dolphins. In both cases, independent evidence shows that snakes evolved from legged ancestors, as did the whale and dolphin. A second example of the same type can be found in the development of the mammalian inner ear. In reptile embryos, two bones develop into the articular bones of the hinge of the jaw, while these two same bones become the hammer and anvil of the inner ear in marsupials. This suggests that, evolutionarily, the inner ear of mammals developed from bones originally found in the jaw of a common ancestor to the reptiles, and the fossil evidence clearly shows this to be the case.
In biology, ontogeny refers to the embryonal development process of a certain species, and phylogeny to a species' evolutionary history. Observers have noted various connections between phylogeny and ontogeny, explained them with evolutionary theory and taken them as supporting evidence for that theory.

Observed connections

Generally, if a structure pre-dates another structure in evolutionarily terms, then it also appears earlier than the other in the embryo. Species which have an evolutionary relationship typically share the early stages of embryonal development and differ in later stages. Examples include:
* The backbone, the common structure among all vertebrates such as fish, reptiles and mammals, appears as one of the earliest structures laid out in all vertebrate embryos.
* The cerebrum in humans, the most sophisticated part of the brain, develops last.
If a structure vanished in an evolutionary sequence, then one can often observe a corresponding structure appearing at one stage during embryonic development, only to disappear or become modified in a later stage. Examples include:
* Whales, believed to have evolved from land mammals, don't have legs, but tiny remnant leg bones lie buried deep in their bodies. During embryonal development, leg extremities first occur, then recede. Similarly, whale embryos (like all mammal embryos) have hair at one stage, but lose most of it later.
* All vertebrates, believed to have evolved from fish, show gill pouches at one stage of their embryonal development.
* The common ancestor of humans and monkeys had a tail, and human embryos also have a tail at one point; it later recedes to form the coccyx.
* The swim bladder in fish presumably evolved from a sac connected to the gut, allowing the fish to gulp air. In most modern fish, this connection to the gut has disappeared. In the embryonal development of these fish, the swim bladder originates as an outpocketing of the gut, and the connection to the gut later disappears.

Explanation

One can explain connections between phylogeny and ontogeny if one assumes that one species changes into another by a sequence of small modifications to its developmental program (specified by the genome). Modifications that affect early steps of this program will usually require modifications in all later steps and are therefore less likely to succeed. Most of the successful changes will thus affect the latest stages of the program, and the program will retain the earlier steps. Occasionally however, a modification of an earlier step in the program does succeed: for this reason a strict correspondence between ontogeny and phylogeny, as expressed in Ernst Haeckel's discredited recapitulation law, fails.

"Ontogeny recapitulates phylogeny", also called the "biogenetic law" or the "theory of recapitulation", is a now discredited hypothesis in biology first espoused in 1866 by Ernst Haeckel. Ontogeny is the development of the embryos of a given species; phylogeny is the evolutionary history of a species. The theory claims that the development of the embryo of every species repeats the evolutionary development of that species.
In order to support his theory, Haeckel produced several embryo drawings which overemphasized similarities between embryos of related species and found their way into many biology textbooks.
Modern biology rejects Haeckel's theory. While for instance the phylogeny of humans as having evolved from fish through reptiles to mammals is generally accepted, no cleanly defined "fish", "reptile" and "mammal" stages of human embryonal development can be discerned.

The fact that the strict recapitulation theory is rejected by modern biologists has sometimes been used as an argument against evolution by creationists. The argument is: "Haeckel's theory was presented as supporting evidence for evolution, Haeckel's theory is wrong, therefore evolution has less support". This argument is not only an oversimplification but misleading because modern biology does recognize numerous connections between ontogeny and phylogeny, explains them using evolutionary theory without recourse to Haeckel's specific views, and considers them as supporting evidence for that theory. See: ontogeny and phylogeny.

Historical impact

Although Haeckel's specific form of recapitulation theory is now discredited among biologists, it did have a strong impact in social and educational theories of the late 19th century. The maturationist theory of G. Stanley Hall was based on the premise that growing children would recapitulate evolutionary stages of development as they grew up and that there was a one to one correspondence between childhood stages and evolutionary history, and that it was counterproductive to push a child ahead of its development stage.

he theory of recapitulation, also called the biogenetic law or embryological parallelism and often expressed as "ontogeny recapitulates phylogeny" is a discredited biological theory. First proposed by Étienne Serres in 1824–26 as what became known as the "Meckel-Serres Law", it attempted to provide a link between comparative embryology and a "pattern of unification" in the organic world. It was supported by Étienne Geoffroy Saint-Hilaire and became a prominent part of his ideas which suggested that past transformations of life could have had environmental causes working on the embryo, rather than on the adult as in Lamarckism. These naturalistic ideas led to disagreements with Georges Cuvier. It was widely supported in the Edinburgh and London schools of higher anatomy around 1830, notably by Robert Edmond Grant, but was opposed by Karl Ernst von Baer's ideas of divergence, and attacked by Richard Owen in the 1830s.
In 1866, the German zoologist Ernst Haeckel proposed that the embryonal development of an individual organism (its ontogeny) followed the same path as the evolutionary history of its species (its phylogeny). This theory, in the highly elaborate and deterministic form advanced by Haeckel, has, since the early twentieth century, been refuted on many fronts. Haeckel's drawings used artistic licence, his theory was associated with Lamarckism[citation needed], it was wrong in supposing that embryos passed through the adult stages of more primitive life-forms, it ignored organs such as teeth which are "held over" to a late developmental stage, and it was used by Haeckel to promote the supremacy of the white European male. However, the basic idea of recapitulation is still widespread - Stephen Jay Gould's first book (Ontogeny and Phylogeny) begins by declaring that many scientific professionals believe, privately and informally, that there is "something to" the notion.

Haeckel's theory

Haeckel formulated his theory as "Ontogeny recapitulates phylogeny". The notion later became simply known as the recapitulation (OED: 'a summing up or brief repetition') theory. Ontogeny is the growth (size change) and development (shape change) of an individual organism; phylogeny is the evolutionary history of a species. Haeckel's recapitulation theory claims that the development of advanced species passes through stages represented by adult organisms of more primitive species. Otherwise put, each successive stage in the development of an individual represents one of the adult forms that appeared in its evolutionary history.

For example, Haeckel proposed that the gill slits (pharyngeal arches) in the neck of the human embryo represented an adult "fishlike" developmental stage as well as signifying a fishlike ancestor. Embryonic pharyngeal arches, the invaginations between the gill pouches or pharyngeal pouches, open the pharynx to the outside. Such gill pouches appear in all tetrapod animal embryos: in mammals, the first gill bar (in the first gill pouch) develops into the lower jaw (Meckel's cartilage), the malleus and the stapes. At a later stage, all gill slits close, only the ear remaining open. But these embryonic pharyngeal arches could not at any stage carry out the same function as the gills of an adult fish.
Haeckel produced several embryo drawings that often overemphasized similarities between embryos of related species. These found their ways into many biology textbooks, and into popular knowledge.

Rejection

Modern biology rejects the literal and universal form of Haeckel's theory. Although humans are generally understood to share ancestors with other taxa, stages of human embryonic development are not functionally equivalent to the adults of these shared common ancestors. In other words, no cleanly defined and functional "fish", "reptile" and "mammal" stages of human embryonal development can be discerned. Moreover, development is nonlinear. For example, during kidney development, at one given time, the anterior region of the kidney is less developed (nephridium) than the posterior region (nephron).
Modern biology does recognize numerous connections between ontogeny and phylogeny[citation needed], and explains them using evolutionary theory without recourse to Haeckel's specific views, and considers them as supporting evidence for that theory.

Historical influence

Although Haeckel's specific form of recapitulation theory is now discredited among biologists, it had a strong influence on social and educational theories of the late 19th century.
English philosopher Herbert Spencer was one of the most energetic promoters of evolutionary ideas to explain many phenomena. He compactly expressed the basis for a cultural recapitulation theory of education in the following claim:
he maturationist theory of G. Stanley Hall was based on the premise that growing children would recapitulate evolutionary stages of development as they grew up and that there was a one-to-one correspondence between childhood stages and evolutionary history, and that it was counterproductive to push a child ahead of its development stage. The whole notion fit nicely with other social Darwinist concepts, such as the idea that "primitive" societies needed guidance by more advanced societies, i.e. Europe and North America, which were considered by social Darwinists as the pinnacle of evolution.[citation needed] An early form of the law was devised by the 19th-century Estonian zoologist Karl Ernst von Baer, who observed that embryos resemble the embryos, but not the adults, of other species.

Modern observations

This section does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (August 2009)
Generally, if a structure pre-dates another structure in evolutionary terms, then it also appears earlier than the other in the embryo. Species which have an evolutionary relationship typically share the early stages of embryonal development and differ in later stages. Examples include:
* The backbone, the common structure among all vertebrates such as fish, reptiles and mammals, appears as one of the earliest structures laid out in all vertebrate embryos.
* The cerebrum in humans, the most sophisticated part of the brain, develops last.
If a structure vanished in an evolutionary sequence, then one can often observe a corresponding structure appearing at one stage during embryonic development, only to disappear or become modified in a later stage. Examples include:
* Whales, which have evolved from land mammals, don't have legs, but tiny remnant leg bones lie buried deep in their bodies. During embryonal development, leg extremities first occur, then recede. Similarly, whale embryos have hair at one stage (like all mammalian embryos), but lose most of it later.
* The common ancestor of humans and monkeys had a tail, and human embryos also have a tail at one point; it later recedes to form the coccyx.
* The swim bladder in fish presumably evolved from a sac connected to the gut, allowing the fish to gulp air. In most modern fish, this connection to the gut has disappeared. In the embryonal development of these fish, the swim bladder originates as an outpocketing of the gut, and is later disconnected from the gut.



Students of biology who have gone to the trouble to memorize this impressive sounding phrase will be disheartened to learn that it has been known to be untrue since it was first proposed as "fact" by Ernst Haeckel nearly 100 years ago! The recapitulation myth, better known as the biogenetic "law", claims that each embryo in its development passes through abbreviated stages that resemble developmental stages of its evolutionary ancestors. The fictitious "gill slits" of human embryos discussed in Myth # 1, for example, are supposed to represent the "fish" or "amphibian" stage of man's evolutionary ancestors. Most professional evolutionists no longer believe this myth. The famous evolutionist Dr. Paul Ehrlich, for example, said: "this interpretation of embryological sequences will not stand close examination. Its' shortcomings have been almost universally pointed out by modern authors, but the idea still has a prominent place in biological mythology." ('The Process of Evolution' 1963, p.66). In his book 'The Beginnings of Life' (1977, p. 32), embryologist Dr. E. Blechschmidt reveals some of his frustration with the persistence of this myth: "The so-called basic law of biogenetics is wrong. No buts or ifs can mitigate this fact. It is not even a tiny bit correct or correct in a different form. It is totally wrong." Yet in a recent (1980) survey of 15 high school biology text books, 9 offered embryological recapitulation as evidence for evolution!


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Species concept

The species is the fundamental unit of organization of the taxonomic system; of interactions between organisms as described by geneticists and ecologists; and of evolution as studied by phylogeneticists.

As a category the term species resists definition; thus, a species concept is adopted as a framework within which biologists of various persuasions delineate the taxa with which they work at the species level. However, no universal concept has been accepted by all biologists for two fundamental reasons: Different groups of organisms in nature are organized differently in terms of reproductive mechanisms and patterns; in degrees of differentiation among species in morphological, genetic, physiological, behavioral, biochemical, and other types of characters; and in the modes of speciation that have given rise to the members of the group. The philosophy, training, working methods, and goals of different of biologists affect the manner in which each perceives the coherence or diversity of the biological world in general and that of the group of organisms in question in particular.

According to the taxonomic concept, a species consists of groups of individuals (populations) that are morphologically similar to one another, and differ morphologically from other such groups. There are several important ideas expressed in this concept. First, there is internal cohesiveness; that is, the members of the species share certain characteristics. Second, there is external distinction because other species have different characteristics, and thus species may be distinguished from one another. Third, the characteristics that a species possesses may be easily observed because they are phenotypic; that is, a species may be identified by its appearance.
Difficulty in applying the taxonomic concept arises with certain groups of organisms. Bacteria are often identified by physiological and biochemical tests requiring sophisticated laboratories and equipment; in addition, the mutation rate in bacteria is so high that the various traits used to identify them can change rapidly. In insects, the morphological differences between species may be very slight and easily overlooked. In certain groups of plants, hybridization and polyploidy have led to a continuous range of variation of characters, in which no discontinuities sufficient to distinguish species can be discerned. Critics claim that the purely phenetic approach of the taxonomic concept may not reflect real genetic or breeding relationships. However, this concept provides guidelines by which species may be recognized by ordinary (nonexperimental) means. The composition of a species so recognized can then be subjected to hypothesis testing within the framework of other concepts. See also Bacterial taxonomy.

According to the biological concept, a species is composed of groups of individuals (populations) that normally interbreed with one another. The fundamental ideas expressed by this concept are that the internal cohesiveness of a species is maintained by the exchange of genes through sexual reproduction (gene flow) and that the distinctness of the species is maintained by reproductive isolation (barriers to gene flow) from other groups of populations. If two populations do not exchange genes, they belong to separate species regardless of their morphological similarity.

This concept works well in those groups of organisms that are exclusively outbreeding, such as birds and mammals. However, it is difficult to apply to plants, in which interbreeding between morphologically very distinct species and even genera is common. Also, those organisms that do not reproduce sexually present problems of classification. Even in sexually reproducing organisms, populations that are morphologically identical but reproductively separated by geographic distance (disjuncts) present problems of classification within the framework of the concept. The populations might interbreed if they were in contact, but this can be determined only under artificial conditions and not in nature. However, the development of the biological species concept has contributed greatly to making taxonomy an evolutionary science because of its emphasis on the identification of genetic, rather than the very possibly superficial phenetic, relationship among organisms.

According to the evolutionary concept, a species is a lineage of ancestor-descendant populations that maintains its identity from other such lineages and that has its own evolutionary tendencies and historical fate. The important ideas expressed in this concept are the following. All organisms, regardless of their mode of reproduction, belong to some evolutionary species. Species need be reproductively isolated from one another to the extent that they maintain their distinction from other species. There may or may not be a morphological discontinuity between species but, if there is, it is reasonable to hypothesize that more than one species is present. If there is not, other data such as that on breeding relationships may be used to recognize species.

The evolutionary concept encompasses the taxonomic concept, the biological concept, and other more narrowly defined concepts—for example, the ecological species, the genetic species, and the paleospecies. It is operational in that it provides guidelines for the recognition of species and for testing of hypotheses concerning membership in each species; it also is compatible with the Linnaean taxonomic hierarchy. As it becomes more widely used by working systematists, problems and difficulties with the concept may appear that will require its refinement. However, the evolutionary concept may in the long run be more acceptable to a wider group of biologists than any other yet proposed.
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Default Speciation

Speciation

Concept

One of the defining characteristics of a species is its reproductive isolation: the fact that among animals and plants that reproduce sexually, it is impossible for members of two different species to mate and produce fertile offspring. Speciation is the process whereby a single species develops over time into two distinct, reproductively isolated species. It is one of the key evolutionary processes and is responsible for the diversity of life that exists on Earth. In the following essay we explore not only the basic facts of speciation and biological diversity but also an example of adaptive radiation, in the form of the wide range of species within the mammalian order.

How It Works

Species and Speciation
The concept of species, discussed in the article devoted to that subject, is an extraordinarily complex one. Owing to limitations of space, that essay only hints at the many details, the competing schools of thought, and the varying definitions of species. Likewise, in the present context, it is possible to examine the concept of speciation only in the most cursory fashion. In addition to consulting the essay on Species for more information, the reader is encouraged to review the article on Taxonomy.
Taxonomy is the area of the biological sciences devoted to the identification, nomenclature, and classification of organisms according to apparent common characteristics. It uses a wide array of specialized rankings for grouping animals, but only seven of them are essential to most biology students. These seven, known as the obligatory hierarchy, are kingdom, phylum, class, order, family, genus, and species. In the case of mammals, it is also useful to refer to subphylum, which in this case is Vertebrata (see the classification of humans in Species), but for the most part it is enough for the beginning student to attain at least some mastery of the obligatory ranks.
Note that species is the most specific of these ranks, which is fitting, because species and specific come from the same Latin root, specie, or "kind." Nonetheless, it is difficult to define species beyond a reference to its place among the categories of the obligatory taxonomy. According to the biological species concept, discussed briefly in Species, a species is any population of individual organisms capable of mating with one another and producing fertile offspring in a natural setting. This is far from the only definition, however.

Interspecific Mating

Occasionally, it is possible to produce an infertile hybrid, such as a mule, which is created by the mating of a male donkey and a female horse, or a hinny, the product of the less common union between a male horse and a female donkey. The infertility is due to genetic disorders that arise when mating takes place between distinct species, and even this imperfect product is possible only by mating two species that are very closely related. Donkeys and horses, for instance, both belong to family Equidae, which makes them very closely connected.

In the taxonomic ranking of humans, this would be equivalent to a human mating with a fellow hominid, or member of family Hominidae. If the long-extinct genus Australopithecus were still around, it is not inconceivable that humans could mate with them and produce at least sterile offspring. Of course, it is unlikely that many humans would want to mate with Australopithecus, the most famous example of which was named "Lucy" after the Beatles' song "Lucy in the Sky with Diamonds." Standing about 3.5-5 ft. (1-1.5 m) tall, Australopithecus was very close in appearance to a modern ape who lived about four million years ago.

All of humans' close relatives are extinct, and today our nearest relatives are members of the order Primates: apes, monkeys, and marsupials. It is impossible to imagine a human mating with one of these animals and producing offspring of any kind. Likewise, it is extremely unlikely that a horse or donkey could mate with a tapir or rhinoceros, which are about as distant in relation to them as other primates are to us. (These species all belong to the order Perissodactyla, herbivorous mammals possessing either one or three hoofed toes on each hind foot.

The Problem of Defining Species

Although the biological species concept is accepted widely, it has its shortcomings, not least of which is the fact that not all species reproduce sexually. Although sexual reproduction is the case with a wide array of animals and even plants, quite a few organisms reproduce by some asexual means: for example, single-cell organisms reproduce by splitting.
Among the competing definitions of species is the phenetic (or morphological) species concept, which relies in part on common sense. According to the phenetic species concept, a species is the smallest possible population of organisms that consistently and continually remains distinct and distinguishable by ordinary methods of observation. There are also a variety of definitions that fall under the heading "phylogenetic species concepts," all of which maintain in one way or another that taxonomic classifications should incorporate the most widely recognized hypotheses regarding the evolutionary lines of descent that produced the organisms in question.

The Process of Speciation

Clearly, there is no hard and fast definition of species, but in general terms, everyone who has some familiarity with the concept has at least a basic knowledge of what does and does not qualify as a species. We will leave finer distinctions to trained taxonomists and other biologists and move on to a fact regarding which there is no disagreement: a wide array of species exists in the world today. Some estimates calculate the number of species in the five kingdoms—animals, plants, monerans, protista, and fungi (see Taxonomy for a very brief identification of each)—at about 1.5 million.
This is only the number of identified species, however. Other figures, based on the probable numbers of unidentified species in the world, put the sum total in the tens of millions. Whatever the case, it is obvious that over the course of evolutionary history (discussed in Evolution and Paleontology), there has been a widespread adaptive radiation—that is, a diversification of species as a result of specialized adaptations by particular populations of organisms.

Speciation events are described as either allopatric or sympatric. Allopatric ("different places") speciation occurs when a population of organisms is divided by a geographic barrier, a great example being the division of squirrel species caused by the formation of the Grand Canyon (see Evolution). Another example is the speciation of the black-throated green warbler, which today consists of one species in the eastern United States, along with three others in the western part of the country. Some scientists speculate that there may once have been a single species of black-throated green warbler, whose population was split by the formation of a glacier during the Pleistocene epoch. The latter was the period of the last ice age, which ended about 10,000 years ago, but the end of the ice age was a slow process. It may be that glaciers, formed in the latter part of that time, helped to separate what became three different western species.
Species share the same gene pool, or the sum of all genetic codes possessed by members of that species. The isolation of two populations slowly results in differences between gene pools, until the two populations are unable to interbreed either because of changes in mating behavior or because of incompatibility of the DNA between the two populations. (Deoxyribonucleic acid, or DNA, contains genetic codes for inheritance. See Genetics for more on this subject.) More rare than allopatric speciation, sympatric ("same place") speciation happens when a group of individuals becomes reproductively isolated from the larger population of the original species. This type of speciation typically results from mutation, or alterations in DNA that result in a genetic change.

Studies of three-spined sticklebacks, a variety of freshwater fish, in British Columbia have revealed what appears to be a fascinating example of sympatric speciation. Evolutionary biologist Dolph Schluter and others have discovered that the region contains two species of stickleback, one with a large mouth that feeds on large prey close to shore, the other with a small mouth that feeds on plankton in open water. Both species jointly inhabit five different lakes. Through DNA analysis, scientists have determined that the lakes were colonized independently by common marine ancestors, meaning that the process of sympatric speciation between the two varieties had to have occurred independently at least five times. This seems to indicate a situation of competition for resources that favored stickleback species at either extreme of size, as opposed to those of medium size and medium-sized mouths.

Rate of Evolutionary Change
Closely tied to speciation is the rate of evolutionary change, or the speed at which new species arise. This is a long process, one that is usually not observable within a human lifetime or even the span of many lifetimes, though bacteria at least have shown some evolutionary change in their growing resistance to antiobiotics (see Infection). DNA analysis (see Genetics and Genetic Engineering for more about DNA) has been used to examine the rate of evolutionary change. To perform such analysis, it is necessary first to determine the percentage of similarity between the organisms under study: the greater the similarity, the more recently the organisms probably diverged from a common stock. Data obtained in this manner then must be corroborated by information obtained from other sources, such as the fossil record and comparative anatomy studies.
At certain times the rate of evolutionary change can be very rapid, leaving little fossil evidence of intermediate forms, a phenomenon known as punctuated equilibrium. This is contrasted with phyletic (that is, evolutionary) gradualism. Of course, the term rapid in this context is relative, since we are talking about vast spans of time. Life on Earth has existed for about 3,000 million years, and the fossil record goes back some 1,000 million years. This is the case, in part, because to leave fossilized remains, an organism must have "hard parts" that can become mineralized to turn into fossils. (
Real-Life Applications

The Diversity of Mammals
One of the most interesting examples of speciation is that which has produced the vast array of species, including humans, that fall within the mammalian class. Mammals began evolving before the dawn of the Cenozoic era about 65 million years ago. The Cenozoic era, which started with a catastrophic event that brought about the mass extinction of the dinosaurs and the end of the Mesozoic era (see Paleontology), is truly the age of the mammal. Just as dinosaurs dominated the Mesozoic, today the world belongs to mammals as to no other class of creature.
Since its humble beginnings in the shadow of the dinosaurs, class Mammalia has undergone a massive radiation to the point that today some 4,625 species of mammal, in about 125 families and 24 orders, are recognized. (That number is changing, as noted later in the context of elephants.) This diversity is tied closely to mammals' enormous mobility, which facilitated their spread throughout the world. Aside from much less complex life-forms, such as arachnids and insects (see Parasites and Parasitology), mammals are believed to be distributed more widely throughout the world than any other comparable taxonomic grouping. Insects may be the most diverse of all animal classes, with numbers of species that may be many times greater than the number of mammals, but considering mammals' much-greater level of physical development and complexity, the diversity of their species is astounding.

Mammals' Early Evolution
In the next section we list the orders of mammals and give very brief descriptions of each. The purpose here is not to provide anything like a comprehensive discussion but rather to illustrate the enormous range of species in a class that includes anteaters, dolphins, humans, elephants, and bats. The fact that all these diverse creatures, and many more, emerged from a common evolutionary lineage is almost as amazing as the fact that this common ancestor was a reptile.
Mammals are believed to have come from the reptilian order Therapsida, which emerged during the Triassic period (from about 245 to 208 million years ago) in the early part of the Mesozoic era. Over the course of many millions of years, these creatures began to develop a number of mammal-like qualities—in particular, endothermy, or the ability to maintain internal temperature regardless of environmental conditions. In other words, these cold-blooded creatures became warm-blooded. This evolutionary process was as complex as it was lengthy. Nor was there a clean break with the past—no moment when the therapsids faded away or when it would have been clear that mammals had taken the place of their reptilian ancestors. Rather, in what must have been a fascinating taxonomic situation, for many millions of years, species that combined aspects of both reptiles and mammals walked the earth.

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Default Speciation

Speciation

The process by which new species of organisms evolve from preexisting species. It is part of the whole process of organic evolution. The modern period of its study began with the publication of Charles Darwin's and Alfred Russell Wallace's Theory of Evolution by Natural Selection in 1858, and Darwin's On the Origin of Species in 1859.

Belief in the fixity of species was almost universal before the middle of the nineteenth century. Then it was gradually realized that all species continuously change, or evolve; however, the causative mechanism remained to be discovered. Darwin proposed a mechanism. He argued that within any species population there is always some heritable variation; the individuals differ among themselves in structure, physiology, and behavior; and natural selection acts upon this variation by eliminating the less fit. Thus if two members of an animal population differ from each other in their ability to find a mate, obtain food, escape from predators, resist the ravages of parasites and pathogens, or survive the rigors of the climate, the more successful will be more likely than the less successful to leave descendants. The more successful is said to have greater fitness, to be better adapted, or to be selectively favored. Likewise among plants: one plant individual is fitter than another if its heritable characteristics make it more successful than the other in obtaining light, water, and nutrients, in protecting itself from herbivores and disease organisms, or in surviving adverse climatic conditions. Over the course of time, as the fitter members of a population leave more descendants than the less fit, their characteristics become more common.

This is the process of natural selection, which tends to preserve the well adapted at the expense of the ill adapted in a variable population. The genetic variability that must exist if natural selection is to act is generated by genetic mutations in the broad sense, including chromosomal rearrangements together with point mutations. See also Genetics; Mutation.
If two separate populations of a species live in separate regions, exposed to different environments, natural selection will cause each population to accumulate characters adapting it to its own environment. The two populations will thus diverge from each other and, given time, will become so different that they are no longer interfertile. At this point, speciation has occurred: two species have come into existence in the place of one. This mode of speciation, speciation by splitting, is probably the most common mode. Two other modes are hybrid speciation and phyletic speciation; many biologists do not regard the latter as true speciation.

Many students of evolution are of the opinion that most groups of organisms evolve in accordance with the punctuated equilibrium model rather than by phyletic gradualism. There are two chief arguments for this view. First, it is clear from the fossil record that many species persist without perceptible change over long stretches of time and then suddenly make large quantum jumps to radically new forms. Second, phyletic gradualism seems to be too slow a process to account for the tremendous proliferation of species needed to supply the vast array of living forms that have come into existence since life first appeared on Earth.
~~~~~~~~~
Speciation is the process by which new species of organisms arise. Earth is inhabited by millions of different organisms, all of which likely arose from one early life-form that came into existence about 3.5 billion years ago. It is the task of taxonomists to decide which out of the multitude of different types of organisms should be considered species. The wide range in the characteristics of individuals within groups makes defining a species more difficult. Indeed, the definition of species itself is open to debate.

Concepts of Species

In the broadest sense, a species can be defined as a group of individuals that is "distinct" from another group of individuals. Several different views have been put forward about what constitutes an appropriate level of difference. Principal among these views are the biological-species concept and the morphological-species concept.

The biological-species concept delimits species based on breeding. Members of a single species are those that interbreed to produce fertile off-spring or have the potential to do so. The morphological-species concept (from the ancient Greek root "morphos," meaning form) is based on classifying species by a difference in their form or function. According to this concept, members of the same species share similar characteristics. Species that are designated by this criteria are known as a morphological species.
Organisms within a species do not necessarily look identical. For example, the domestic dog is considered to be one species, even though there is a huge range in size and appearance among the different breeds. For naturally occurring populations of organisms that we are much less familiar with, it is much more difficult to recognize the significance of any character differences observed. Therefore deciding what characteristics should be used as criteria to designate a species can be difficult.

Speciation Mechanisms: Natural Selection and Genetic Drift

Before the development of the modern theory of evolution, a widely held idea regarding the diversity of life was the "typological" or "essentialist" view. This view held that a species at its core had an unchanging perfect "type" and that any variations on this perfect type were imperfections due to environmental conditions. Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) independently developed the theory of evolution by natural selection, now commonly known as Darwinian evolution.
The theory of Darwinian evolution is based on two main ideas. The first is that heritable traits that confer an advantage to the individual that carries them will become more widespread in a population through natural selection because organisms with these favorable traits will produce more offspring. Since different environments favor different traits, Darwin saw that the process of natural selection would, over time, make two originally similar groups become different from one another, ultimately creating two species from one. This led to the second major idea, which is that all species arise from earlier species, therefore sharing a common ancestor.

When so much change occurs between different groups that they are morphologically distinct or no longer able to interbreed, they may be considered different species; this process is known as speciation. A species as a whole can transform over time into a new species (vertical evolution) or split into more separate populations, each of which may develop into new species (adaptive radiation).

Modern population geneticists recognize that natural selection is not the only factor causing genetic change in a population over time. Genetic drift is the random change in the genetic composition of a small population over time, due to an unequal genetic contribution by individuals to succeeding generations. It is thought that genetic drift can result in new species, especially in small isolated populations.

Isolating Mechanisms

Whether natural selection and genetic drift lead to new species depends on whether there is restricted gene flow between different groups. Gene flow is the movement of genes between separate populations by migration of individuals. If two populations remain in contact, gene flow will prevent them from becoming separate species (though they may both develop into a new species through vertical evolution).
Gene flow is restricted through geographic effects such as mountain ranges and oceans, leading to geographic isolation. Gene flow can also be prevented by biological factors known as isolating mechanisms. Biological isolating mechanisms include differences in behavior (especially mating behavior), and differences in habitat use, both of which lead to a decrease in mating between individuals from different groups.
When geographic separation plays a role in speciation, this is known as allopatric speciation, from the Greek roots allo, meaning separate, and "patric," meaning country. In allopatric speciation, natural selection and genetic drift can act together.

For example, imagine a mud slide that causes a river to back up into a valley, separating a population of rodents into two, one restricted to the shady side of the river, the other to the sunny side. Because coat thickness is a genetically inherited trait, eventually, through natural selection, the population of animals on the cooler side may develop thicker coats. After many generations of separation, the two groups may look quite different and may have evolved different behaviors as well, to allow them to survive better in their respective habitats. Genetic drift may occur especially if either or both populations remain small. Eventually these two populations may be so different as to warrant designation as different species.

It is also possible for new species to form from a single population without any geographic separation. This is known as "ecological" or "sympatric" (from the Greek root sym, meaning same) speciation, and it results in ecological differences between morphologically similar species inhabiting the same area. Sympatric speciation can occur in flowering plants in a single generation, due to the formation of a polyploid. Polyploidy is the complete duplication of an organism's genome, for example from n chromosomes to 4n. Even higher multiples of n are possible. This increase in a plant's DNA content makes it reproductively incompatible with other individuals of its former species.
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Default Speciation

What Is a Species?

One of the best definitions is that of the evolutionary biologist Ernst Mayr:
A species is an actually or potentially interbreeding population that does not interbreed with other such populations when there is opportunity to do so.
Note: sometimes breeding may take place (as it can between a horse and a donkey) but if so, the offspring are not so fertile and/or well adapted as the parents (the mule produced is sterile).

Allopatric Speciation: the Role of Isolation in Speciation

The formation of two or more species often (some workers think always!) requires geographical isolation of subpopulations of the species. Only then can natural selection or perhaps genetic drift produce distinctive gene pools.
It is no accident that the various races (or "subspecies") of animals almost never occupy the same territory. Their distribution is allopatric ("other country").
The seven distinct subspecies or races of the yellowthroat Geothlypis trichas in the continental U.S. would soon merge into a single homogeneous population if they occupied the same territory and bred with one another.

Darwin's Finches

As a young man of 26, Charles Darwin visited the Galapagos Islands off the coast of Ecuador.
Among the animals he studied were 13 species of finches found nowhere else on earth.
* Some have stout beaks for eating seeds of one size or another (#2, #3, #6).
* Others have beaks adapted for eating insects or nectar.
* One (#7) has a beak like a woodpecker's. It uses it to drill holes in wood, but lacking the long tongue of a true woodpecker, it uses a cactus spine held in its beak to dig the insect out.
* One (#12) looks more like a warbler than a finch, but its eggs, nest, and courtship behavior is like that of the other finches.
Darwin's finches. The finches numbered 1–7 are ground finches. They seek their food on the ground or in low shrubs. Those numbered 8–13 are tree finches. They live primarily on insects.
1. Large cactus finch (Geospiza conirostris)
2. Large ground finch (Geospiza magnirostris)
3. Medium ground finch (Geospiza fortis)
4. Cactus finch (Geospiza scandens)
5. Sharp-beaked ground finch (Geospiza difficilis)
6. Small ground finch (Geospiza fuliginosa)
7. Woodpecker finch (Cactospiza pallida)
8. Vegetarian tree finch (Platyspiza crassirostris)
9. Medium tree finch (Camarhynchus pauper)
10. Large tree finch (Camarhynchus psittacula)
11. Small tree finch (Camarhynchus parvulus)
12. Warbler finch (Certhidia olivacea)
13. Mangrove finch (Cactospiza heliobates)


Since Darwin's time, these birds have provided a case study of how a single species reaching the Galapagos from Central or South America could — over a few million years — give rise to the 13 species that live there today.
Several factors have been identified that may contribute to speciation.

Ecological opportunity

When the ancestor of Darwin's finches reached the Galapagos, it found
* no predators (There were no mammals and few reptiles on the islands.)
* few, if any, competitors. There were only a handful of other types of songbirds. In fact, if true warblers or woodpeckers had been present, their efficiency at exploiting their niches would surely have prevented the evolution of warblerlike and woodpeckerlike finches.

Geographical Isolation (allopatry)

The proximity of the various islands has permitted enough migration of Darwin's finches between them to enable distinct island populations to arise. But the distances between the islands is great enough to limit interbreeding between populations on different islands. This has made possible the formation of distinctive subspecies (= races) on the various islands.
The importance of geographical isolation is illuminated by a single, fourteenth, species of Darwin's finches that lives on Cocos Island, some 500 miles to the northeast of the Galapagos.
The first immigrants there must also have found relaxed selection pressures with few predators or competitors.
How different the outcome, though. Where one immigrant species gave rise to at least 13 on the scattered Galapagos Islands, no such divergence has occurred on the single, isolated Cocos Island.

Evolutionary Change

In isolation, changes in the gene pool can occur through some combination of
* natural selection
* genetic drift
* founder effect
These factors may produce distinct subpopulations on the different islands. So long as they remain separate (allopatric) we consider them races or subspecies. In fact, they might not be able to interbreed with other races but so long as we don't know, we assume that they could.
How much genetic change is needed to create a new species?
Perhaps not as much as you might think. For example, changes at one or just a few gene loci might do the trick. For example, a single mutation altering flower color or petal shape could immediately prevent cross-pollination between the new and the parental types (a form of prezygotic isolating mechanism).

Reunion

The question of their status — subspecies or true species — is resolved if they ever do come to occupy the same territory again (become sympatric). If successful interbreeding occurs, the differences will gradually disappear, and a single population will be formed again. Speciation will not have occurred.
If, on the other hand, two subspecies reunite but fail to resume breeding, speciation has occurred and they have become separate species.
An example: The medium tree finch Camarhynchus pauper is found only on Floreana Island. Its close relative, the large tree finch, Camarhynchus psittacula, is found on all the central islands including Floreana.
Were it not for its presence on Floreana, both forms would be considered subspecies of the same species. Because they do coexist and maintain their separate identity on Floreana, we know that speciation has occurred.
Isolating Mechanisms
What might keep two subpopulations from interbreeding when reunited geographically? There are several mechanisms.

Prezygotic Isolating Mechanisms

These act before fertilization occurs.
* Sexual selection — a failure to elicit mating behavior. On Floreana, Camarhynchus psittacula has a longer beak than Camarhynchus pauper, and the Grants have demonstrated that beak size is an important criterion by which Darwin's finches choose their mates.
* Two subpopulations may occupy different habitats in the same area and thus fail to meet at breeding time.
* In plants, a shift in the time of flowering can prevent pollination between the two subpopulations.
* Structural differences in the sex organs may become an isolating mechanism.
* The sperm may fail to reach or fuse with the egg.

Postzygotic Isolating Mechanisms

These act even if fertilization does occur.
* Even if a zygote is formed, genetic differences may have become so great that the resulting hybrids are less viable or less fertile than the parental types. The sterile mule produced by mating a horse with a donkey is an example.
* Sterility in the males produced by hybridization is more common than in females. In fact, it is the most common postzygotic isolating mechanism.
* When Drosophila melanogaster attempts to mate with its relative Drosophila simulans, no viable males are even produced. Mutations in a single gene (encoding a component of the nuclear pore complex) are responsible.

Speciation by Hybridization

Hybridization between related angiosperms is sometimes followed by a doubling of the chromosome number. The resulting polyploids are now fully fertile with each other although unable to breed with either parental type. A new species has been created. This appears to have been a frequent mechanism of speciation in angiosperms.
Even without forming a polyploid, interspecific hybridization can occasionally lead to a new species of angiosperm. Two species of sunflower, the "common sunflower", Helianthus annuus, and the "prairie sunflower", H. petiolaris, grow widely over the western half of the United States. They can interbreed, but only rarely are fertile offspring produced.

However, Rieseberg and colleagues have found evidence that successful hybridization between them has happened naturally in the past. They have shown that three other species of sunflower (each growing in a habitat too harsh for either parental type) are each the product of an ancient hybridization between Helianthus annuus and H. petiolaris. Although each of these species has the same diploid number of chromosomes as the parents (2n = 34), they each have a pattern of chromosome segments that have been derived, by genetic recombination, from both the parental genomes. They demonstrated this in several ways, notably by combining RFLP analysis with the polymerase chain reaction (PCR).
They even went on to produce (at a low frequency) annuus x petiolaris hybrids in the greenhouse that mimicked the phenotypes and genotypes of the natural hybrids. (You can read about the results of these monumental studies in the 29 August 2003 issue of Science.)

Another example. In Pennsylvania, hybrids between
* a species of fruit fly (not Drosophila) that feeds on blueberries and
* another species (again, not Drosophila) that feeds on snowberries
feed on honeysuckle where they neither
* encounter competition from their parental species nor
* have an opportunity to breed with them (no introgression).
(You can read about the this study in the 28 July 2005 issue of Nature.)
So speciation can occur as a result of hybridization between two related species, if the hybrid
* receives a genome that enables it to breed with other such hybrids but
* not breed with either parental species;
* can escape to a habitat where it does not have to compete with either parent;
* is adapted to live under those new conditions.

Intense Competition

The process of speciation is often hastened when two formerly isolated groups are reunited. Even if they no longer interbreed, they are probably still similar in many ways, including their requirements for the necessities of life.
Thus the reuniting of the two groups may create an intense selection pressure leading to one of two possible outcomes.
1. The competition may be so intense that one species becomes eliminated entirely; that is, it is driven to extinction on that island. This may have occurred to C. pauper on some of the central islands.
2. Alternatively, the increased selection pressure may lead to character displacement and so lessen the competition between them.

Adaptive Radiation

The process described above can occur over and over. In the case of Darwin's finches, it must have been repeated a number of times forming new species that gradually divided the available habitats between them. From the first arrival have come a variety of ground-feeding and tree-feeding finches as well as the warblerlike finch and the tool-using woodpeckerlike finch.
The formation of a number of diverse species from a single ancestral one is called an adaptive radiation.
House mice on the island of Madeira
A report in the 13 January 2000 issue of Nature describes a study of house mouse populations on the island of Madeira off the Northwest coast of Africa. These workers (Janice Britton-Davidian et al) examined the karyotypes of 143 house mice (Mus musculus domesticus) from various locations along the coast of this mountainous island. Their findings:
* There are 6 distinct populations (shown by different colors)
* Each of these has a distinct karyotype, with a diploid number less than the "normal" (2N=40).
* The reduction in chromosome number has occurred through Robertsonian fusions. Mouse chromosomes tend to be acrocentric; that is, the centromere connects one long and one very short arm. Acrocentric chromosomes are at risk of translocations that fuse the long arms of two different chromosomes with the loss of the short arms.
* The different populations are allopatric; isolated in different valleys leading down to the sea.
* The distinct and uniform karyotype found in each population probably arose from genetic drift rather than natural selection.
* The 6 different populations are technically described as races because there is no opportunity for them to attempt interbreeding.
* However, they surely meet the definition of true species. While hybrids would form easily (no prezygotic isolating mechanisms), these would probably be infertile as proper synapsis and segregation of such different chromosomes would be difficult when the hybrids attempted to form gametes by meiosis.
Sympatric Speciation
Sympatric speciation refers to the formation of two or more descendant species from a single ancestral species all occupying the same geographic location.
Some evolutionary biologists don't believe that it ever occurs. They feel that interbreeding would soon eliminate any genetic differences that might appear.
But there is some compelling (albeit indirect) evidence that sympatric speciation can occur.
The three-spined sticklebacks, freshwater fishes, that have been studied by Dolph Schluter (who received his Ph.D. for his work on Darwin's finches with Peter Grant) and his current colleagues in British Columbia, provide an intriguing example that is best explained by sympatric speciation.
They have found:
* Two different species of three-spined sticklebacks in each of five different lakes.
o a large benthic species with a large mouth that feeds on large prey in the littoral zone
o a smaller limnetic species — with a smaller mouth — that feeds on the small plankton in open water.
* DNA analysis indicates that each lake was colonized independently, presumably by a marine ancestor, after the last ice age.
* DNA analysis also shows that the two species in each lake are more closely related to each other than they are to any of the species in the other lakes.
* Nevertheless, the two species in each lake are reproductively isolated; neither mates with the other.
* However, aquarium tests showed that
o the benthic species from one lake will spawn with the benthic species from the other lakes and
o likewise the limnetic species from the different lakes will spawn with each other.
o These benthic and limnetic species even display their mating preferences when presented with sticklebacks from Japanese lakes; that is, a Canadian benthic prefers a Japanese benthic over its close limnetic cousin from its own lake.
* Their conclusion: in each lake, what began as a single population faced such competition for limited resources that
o disruptive selection — competition favoring fishes at either extreme of body size and mouth size over those nearer the mean — coupled with
o assortative mating — each size preferred mates like it
favored a divergence into two subpopulations exploiting different food in different parts of the lake.
* The fact that this pattern of speciation occurred the same way on three separate occasions suggests strongly that ecological factors in a sympatric population can cause speciation.
Sympatric speciation driven by ecological factors may also account for the extraordinary diversity of crustaceans living in the depths of Siberia's Lake Baikal.

Parapatric Speciation
There is another possible way for new species to arise in the absence of geographical barriers.
* If a population ranges of a vast area (e.g., the Amazon basin in south America) and
* if the individuals in that population can disperse over only a small portion of this range,
then gene flow across these great distances would be reduced.
Genetic isolation arising simply from the great distance separating subpopulations could thus lead to "parapatric" speciation. Some workers feel that the enormous species diversity of the rain forests of South America (greater than that of Africa and Asia combined!) arose from parapatric speciation.
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