Genetic drift

Genetic drift is one of the three forces that affect evolution in a big way. The other two are selection and mutation. Random Genetic Drift causes changes in the phenotypes and genotype over a period of time. It also determines the kind and degree of variation within a population of a species. In the real world there is also a lot dependant on the factors of fortune chance with the reproduction and survival of any organism being subject to accidents that are entirely unpredictable. Genetic drift is a kind of statistical effect that is the outcome of influence that it brings about making an allele rarer or more common over several generations. Genetic drift is not predisposed towards a particular direction and a decrease or an increase is equally likely.

Genetic drift is a mechanism of evolution that acts in concert with natural selection to change the characteristics of species over time. It is a stochastic effect that arises from the role of random sampling in the production of offspring. Like selection, it acts on populations, altering the frequency of alleles and the predominance of traits amongst members of a population, and changing the diversity of the group. drift is observed most strongly in small populations and results in changes that need not be adaptive.

1 Allele frequencies
2 drift versus selection
3 genetic drift in populations


Allele frequencies

From the perspective of population genetics, drift is a "sampling effect". To illustrate: on average, coins turn up heads or tails equally. Yet just a few tosses in a row are unlikely to produce heads and tails in equal number. The numbers are no more likely to be exactly equal for a large number of tosses in a row, but the inequality can be very small in percentage terms. As an example, ten tosses turn up 70% heads about once in every six tries, but the chance of a hundred tosses in a row producing 70% heads is only about one in 25,000.

Similarly, in a breeding population, if an allele has a frequency of p, probability theory dictates that (if natural selection is not acting) in the following generation, a fraction p of the population will inherit that particular allele. However, as with the coin toss above, allele frequencies in real populations are not probability distributions; rather, they are a random sample, and are thus subject to the same statistical fluctuations (sampling error ).

When the alleles of a gene do not differ with regard to fitness, on average the number of carriers in one generation is proportional to the number of carriers in the last. But the average is never tallied, because each generation parents the next one only once. Therefore the frequency of an allele among the offspring often differs from its frequency in the parent generation. In the offspring generation, the allele might therefore have a frequency slightly different from p (p'). In this situation, the allele frequencies are said to have drifted. Note that the frequency of the allele in the following generations will now be determined by the new frequency p'.

As in the coin toss example above, the size of the breeding population (the effective population size) governs the strength of the drift effect. When the effective population size is small, genetic drift will be stronger.

Drifting alleles usually have a finite lifetime. As the frequency of an allele drifts up and down over successive generations, eventually it drifts till fixation - that is, it either reaches a frequency of zero, and disappears from the population, or it reaches a frequency of 1 and becomes the only allele in the population. Subsequent to the latter event, the allele frequency can only change by the introduction of a new allele by a new mutation.

The lifetime of an allele is governed by the effective population size. In a very small population, only a few generations might be required for genetic drift to result in fixation. In a large population, it would take many more generations. On average, an allele will be fixed in 4Ne generations, where Ne is the effective population size.

Drift versus selection

Genetic drift and natural selection rarely occur in isolation of each other; both forces are always at play in a population. However, the degree to which alleles are affected by drift and selection varies according to circumstance.

In a large population, where genetic drift occurs very slowly, even weak selection on an allele will push its frequency upwards or downwards (depending on whether the allele is beneficial or harmful). However, if the population is very small, drift will predominate. In this case, weak selective effects may not be seen at all as the small changes in frequency they would produce are overshadowed by drift.

Genetic drift in populations

Drift can have profound and often bizarre effects on the evolutionary history of a population. These effects may be at odds with the survival of the population.

In a population bottleneck, where the population suddenly contracts to a small size and then grows again to a large population (believed to have occurred in the history of human evolution), genetic drift can result in sudden and dramatic changes in allele frequency that occur independently of selection. In such instances, many beneficial adaptations may be eliminated.

Similarly, migrating populations may see founder's effect, where a few individuals with a rare allele in the originating generation can produce a population that has allele frequencies that seem to be at odds with natural selection. Founder's effects are sometimes held to be responsible for high frequencies of some genetic diseases.

How does Darwin's theory of natural selection explain the origin of species?

If evolution was a car, the theory of natural selection would be the engine. The basic ideas of evolution were discussed long before there was any scientific research done to support them. The evolutionary concept was never able to gain any real steam because it lacked a mechanism. That is, scientists wanted to believe that species evolved from one form to another, but had no plausible process to make it happen. The theory of natural selection provides that reasonable method of evolution.

Natural selection essentially states that "the strong survive." The basic idea is that when change occurs, those organisms best suited to the new circumstances will thrive. Those who are not ideally suited will not be able to compete. Charles Darwin proposed this principle after observing some population variations in birds. He noticed that animals within a species often had slightly varied traits, and that those traits made some more suited to certain conditions. Darwin's theory was that, over time, the better suited animals would thrive and the others would die out completely. The resulting population would be entirely made up of those animals with the "better" trait. Over time, he reasoned, this could result in a species changing enough traits to eventually become a totally different creature, like a fish becoming a frog.

There have been some concerns expressed about the real meanings of the theory of natural selection. There is no doubt that variations within a single species make some members better suited to handle different circumstances. For instance, there's a popular story in science texts about moths. These moths lived in cities around the time of the industrial revolution and had to deal with increased pollution. Lighter-colored moths stood out on soot-stained buildings and trees, and thus, were easier targets for birds. The darker moths found it easier to survive, because they blended into the darkened environment. As a result, the population of light-colored moths dwindled over time, and the darker-colored moths increased. The dominance of the darker moths is used as an example of natural selection.

There is an important point to be made about the theory of natural selection, however. Once conditions return to "normal," the balance of that species will return to "normal" as well. Birds with unusually heavy beaks may become dominant during dry years, since they can more easily break open nut shells and tree bark. The "normal" birds, with regular beaks, will struggle and diminish. Yet, once the drought is over, the population tends back to normal-beaked birds. The darker moths who were more suited to the polluted times made up most of the moth population, but when the pollution began to fade, the moth population returned to its "normal" state.

Why does this happen? Species have shown to be genetically stable. In fact, genetic defects that change the form or function of creatures usually result in death. The examples of the moths and birds show that each species has some variations, and that those variations can favor different animals at different times. However, they also show that the same variations are possible generation after generation - which is why the populations can change right back to where they were. There are no new species or new variations being produced, just more or less of those that already existed.

There has been no scientific observation of any permanent change in species. There are plenty of proven cases of adaptation, which involves non-genetic changes. There are examples of natural selection changing the balance of populations within a species. Yet there are no known instances of a natural population experiencing a permanent, meaningful change. Observed genetic mutations are, in the natural world, crippling and usually fatal. While there is no doubt about the short-term function of natural selection, its long-term effects are not fully understood. While scientists prefer to point to the examples of birds and moths as proof of the theory of natural selection, they often refuse to see the same examples as contradictory to evolution itself.

Theory of Natural Selection


In the 19th century, a man called Charles Darwin, a biologist from England, set off on the ship HMS Beagle to investigate species of the island.

After spending time on the islands, he soon developed a theory that would contradict the creation of man and imply that all species derived from common ancestors through a process called natural selection. Natural selection is considered to be the biggest factor resulting in the diversity of species and their genomes. The principles of Darwin's work and his theory are stated below.

* One of the prime motives for all species is to reproduce and survive, passing on the genetic information of the species from generation to generation. When species do this they tend to produce more offspring than the environment can support.
* The lack of resources to nourish these individuals places pressure on the size of the species population, and the lack of resources means increased competition and as a consequence, some organisms will not survive.
* The organisms who die as a consequence of this competition were not totally random, Darwin found that those organisms more suited to their environment were more likely to survive.
* This resulted in the well known phrase survival of the fittest, where the organisms most suited to their environment had more chance of survival if the species falls upon hard times. (This phrase if often associated with Darwin, though on closer inspection Herbert Spencer puts the phrase in a more accurate historical context.)
* Those organisms who are better suited to their environment exhibit desirable characteristics, which is a consequence of their genome being more suitable to begin with.

This 'weeding out' of less suited organisms and the reward of survival to those better suited led Darwin to deduce that organisms had evolved over time, where the most desirable characteristics of a species are favoured and those organisms who exhibit them survive to pass their genes on.

As a consequence of this, a changing environment would mean different characteristics would be favourable in a changing environment. Darwin believed that organisms had 'evolved' to suit their environments, and occupy an ecological niche where they would be best suited to their environment and therefore have the best chance of survival.

As the above indicates, those alleles of a species that are favoured in the environment will become more frequent in the genomes of the species, due to the organisms higher likeliness of surviving as part of the species at large

Theory of Natural Selection


In the 19th century, a man called Charles Darwin, a biologist from England, set off on the ship HMS Beagle to investigate species of the island.

After spending time on the islands, he soon developed a theory that would contradict the creation of man and imply that all species derived from common ancestors through a process called natural selection. Natural selection is considered to be the biggest factor resulting in the diversity of species and their genomes. The principles of Darwin's work and his theory are stated below.

* One of the prime motives for all species is to reproduce and survive, passing on the genetic information of the species from generation to generation. When species do this they tend to produce more offspring than the environment can support.
* The lack of resources to nourish these individuals places pressure on the size of the species population, and the lack of resources means increased competition and as a consequence, some organisms will not survive.
* The organisms who die as a consequence of this competition were not totally random, Darwin found that those organisms more suited to their environment were more likely to survive.
* This resulted in the well known phrase survival of the fittest, where the organisms most suited to their environment had more chance of survival if the species falls upon hard times. (This phrase if often associated with Darwin, though on closer inspection Herbert Spencer puts the phrase in a more accurate historical context.)
* Those organisms who are better suited to their environment exhibit desirable characteristics, which is a consequence of their genome being more suitable to begin with.

This 'weeding out' of less suited organisms and the reward of survival to those better suited led Darwin to deduce that organisms had evolved over time, where the most desirable characteristics of a species are favoured and those organisms who exhibit them survive to pass their genes on.

As a consequence of this, a changing environment would mean different characteristics would be favourable in a changing environment. Darwin believed that organisms had 'evolved' to suit their environments, and occupy an ecological niche where they would be best suited to their environment and therefore have the best chance of survival.

As the above indicates, those alleles of a species that are favoured in the environment will become more frequent in the genomes of the species, due to the organisms higher likeliness of surviving as part of the species at large.

Examples of Natural Selection

Darwin's Finches

Darwin's finches are an excellent example of the way in which species' gene pools have adapted in order for long term survival via their offspring. The Darwin's Finches diagram below illustrates the way the finch has adapted to take advantage of feeding in different ecological niche's.
Their beaks have evolved over time to be best suited to their function. For example, the finches who eat grubs have a thin extended beak to poke into holes in the ground and extract the grubs. Finches who eat buds and fruit would be less successful at doing this, while their claw like beaks can grind down their food and thus give them a selective advantage in circumstances where buds are the only real food source for finches.


Industrial Melanism

Polymorphism pertains to the existence of two distinctly different groups of a species that still belong to the same species. Alleles for these organisms over time are governed by the theory of natural selection, and over this time the genetic differences between groups in different environments soon become apparent, as in the case of industrial melanism.

Industrial melanism occurs in a species called the peppered moth, where the occurrence has become of more frequent occurrence since the beginning of the industrial age. The following argument elaborates the basis of principles involved in natural selection as far as industrial melanism is concerned.

* Pollution, which is more common in today's world since the industrial age causes a change in environment, particularly in the 1800's when soot would collect on the sides of buildings from chimneys and industries and make them a darker colour.
* The resultant effect was that the peppered moth, which had a light appearance was more visible against the darker backgrounds of sooty buildings.
* This meant that predators of the peppered moth could find them more easily as they are more visible against a dark background.
* Due to mutations, a new strain of peppered moth came to existence, where their phenotype was darker than that of the white peppered moth.
* This meant that these new, darker peppered moths were once again harder to track down by their prey in environments where industry has taken its toll.
* In this instance, natural selection would favour the darker moths in polluted environments and the whiter moths in the lesser polluted environments due to their ability to merge in with their environmental colours and lessen the chances of them being prone to a predator.

Sickle Cell Trait

Consider this argument of natural selection in the case of sickle cell trait, a genetic defect common in Africa.

* Sickle cell trait is a situation that occurs in the presence of a recessive allele coding for haemoglobin, a substance in the blood responsible for the transport of gases like oxygen. The presence of the allele is either partially expressed recessively (sickle cell), or fully expressed by a complete recessive expression which results in full blown anaemia. If this particular allele is dominant, no sickle cell trait is expressed in the phenotype.
* The above occurrences in the case of a recessive allele result in structural defects of red blood cells, severely reducing the organisms capacity to uptake oxygen.
* It was pointed out that in Africa, there is a high frequency of this mutation, where cases of malaria were high.
* A substantiated link was made noting those who suffer sickle cell trait or anaemia were immune to the effects of malaria.
* This is yet again natural selection at work. Although sickle cell trait or anaemia are not advantageous characteristics on their own, they prove to be advantageous in areas where malaria proves to be a greater threat to preserving the genome (i.e. surviving).
* The incomplete dominance of this genetic expression proves favourable either way.

This is how science has understood natural selection since the first studies involving Darwin. In the 21st century, humans selectively breed species to create hybrid species possessing the best genes of both parents via a process known as selective breeding.

INDUSTRIAL MELANISM
Industrial melanism is where natural selection pressures due to man-made influences have led to color changes in certain species. The industrial revolution across Europe and North America, and the burning of coal, led to every surface, in certain areas, being covered with black soot.
Species relying on camouflage as a means of avoiding predation had to adapt or risk local extinction. In many developing countries, coal burning is still fueling this process.

THE PEPPERED MOTH


The peppered moth, Biston betularia f. typica, (Fig. 1) is a species that is largely light colored, with dark speckled patches, enabling them to lie camouflaged against lichen growth on the bark of trees. There is a sub-species of this moth, f. carbonaria, (Fig. 2) where a genetic mutation has led to the moth being dark with light patches.
In a normal environment, the dark subspecies tended not to last long, as it’s visibility against the light background would lead to it being more easily spotted by predators and eaten. This result of this is that it would be less likely to pass down it’s coloration through the generations.


INDUSTRIAL MELANISM IN ACTION

Normally, evolutionary pressures change only slowly, meaning that genetic change and natural selection moves very slowly. This began to change with the increasing industrialization of human societies. The Industrial Revolution, in Britain, burned vast amounts of coal, producing sulfur dioxide that killed off all the lichens. Factories also threw out huge amounts of black soot, covering every building and every tree with black grime.
All of a sudden, evolutionary pressure on the peppered moth began to change. Light colored moths resting on a tree now stood out against the black background and were more likely to be eaten. The darker variant, on the other hand, was now camouflaged, and more likely to survive and breed. In a textbook case of industrial melanism , in just a few generations, the dark variant became by far the most common.

In just over 50 years, the dark variety went from making up just 2% of the population to making up over 95%, a change that could not be explained by any theory other than natural selection and industrial melanism. Genetic drift, where random influences can change the genetic make up of a population over time, is far too slow a process to account for this.

In genetic terms, the gene for dark color, as in most species, is dominant; once the pressure of predation was removed, this variant quickly spread. This is borne out by the fact that the American variant of the species changed in exactly the same way, a process known as convergent evolution.

RE-ADAPTATION

Strangely enough, now that modern industry in Europe is using cleaner technologies, the moth is now returning back to the typical variety, as the selection pressure from predation has now reversed. Because the allele for the lighter color is recessive and requires a copy from both parents, it is a slower process than the initial change. This is known as reverse industrial melanism.

MICRO-EVOLUTION

The incidence of industrial melanism is a process called micro-evolution, where selection pressures within a species lead to changes. In time, when mixed with genetic drift, other mutations and other possible selection pressures, this process of micro-evolution could have led to speciation within the peppered moth population.
If you have two separate populations living in sooty areas and natural areas, with little mixing between the two, random fluctuations could well lead to them becoming distinct species, as with Darwin’s finches.

Mimicry

Similarity between organisms that confers a survival advantage on one.

In Batesian mimicry, an organism lacking defenses mimics a species that does have defenses. In Müllerian mimicry, all species in a group are similar even though all individually have defenses. In aggressive mimicry, a predatory species mimics a benign species so that it can approach its prey without alarming it, or a parasitic species mimics its host. Some plant species mimic the colour patterns and scents of animals for the purposes of pollination and dispersal. Mimicry differs from camouflage in that camouflage hides the organism, whereas mimicry benefits the organism only if the organism is detected.

in biology, phenomenon characterized by the superficial resemblance of two or more organisms that are not closely related taxonomically. This resemblance confers an advantage—such as protection from predation—upon one or both organisms through some form of “information flow” that passes between the organisms and the animate agent of selection. The agent of selection (which may be, for example, a predator, a symbiont, or the host of a parasite, depending on the type of mimicry encountered) interacts directly with the similar organisms and is deceived by their similarity. This type of natural selection distinguishes mimicry from other types of convergent resemblance that result from the action of other forces of natural selection (e.g., temperature, food habits) on unrelated organisms.

In the most studied mimetic relationships the advantage is one-sided, one species (the mimic) gaining advantage from a resemblance to the other (the model). Since the discovery of mimicry in butterflies in the mid-19th century, a great many plants and animals have been found to be mimetic. In many cases the organisms involved belong to the same class, order, or even family, but numerous instances are known of plants mimicking animals and vice versa. Although the best-known examples of mimicry involve similarity of appearance, investigations have disclosed fascinating cases in which the resemblance involves sound, smell, behaviour, and even biochemistry.

A key element in virtually every mimetic situation is deception by the mimic, perpetrated upon a third party, which mistakes the mimic for the model. This third party may be the collective potential predators upon the mimic, potential prey of a predacious mimic, or even one sex of the mimic’s own species. In some cases, such as host mimicry by parasites, the organism deceived is the model.

Because of the variety of situations in which mimicry occurs, a formal definition must rest upon the effect of certain key communicative signals upon the appropriate receiver and the resultant evolutionary effect upon the emitters of the signals. Mimicry may be defined as a situation in which virtually identical signals, emitted by two different organisms, have in common at least one receiver that reacts in the same manner to both signals because it is advantageous to react in that manner to one of them (that of the model), although it may be disadvantageous to react thus to the counterfeit signal.

The distinction between camouflage and mimicry is not always clear when only the model and the mimic are at hand. When the receiver is known and its reactions understood, however, the distinction is quite clear: in mimicry the signals have a special significance for the receiver and for the sender, which has evolved the signals in order to be perceived by the receiver; in camouflage the sender seeks to avoid detection by the receiver through imitation of what is neutral background to the receiver. For information on camouflage, see coloration: Camouflage.

Basic types of mimicry » Batesian mimicry
In 1862 the English naturalist Henry W. Bates published an explanation for unexpected similarities in appearance between certain Brazilian forest butterflies of two distinct families. Members of one family, the Heliconiidae, are unpalatable to birds and are conspicuously coloured; members of the other family, the Pieridae, are edible to predators. Bates concluded that the conspicuous coloration of the inedible species must serve as a warning for predators that had learned of their inedibility through experience. The deceptively similar colour patterns of the edible species would provide protection from the same predators. This form of mimicry, in which a defenseless organism bears a close resemblance to a noxious and conspicuous one, is called Batesian, in honour of its discoverer

Mimicry
In mimicry a message (feature or signal) of one organism, the mimic, resembles some message of another organism which usually belongs to a different species, some feature of the environment, or a generalization of either of those, that is called the model. This resemblance should have some functionality for the mimic by being deceptive for a third participant, the receiver, whose recognition and response is relevant for the mimic. Some researchers use the notion mimicry system to emphasize the systemic nature of mimicry and the relatedness of the three participants.

There is a remarkable variety among mimicry cases in nature. Mimicry exists in most animal classes and also in many plants. Mimetic messages can be transferred in visual, auditory, chemical, tactile and other channels or frequencies that animals use for communication. Mimicry can also be based on many ecological relations such as predation, symbiosis, parasitism, and it can employ different life functions such as foraging, reproduction, and defense.

To organize such high diversity, researchers have proposed many mimicry types and typologies. Historically, the oldest and best-known mimicry types are Batesian mimicry (resemblance of harmless species to some non-edible species that signals their unsuitability to possible predators), Müllerian mimicry (resemblance of aposematic signals of different non-edible species), and aggressive or Peckhamian mimicry (resemblance of predators’ messages to messages of some species, or to some objects, that are harmless to their prey).

Biologists, who have carried out most of the research in mimicry, pay much attention to evolutionary aspects of the phenomenon. The main aspects in the mimicry research of modern biology include: dynamics of mimic and model populations in various selective situations and environmental conditions; behavior of signal receivers with respect to mimics and models, receivers’ abilities to discriminate and learn; variability of mimetic features including genetic and geographical variability of mimics and models. There are also alternative explanations to mimicry that do not rely on evolutionary concepts. Researchers have explained mimicry, for instance, as coincidence because of limited structural combinations in living organisms or as similarity caused by influences of physical conditions in similar living environment.

Also paradigms outside of the natural sciences use the concept of mimicry. For instance in postcolonial studies mimicry has been understood as disruptive imitations that are characteristic of postcolonial cultures. In psychology many authors use mimicry to indicate unconscious imitations between humans, especially related to facial gestures and body movements. In a semiotic context several authors (Sebeok, Nöth, Deacon) have discussed biological mimicry in terms of sign categories and sign processes. There are indeed many directions in mimicry studies, where a semiotic approach can be productive. For instance, semiotics can be applied in analyzing long and complex mimicry displays. Uexküllian biosemiotics, that pays attention to meanings in nature, can be successful in analyzing abstract mimicry, where abstract features, such as ocular shapes, movements or body types common to larger groups, are imitated rather than species-specific characteristics. The Peircean typology of signs opens up new perspectives for classifying mimicry resemblances.

Mechanisms of Evolution

Evolution does not occur in individuals but in populations. A population is an interbreeding group of individuals of one species in a given geographic area. A population evolves because the population contains the collection of genes called the gene pool. As changes in the gene pool occur, a population evolves.

Mutation

Mutation, a driving force of evolution, is a random change in a population's gene pool. It is a change in the nature of the DNA in one or more chromosomes. Mutations give rise to new alleles; therefore, they are the source of variation in a population.
Mutations may be harmful, but they may also be beneficial. For example, a mutation may permit organisms in a population to produce enzymes that will allow them to use certain food materials. Over time, these types of individuals survive, while those not having the mutations perish. Therefore, natural selection tends to remove the less-fit individuals, allowing more-fit individuals to survive and form a population of fit individuals.

Gene flow

Another mechanism of evolution may occur during the migration of individuals from one group to another. When the migrating individuals interbreed with the new population, they contribute their genes to the gene pool of the local population. This establishes gene flow in the population. Gene flow occurs, for example, when wind carries seeds far beyond the bounds of the parent plant population. As another example, animals may be driven off from a herd. This forces them to migrate to a new population, thereby bringing new genes to a gene pool. Gene flow tends to increase the similarity between remaining populations of the same species because it makes gene pools more similar to one another.

Genetic drift

Another mechanism for evolution is genetic drift. Genetic drift occurs when a small group of individuals leaves a population and establishes a new one in a geographically isolated region. For example, when a small population of fish is placed in a lake, the fish population will evolve into one that is different from the original. Fitness of a population is not considered in genetic drift, nor does genetic drift occur in a very large population.

Natural selection

Clearly, the most important influence on evolution is natural selection, which occurs when an organism is subject to its environment. The fittest survive and contribute their genes to their offspring, producing a population that is better adapted to the environment. The genes of less-fit individuals are eventually lost. The important selective force in natural selection is the environment.

Environmental fitness may be expressed in several ways. For example, it may involve an individual's ability to avoid predators, it may imply a greater resistance to disease, it may enhance ability to obtain food, or it may mean resistance to drought. Fitness may also be measured as enhanced reproductive ability, such as in the ability to attract a mate. Better-adapted individuals produce relatively more offspring and pass on their genes more efficiently than less-adapted individuals.

Several types of natural selection appear to act in populations. One type, stabilizing selection, occurs when the environment continually eliminates individuals at extremes of a population. Another type of natural selection is disruptive selection. Here, the environment favors extreme types in a population at the expense of intermediate forms, thereby splitting the population into two or more populations. A third type of natural selection is directional selection. In this case, the environment acts for or against an extreme characteristic, and the likely result is the replacement of one gene group with another gene group. The development of antibiotic-resistant bacteria in the modern era is an example of directional selection.

Species development

A species is a group of individuals that share a number of features and are able to interbreed with one another. (When individuals of one species mate with individuals of a different species, any offspring are usually sterile.) A species is also defined as a population whose members share a common gene pool.
The evolution of a species is speciation, which can occur when a population is isolated by geographic barriers, such as occurred in the isolation of Australia, New Zealand, and the Galapagos Islands. The variety of life forms found in Australia but nowhere else is the characteristic result of speciation by geographic barriers.

Speciation can also occur when reproductive barriers develop. For example, when members of a population develop anatomical barriers that make mating with other members of the population difficult, a new species can develop. The timing of sexual activity is another example of a reproductive barrier. Spatial difference, such as one species inhabiting treetops while another species occurs at ground level, is another reason why species develop.

Gradual versus rapid change

Darwin's theory included the fact that evolutionary changes take place slowly. In many cases, the fossil record shows that a species changed gradually over time. The theory that evolution occurs gradually is known as gradualism.
In contrast to gradualism is the theory of punctuated equilibrium, which is a point of discussion among scientists. According to the theory of punctuated equilibrium, some species have long, stable periods of existence interrupted by relatively brief periods of rapid change.
Both groups of scientists agree that natural selection is the single most important factor in evolutionary changes in species. Whether the change is slow and gradual, or punctuated and rapid, one thing is certain: Organisms have evolved over time.

THE MECHANISMS OF EVOLUTION"

There are many mechanisms that lead to evolutionary change. One of the most important mechanism in evolution is natural selection which is the differential success in the reproduction of different phenotypes resulting from the interaction of organisms with their environment. Natural selection occurs when a environment makes a individual adapt to that certain environment by variations that arise by mutation and genetic recombination. Also it favors certain traits in a individual than other traits so that these favored traits will be presented in the next generation. Another mechanism of evolution is genetic drift. Genetic drift is a random change in a small gene pool due to sampling errors in propagation of alleles or chance. Genetic drift depends greatly on the size of the gene pool. If the gene pool is large, the better it will represent the gene pool of the previous generation. If it is small, its gene pool may not be accurately represented in the next generation due to sampling error. Genetic drift usually occurs in small populations that contain less than 100 individuals, but in large populations drift may have no significant effect on the population. Another mechanism is gene flow which is when a population may gain or lose alleles by the migration of fertile individuals between populations. This may cause the allele frequencies in a gene pool to change and allow the organism to evolve. The most obvious mechanism would have to be mutation that arises in the gene pool of a population or individual. It is also the original source of the genetic variation that serves as raw material for natural selection.

Not only are there mechanisms of evolution, but there is also evidence to prove that these mechanisms are valid and have helped create the genetic variety of species that exists today. Antibiotic resistance in bacteria is one example of evolutionary evidence. In the 1950's, Japanese physicians realized that a antibiotic given to patients who had a infection that caused severe diarrhea was not responding. Many years later, scientists found out that a certain strain of bacteria called Shigella contained the specific gene that conferred antibiotic resistance. Some bacteria had genes that coded for enzymes that specifically destroyed certain antibiotics such as ampicillin. From this incident, scientists were able to deduce that natural selection helped the bacteria to inherit the genes for antibiotic resistance.

Scientists have also been able to use biochemistry as a source of evidence. The comparison of genes of two species is the most direct measure of common inheritance from shared ancestors. Using DNA-DNA hybridization, whole genomes can be compared by measuring the extent of hydrogen bonding between single-stranded DNA obtained from two sources. The similarity of the two genes can be seen by how tightly the DNA of one specie bonds to the DNA of the other specie. Many taxonomic debates have been answered using this

method such as whether flamingos are more closely related to storks or geese. This method compared the DNA of the flamingo to be more closely related to the DNA of the stork than the geese. The only disadvantage of this method is that it does not give precise information about the matchup in specific nucleotide sequences of the DNA which restriction mapping does. This technique uses restriction enzymes that recognizes a specific sequence of a few nucleotides and cleaves DNA wherever such sequences are found in the genome. Then the DNA fragments are separated by electrophoresis and compared to the other DNA fragments of the other species. This technique has been used to compare mtDNA from people of several different ethnicity's to find out that the human species originated from Africa. The most precise and powerful method for comparing DNA from two species is DNA sequencing which determines the nucleotide sequences of entire DNA segments that have been cloned by recombinant DNA techniques. This type of comparison tells us exactly how much divergence there has been in the evolution of two genes derived from the same ancestral gene. In 1990, a team of researchers used PCR(polymerase chain reaction) a new technique to compare a short piece of ancient DNA to homologous DNA from a certain plant. Scientists have also compared the proteins between different species such as in bats and dolphins.

The oldest type of evidence has been the fossil record which are the historical documents of biology. They are preserved remnants found in sedimentary rocks and are preserved by a process called pretrification. To compare fossils the ages must be determined first by relative dating. Fossils are preserved in strata, rock forms in layers that have different periods of sedimentation which occurs in intervals when the sea level changes. Since each fossils has a different period of sedimentation it is possible to find the age of the fossil. Geologists have also established a time scale with a consistent sequence of geological periods. These periods are: the Precambrian, Paleozoic, Mesozoic and the Cenozoic eras. With this time scale, geologists have been able to deduce which fossils belong in what time scale and determine if a certain specie evolved from another specie. Radioactive dating is the best method for determining the age of rocks and fossils on a scale of absolute time. All fossils contain isotopes of elements that accumulated in the organisms when they were alive. By determining an isotope's half-life which is the number of years it takes for 50% of the original sample to decay, it is possible to determine the fossil's age.

Mutation

Mutations are alterations of genetic material. They occur frequently during DNA duplication in cell division. This should not be surprising considering the fact that mitosis and meiosis are essentially mechanical processes with millions of operations that must be precisely completed in order for duplicate DNA molecules to be created. There are four common categories of mutations:
1. DNA base substitutions and deletions
2. unequal crossing-over and related structural modifications of chromosomes
3. partial or complete gene duplication
4. irregular numbers of chromosomes
Substitutions and deletions of single bases are common. For example, an adenine can be accidently substituted for a guanine. Such small errors in copying DNA are referred to as point mutations. There is a self correcting mechanism in DNA replication that repairs these small errors, but it does not always find every one of them.
Structural modifications of chromosomes generally occur as a consequence of the crossing-over process during cell division. Normally, there is an equal exchange of end sections of homologous chromosomes. Occasionally, there is a reunion of an end section onto a chromosome that is not homologous. Likewise, there can be an orphaned end section that does not reattach to any chromosome. The genes on such orphans are functionally lost.
Sometimes, extra copies of one or more genes are produced when a DNA molecule is replicated. More often, however, sections of the far more common non-protein coding DNA regions are duplicated. This duplication of large sections of DNA is an important source of genetic variation for a species. Spare copies of genes or inactive genes can mutate and change their function over time thereby producing a new variation that natural selection can favor or reject. Large-scale evolutionary changes in a species line generally occur in this way. Very likely, an explosion of gene duplications 7-12 million years ago led to the branching off of gorillas and then chimpanzees from the evolutionary line that ultimately became modern humans.
Irregular numbers of chromosomes can occur as a consequence of errors in meiosis and the combining of parental chromosomes at the time of conception. Such is the case when there are three instead of two autosomes for pair 21. This specific error is characteristic of Down syndrome.



In order for a mutation to be inherited, it must occur in the genetic material of a sex cell. It is now known that the frequency of new mutations in humans ranges from 100 to 200 for every individual. it is to be expected then that most sex cells also contain gene mutations of some sort. In other words, mutations are probably common occurrences even in healthy people. Most probably do not confer a significant advantage or disadvantage because they are point mutations that occur in non-gene coding regions of DNA molecules. They are relatively neutral in their effect. However, some mutations are extremely serious and can result in death before birth, when an individual is still in the embryonic or early fetal stages of development.
Mutations can occur naturally as a result of occasional errors in DNA replication. They also can be caused by exposure to radiation, alcohol, lead, lithium, organic mercury, and some other chemicals. Viruses and other microorganisms may also be responsible for them. Even some commonly prescribed drugs are thought to be mutagens.


1. androgens (steroid hormones that control the development and maintenance of masculine characteristics)
2. ACE inhibitors (a class of blood pressure medication)
3. streptomycin and tetracycline (two classes of antibiotics)
4. vitamin A
Mutations appear to be spontaneous in most instances. That does not mean that they occur without cause but, rather, that the specific cause is almost always unknown. Subsequently, it is usually very difficult for lawyers to prove in a court of law that a mutagen is responsible for causing a specific mutation in people. With the aid of expert scientific testimony, they can often demonstrate that the mutagen can cause a particular kind of mutation. However, that is not the same thing as proving that a plaintiff's mutation was caused by that mutagen instead of some others.
The great diversity of life forms that have been identified in the fossil record is evidence that there has been an accumulation of mutations producing a more or less constant supply of both small and large variations upon which natural selection has operated for billions of years. Mutation has been the essential prerequisite for the evolution of life.
In order for a mutation to be subject to natural selection, it must be expressed in the phenotype of an individual. Selection favors mutations that result in adaptive phenotypes and eliminates nonadaptive ones. Even when mutations produce recessive alleles that are seldom expressed in phenotypes, they become part of a vast reservoir of hidden variability that can show up in future generations. Such potentially harmful recessive alleles add to the genetic load of a population.