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.
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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.