Modes of Speciation
With the 1859 publication of On the Origin of Species, Charles Darwin put forth the idea that all species have descended, with modification, from a common population of ancestors. All life on earth is therefore united by evolutionary history. This idea, which also is credited to Alfred Wallace, revolutionized scientific thought and is the cornerstone of modern evolutionary theory. However, what Darwin didn't fully understand, what he called the "mystery of mysteries," is how new species arise. We now know that there are many modes, mechanisms, and evolutionary forces that play a role in the formation of new species.
To understand speciation (the formation of new species) we must first have an idea of what species are. Many different definitions have been proposed; none of them are universally applicable and all of them are useful in particular contexts. At present, one of the most widely used definitions of species is based on the biological species concept. This definition is appropriate for our discussion of speciation because it not only defines species, but it also emphasizes the process by which species arise. The biological species concept states that "species are groups of interbreeding natural populations that are reproductively isolated from other such groups." Reproductive isolation refers to biological differences between two populations that impede or greatly reduce gene exchange between them. Most biologists recognize that some gene flow occurs between populations and do not require complete reproductive isolation in order to recognize a population as a distinct species. The study of speciation can therefore be thought of as the study of how a population evolves genetic distinctiveness and becomes reproductively separate from other populations.
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