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Parallel Speciation by Natural Selection

Natural selection plays a prominant role in most theories of speciation. Mayr (1963) and others viewed selection as one of the causes of genetic divergence between allopatric populations that yielded reproductive isolation as an incidental by-product. Selection is also invoked in models of parapatric (Endler 1977), reinforcement (Dobzhansky 1951), founder event (Barton 1989), and sympatric (Diehl and Bush 1989) speciation. Yet there is surprisingly ittle evidence from nature in support of its role in any scenario. Our purpose here is to introduce an unappreciated form of evidence for natural selection that we call "parallel speciation." This we define as the repeated independent evolution of the same reproductive isolating mechanism. Below, we explain the phenomenon, list criteria needed to demonstrate it, and identify several possible examples from nature. We discuss broad implications of parallel speciation and review several mechanisms that can produce it. Throughout we use the concept of biological species (Mayr 1963): populations belong to the same species only if they are not reproductively isolated. We add the proviso that reproductive isolation between good species may be imperfect.