Our understanding of the origin of species, or speciation, is sometimes viewed as incomplete, a "mystery of mysteries." We in fact know a lot about speciation, especially when we consider its two basic components, the geography of speciation and the biology of speciation (changes in phenotype and genotype that occur during the process). Our understanding of the geography of speciation is quite clear. The process involves the separation of a once-continuous range into two or more geographically isolated, or allopatric parts, which over time accrue genetic changes that result in new daughter species. Current distributions show that recently evolved species are currently allopatric, supporting the role of allopatry in speciation. However, many species originated in the early-to midPleistocene, meaning that they persisted through the environmental perturbations of multiple glacial cycles. It has been assumed that species maintained allopatric distributions during these episodes of glacial advance and retreat. I used Grinnelian niche models to estimate species distributions at the Last Glacial Maximum and the Last Interglacial. For a pair of recently isolated warbler species, and a pair of relatively old sister species of gnatcatchers, allopatry was observed at all time periods. Thus, there is no mystery about the geography of speciation: at least in birds, allopatry predominates. The sentiment that speciation is mysterious comes, I argue, from the biological species concept, which requires populations to be reproductively isolated before recognizing them as species. Reproductive isolation is a complicated process that rarely occurs the same way twice, and I argue that this lack of generality has been misinterpreted as a mystery.Keywords Species concepts . Speciation . Allopatry . Niche models Speciation, or the origin of species, is often called Darwin's "mystery of mysteries." In fact, Via (2009:9939) remarked "The origin of species is only slightly less mysterious now than it was 150 years ago when Darwin published his famous book." In part, this characterization is discouraging because in fact we know a great deal about how new species form. I believe that the perception that speciation is a mystery stems from the definition of species themselves. Various species concepts differ in their complexity and the types of biological changes considered necessary and sufficient to judge that speciation has occurred. The more complicated the species concept, the more mysterious speciation becomes. To dismantle the perception that speciation is mysterious, it is useful to divide the origin of species into two components, which we might for simplicity call the geography and the biology of speciation (Bush 1975). In this essay, I concentrate on our understanding of the geography of speciation in birds, arguably one of the best known vertebrate groups. I show that geography of speciation is straightforward under all species concepts, and that in fact speciation is only mysterious under certain definitions of species.
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