Geography of Speciation

Speciation is the process of a single lineage splitting into two or more daughter lineages. Geography is key to this process. When populations are spatially isolated, they no longer exchange genes and can diverge. A byproduct of this divergence is the evolution of intrinsic reproductive barriers—traits that prevent species from interbreeding and signal the completion of speciation. Allopatric speciation is the origin of new species in spatially isolated populations. Allopatric speciation is the simplest and easiest model of speciation because there is no gene flow between populations that can erode divergence. Given enough time, allopatric speciation is expected, and it is widely agreed to be the most common mode of speciation. Indeed, allopatric speciation was described by Coyne and Orr in Speciation (2004) as “so plausible that it hardly seems worth documenting.” Speciation can of course occur in other geographic contexts, with differing levels of gene flow. A single panmictic population can be split into two isolated populations through disruptive selection, known as sympatric speciation, or new species can arise from populations that do not directly overlap, but are in close enough proximity to exchange genes, known as parapatric speciation. These three geographic modes represent different points on a continuum of spatial isolation that determines the connectivity, or amount of gene flow, among diverging populations. However, these are hardly discrete categories, and the term “speciation-with-gene-flow” (also “divergence-with-gene-flow”) is often used to encompass any geographic model of speciation other than strict allopatry. This article will provide an overview of the various geographic contexts for speciation, with a focus on cases where spatial isolation ranges from complete (allopatric speciation) to some form of contact between diverging populations.