Speciation in nature: the threespine stickleback model systems

The threespine stickleback Gasterosteus aculeatus species complex is an important natural model for speciation research because it includes several replicated sets of coexisting, divergent forms that are also experimentally tractable. Recent research has begun to emphasize lesser known divergences within the complex in addition to the well-studied limnetic–benthic pairs, as well as exploring a broader range of speciation mechanisms. With the goals of making general inferences about speciation in nature and bringing this body of research to a wider audience, we have surveyed studies from the entire species complex. We find that stickleback speciation is often rapid, that the geographical context of speciation is variable and often complex, and that many, diverse traits have often diverged early in the speciation process. We find no unambiguous evidence of founder-effect speciation, but much evidence that divergent natural and sexual selection have been central to the evolution of reproductive isolation in this species complex.