Integration of populations and differentiation of species.

The framework for modern studies of speciation was established as part of the Neo-Darwinian synthesis of the early twentieth century. Here we evaluate this framework in the light of recent empirical and theoretical studies. Evidence from experimental studies of selection, quantitative genetic studies of species' differences, and the molecular evolution of 'isolation' genes, all agree that directional selection is the primary cause of speciation, as initially proposed by Darwin. Likewise, as suggested by Dobzhansky and Mayr, gene flow does hold species together, but probably more by facilitating the spread of beneficial mutants and associated hitchhiking events than by homogenizing neutral loci. Reproductive barriers are important as well in that they preserve adaptations, but as has been stressed by botanists for close to a century, they rarely protect the entire genome from gene flow in recently diverged species. Contrary to early views, it is now clear that speciation can occur in the presence of gene flow. However, recent theory does support the long-held view that population structure and small population size may increase speciation rates, but only under special conditions and not because of the increased efficacy of drift as suggested by earlier authors. Rather, low levels of migration among small populations facilitates the rapid accumulation of beneficial mutations that indirectly cause hybrid incompatibilities.