Ecological speciation in postglacial fishes

A venerable view of speciation is that reproductive isolation ultimately evolves from contrasting selection pressures between populations exploiting different resource environments. Yet, this 'ecological' mode of species origins has been tested rarely in nature. Here I describe emerging evidence of divergent selection and speciation in fishes of postglacial lakes. Examples of very closely related species pairs within such lakes are known from over a dozen independent fish lineages. These pairs display four remarkable attributes which together raise the possibility of ecological speciation: rapid evolution of assortative mating in sympatry and/or allopatry; persistence in sympatry despite a history of gene flow; a high degree of niche differentiation (usually one species is planktivorous and the other is benthic) accompanied by differences in body size and shape; and high intrinsic viability and fertility of interspecific hybrids. More direct evidence for ecological speciation comes from preliminary confirmation of three predictions: selection against hybrids has an ecological basis (morphologically intermediate hybrid sticklebacks have a reduced ability to exploit the main resources on which the parent species are specialized); premating isolation is linked to the morphological traits that have diverged between species (hybridization in sticklebacks occurs only between the morphologically most similar individuals of the two parent species); premating isolation mechanisms evolve in parallel in similar environments (interspecific mate preferences appear to have diverged in parallel in stickleback species pairs that evolved independently). These findings argue that knowledge of ecological environments is essential to understanding the origin of species in adaptive radiation, and perhaps more generally. Field studies of selection on traits determining reproductive isolation are sorely needed, and would complement (and perhaps transform) traditional genetic approaches to speciation.