Is speciation adaptive?

The results of a research study on the origin of new species by the synergistic interaction of natural and sexual selection are published in Science, 18 December 2009.

Darwin suggested that the action of natural selection can produce new species, but 150 years after the publication of his famous book, ‘On the Origin of Species’, debate still continues on the mechanisms of speciation. Disruptive natural selection can explain the diversification of a species into ecotypes that are adapted to different ecological conditions (say, to wet versus dry habitats). This does not lead to speciation unless gene flow among ecotypes due to hybridization (e.g., the mating of wet-adapted individuals with dry-adapted individuals) is prevented.

Sander van Doorn (University of Bern, Switzerland), Pim Edelaar (Estación Biológica de Doñana, Spain) and Franjo Weissing (University of Groningen, Netherlands) demonstrate how gene flow among ecotypes can be prevented by the interplay of natural and sexual selection. By means of a mathematical model, they show that disruptive natural selection triggers the evolution of female preferences for male traits that indicate the male’s degree of adaptation to the local ecological conditions. Once this process of sexual selection gets off the ground, it strengthens natural selection. By working in concert and mutually reinforcing each other, natural and sexual selection eventually achieve the evolution of locally adapted ecotypes which do no longer hybridize and, hence, correspond to separate species. This mutual reinforcement is in striking contrast to the common idea (originating with Darwin) that sexual selection typically counteracts natural selection.

It is generally agreed that evolution within a species is largely adaptive. This research indicates that also the speciation process itself will often be adaptive, that is, fully driven by selection. Hence, the new insights may be viewed as a first step to extend ‘Darwin’s brilliant idea’ from the domain of micro-evolution (evolution within species) to the domain of macro-evolution (diversification of the tree of life). The ideas presented in the paper may soon be tested in the field in crossbill birds, sticklebacks and other species where biologists are currently investigating the relationship between mate attractiveness and adaptation to the local environment.

Contact: Franjo Weissing, tel. +3150-363 4615/2131, f.j.weissing rug.nl