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Choosing mates wisely
Hybridization between species has long been seen as an accidental contributor to evolution in some cases and as a dead end in others. New evidence is emerging, however, that hybridization may have played important, and nonrandom, roles in adaptation. Chen and Pfennig describe just such a case where female Plains spadefoot toads preferentially choose males from another toad species, the Mexican spadefoot, as mates, but only under certain environmental conditions (see the Perspective by Zuk). The offspring of this preferred hybrid mating event have higher fitness than nonhybrids in the same environment. Thus, not only do hybrids have an advantage, but females of one species are exerting a selective influence on the other species.
Abstract
Hybridization—interbreeding between species—is generally thought to occur randomly between members of two species. Contrary to expectation, female plains spadefoot toads (Spea bombifrons) can increase their evolutionary fitness by preferentially mating with high-quality males of another species, the Mexican spadefoot toad (Spea multiplicata). Aspects of Mexican spadefoot males’ mating calls predict their hybrid offspring’s fitness, and plains spadefoot females prefer Mexican spadefoot males on the basis of these attributes, but only in populations and ecological conditions where hybridization is adaptive. By selecting fitness-enhancing mates of another species, females increase hybridization’s benefits and exert sexual selection across species. Nonrandom mating between species can thereby increase the potential for adaptive gene flow between species so that adaptive introgression is not simply happenstance.
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