Female choice regarding their potential mate's genetic and culturally transmitted traits drives the appearance of new species in Iberá Seedeater songbirds, a new study finds. The results shed light on how a single population can split into two distinct species - a fundamental yet unresolved question in evolutionary biology. Understanding the process of speciation in the animal kingdom remains challenging, particularly when the two newly emerging species share a habitat. Because post-mating barriers to reproduction, like genetic incompatibilities, for example, often take a long time to evolve between taxa, pre-mating isolation - ecological or behavioral adaptations that prevent closely related individuals from interbreeding - is thought to be a powerful force in maintaining reproductive barriers between co-occurring species early in their divergence. Observing the mating decisions among co-occurring wild populations at the beginning of speciation could provide an opportunity to understand better how behavioral isolation fuels divergence and rapid speciation. Here, Sheela Turbek and colleagues show how this process occurs in two species of southern capuchino seedeaters (Sporophila) - a lineage of songbird that represents one of the most recent and rapid avian radiations - in an overlapping terrain. Turbek et al. found that, despite their striking genomic similarity, and in particular, females that are largely indistinguishable between species, the recently discovered Iberá Seedeater (S. iberansis) and the Tawny-bellied Seedeater (S. hypoxantha) maintain reproductive isolation from one another. Genomic analysis of the seedeaters revealed that they only differ in three small genomic regions, two of which contain mutations associated with male plumage patterning. Behavioral experiments involving decoy birds showed that the two species also sing distinct songs, which provides another important sexual signal between mates. According to the authors, the findings suggest that females select mates based on genetically inherited plumage color and culturally inherited songs, and this is the primary mechanism promoting and maintaining early and rapid divergence between these two seedeater species. In a Perspective, Erich Jarvis discusses the study and its findings in greater detail. For reporters interested in trends, a 2017 Science study of Galapagos finches showed how a cross-island migrant bred with a native species to form a self-perpetuating hybrid population that was reproductively isolated from both parental species. https://science.sciencemag.org/content/359/6372/224. And a 2017 Science Advances study featuring some authors overlapping with those on Turbek et al. determined how southern capuchino seedeaters, despite having very little underlying genetic diversity, exhibit such striking differences in plumage, male to male. http://advances.sciencemag.org/content/3/5/e1602404
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Journal
Science