Female choice regarding their potential mate's genetic and culturally transmitted traits drives the appearance of new species in capuchino seedeater songbirds.
According to a new study published in the March 25 issue of Science, slight differences in male coloration and song are enough to keep the two nearly identical bird species from mating with each other across the generations.
The findings shed light on how one population of individuals can rapidly split and become two distinct species — a fundamental yet unresolved question in evolutionary biology.
Usually, new species are thought to arise through the isolation of populations over long periods of time, which allows for the slow accumulation of genetic mutations that prevent them from interbreeding with their closely related cousins.
However, the ten species of South American capuchino seedeaters all split from a single common ancestor within the last million years. This recent and rapid radiation occurred even though the emerging populations of new seedeater species appear to have lacked any geographic or biological barriers to reproduction.
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 — are thought to be a powerful force in maintaining reproductive barriers between populations early in their divergence.
According to lead author Sheela Turbek, an evolutionary biologist at the University of Colorado, it's unclear what barriers enabled these populations to remain reproductively isolated and distinct despite living side by side in the same habitats.
"This group is fascinating from an evolutionary perspective because the various capuchino species feed on the same grasses, reproduce at the same time, breed in similar habitats and are virtually identical on a genetic level, yet somehow manage to coexist in Iberá National Park," said Turbek.
Turbek and her team of researchers focused on two types of seedeaters native to Iberá National Park in northeastern Argentina: the Tawny-bellied seedeater and Iberá seedeater, the latter species only recently discovered in 2001. According to Turbek, the two species are nearly identical, varying only in male plumage coloration and song, and have the ability to produce viable offspring should they reproduce.
"Given that the females of the Iberá seedeater and other cupuchino species are impossible to distinguish to the human eye, it was unclear if the Iberá seedeater interbreeds with other co-occurring species," said Turbek.
To unravel the mating behaviors of these birds, Turbek spent several years looking for Tawny-bellied and Iberá seedeaters in the wild, searching for nests, marking individual birds and collecting blood samples from adults and nestlings. Using 3D printed models of male birds from each species embedded with speakers that played recordings of their unique songs, the researchers also observed the territorial behaviors and interactions between the two birds.
They found that both capuchino species responded most aggressively to the song and coloration of their own species, and less so to others, suggesting that these traits are used by the seedeaters to distinguish one another to avoid interbreeding. What's more, genetic analysis of the seedeaters suggests that despite all their genetic, ecological and biological similarities, the Iberá seedeater and the Tawny-bellied seedeater have only ever mated with members of their own species.
According to the findings, the two seedeater species can only be genetically distinguished by three small regions of their genome — two of which contain mutations associated with plumage coloration in other bird species and likely underly the plumage variation between seedeater species. These mutations likely arose through the reshuffling of genetic variation from previous seedeater lineages.
As these mutations grew in an ancestor population, choosy females decided they liked the look of some birds over others, and selected their preferred mates based on coloration, creating reproductive isolation in one segment of the population and thus driving speciation.
"We found that the reshuffling of existing genetic variation into novel combinations that encode traits used in mate choice can maintain species boundaries among co-occurring and closely related organisms," said Turbek.
"As more and more studies analyze rapid radiations using whole-genome sequencing, we may find that this mechanism of speciation is more common than previously thought," said Turbek.
[Credit for associated images: Sheela Turbeck]