A yellow (left) and black (right) Gouldian finch take a break together on a branch. (Credit: Nigel Jacques)
A highly endangered rainbow-colored Australian finch is breaking all the rules when it comes to avian evolution, according to by scientists from CU Boulder, the Cornell Lab of Ornithology, the University of Sheffield in the United Kingdom and other institutions.
A red (top) and black (bottom) Gouldian finch. (Credit: Marc Gardner)
“This study helps us understand the genes that control feather color in Gouldian finches and also shows that, in this case at least, it is a gene that has been implicated in feather color in another group of birds,” says co-author Scott Taylor, an assistant professor of Ecology and Evolutionary Biology at CU Boulder. “Understanding the genetic basis of feather color in birds helps us understand their evolution more broadly.”
There appears to be an underlying selection mechanism at work among Gouldian finches—a mechanism that allows this species to produce and maintain individuals with red heads, black heads, and yellow heads. The authors reveal what this additional evolutionary process might be in the journal Nature Communications. "Most people have heard of natural selection," says lead author Kang-Wook Kim at the University of Sheffield. "But 'survival of the fittest' cannot explain the color diversity we see in the Gouldian Finch. We demonstrate that there is another evolutionary process—balancing selection—that has maintained the black or red head color over thousands of generations."
The yellow-headed type (actually more orange) is produced by a completely different mechanism that is not yet understood. Yellow-headed Gouldian Finches make up only 1 to 2 percent of the wild population.
"Having distinct multiple color types—a polymorphism—maintained within a species for a long time is extremely rare," explains co-author David Toews, who did this work as a postdoctoral researcher at the Cornell Lab and who is now at Pennsylvania State University. "Natural selection is typically thought of in a linear fashion—a mutation changes a trait which then confers some reproductive or survival advantage, which results in more offspring, and the trait eventually becomes the sole type in the population."
Studies have shown the red-headed finches have the apparent advantage. Female Gouldian Finches of all colors prefer the red-headed males, who also happen to be more dominant in the social hierarchy. So why hasn’t the black-headed type disappeared? It turns out there are disadvantages to having a red head, too, such as higher levels of stress hormones and poorer reproductive outcomes.
"If advantages are cancelled out by concurrent disadvantages, these two color types can be maintained—that’s balancing selection," Toews explains. "Red forms are not as common in the wild, so the counterbalancing pressure reduces the advantage of being red. That’s super cool."
The researchers zeroed in on a particular gene called follistatin which is found on the Gouldian Finch sex chromosome and regulates melanin to produce either red or black-headed finches. For the yellow morph, a different gene, not located on the sex chromosome, is controlling the head pigmentation, but it hasn’t yet been found and it’s not clear what forces are allowing the yellow morph to persist in the wild.
In another twist, Taylor and Toews have done previous research that revealed the genes likely governing the plumage differences between North American Blue-winged and Golden-winged Warblers—and one of those regions is in the same spot on the sex chromosome that differs among Gouldian Finches with different head colors.
"The probability that we’d locate the exact gene region that governs plumage differences in both the Gouldian Finch and the two warblers was almost zero," says Toews. "But now that we've done it, it opens up the possibility that the same region in other species may also be controlling plumage color."
