The bright, eye-catching colors donned by African glossy starlings are a boon for the birds when it comes to competing for a mate. But the flashy colors are also good for something else: quickly driving the formation of new, diverse starling species throughout evolutionary history. In a PNAS Early Edition paper published this week, researchers reported that the African starlings possess a unique color palette that lets them evolve new colors up to 40 times faster than other birds. And the starling lineages with the most changeable colors also diversify into new species more quickly. The discovery suggests a general rule that a species with more malleable ornamental features may diversify more quickly than other species.
Two distinct mechanisms govern the hues of birds: pigment molecules, which each encode a narrow range of shades, and structural elements that come together in various ways to reflect light and can lead to an array of iridescent colors. Most iridescent birds have the same structure embedded in their feathers to form this rainbow of colors: rod-shaped melanosomes.
“You can get new colors just by changing how the melanosomes are organized or spaced out,” explains first author of the new paper Rafael Maia, a biology graduate student at the University of Akron.
When Maia and his colleagues at Akron and Columbia University peered at African starling feathers under a microscope, however, they saw melanosomes that varied from the classic solid rod shape. Some were flattened and solid, others rod-shaped but hollow, and some both flattened and hollow.
The researchers went on to show that by having all four types, the African starlings can create a wider diversity of colors than birds with only solid rod-shaped melanosomes, through only slight rearrangements to the physical patterns of the melanosomes. The flexibility of the colors made the team wonder whether the colors change more frequently during evolution, and what effect this had
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on the starling lineages.
“We know that in birds these colors are very important in identifying a suitable mate and seeing if a mate is of the same species,” says Maia. “So if the color can evolve very fast, it could theoretically lead to two groups that don’t recognize each other as mates, even if they’re still genetically compatible.”
By comparing species of starlings, Maia found that bird lineages with the more complex melanosomes had changed color throughout evolution anywhere from 10 to 40 times faster than lineages that only had rod-shaped melanosomes. Then, they showed that the bird family trees with the unique melanosomes also branched off into new species more often. It’s the first time such a link has been made between speciation rate and the evolvability of an ornamental feature of an organism, Maia says.
“We showed that the groups of species that have more flexible ornaments are speciating faster,” Maia says. “And I think this is likely to extend beyond startling colors to other physical features and ways of communicating.”
The same mechanism could be at play in bird songs, ornamental feathers, electrical organs of fish, and insects, he suggests. In each case, if a feature was highly flexible, it could lead to the division of a species by changing sexual selection. A starling with an altered melanocyte structure that leads to a color change (or a bird singing a new tune) could be seen as not appealing to starlings that haven’t changed, even if they’re completely genetically compatible and able to produce offspring. So the lineages would quickly diverge.
“A lot of studies looking at the link between sexual selection and diversification have focused on the selective pressures and the preferences for specific ornaments,” Maia points out. “But I think the evolvability of these ornamental features adds a whole new perspective.”