Catching one species in the act of becoming two is no easy feat. Yet evolutionary biologists working in the Solomon Islands may have done just that. They have found that a single genetic change turns a small, brown-bellied bird black, possibly leading it to mate with like-colored birds--and setting it on the road to becoming a new species.
The late Ernst Mayr, a famous Harvard University evolutionary biologist, was the first to notice the speciation potential of flycatcher birds in the South Pacific's Solomon Islands. During the 1940s, he described differences in the body size and plumage of several populations of the flycatcher (Monarcha castaneiventris) and asserted that there existed at least five subspecies. Evolutionary biologist J. Albert Uy of Syracuse University in New York state and colleagues decided to see just how different two of these subspecies were. One lives on a larger island and has a reddish-brown belly with an iridescent blue-black back and head, and the other, which is all blue-black, lives on smaller islands about 10 kilometers away.
To track down the gene underlying the color change, Uy and his colleagues took a cue from black sheep and pigs. These animals have a mutation in the gene for the melanocortin-1 receptor, a protein that helps control how much black pigment is produced. The researchers sequenced part of that gene from 28 black birds and 19 brown-bellied ones. They found a few differences but only one that mattered: a genetic change that altered a single amino acid in the resulting protein. It seems this change permanently activates the protein so that more black than brown pigment is produced.
Next, the researchers evaluated whether this color change might make any difference to the birds. They put stuffed birds of either color into the territories of live flycatchers. Flycatchers are not bothered by most foreign birds, but they will attack potential rivals of the same species. Black bird decoys drew angry responses from black birds but little reaction from brown-belly birds and vice versa, Uy and his colleagues report in the August issue of The American Naturalist.
The researchers have not been able to test directly whether black birds prefer black mates, a behavior that could cause the species to split, but other research shows that at least some territorial birds rely on the same cues to pick mates as to detect intruders. Future experiments will look at the potential advantage of being black on those smaller islands and just what role the coloration plays in mate preference, says Uy.
"The paper is exciting because it makes the link between a single nucleotide change and a trait involved in species recognition, which then has the potential implications for mate choice and ultimately speciation," says Hopi Hoekstra, an evolutionary biologist at Harvard. But Trevor Price, an evolutionary biologist at the University of Chicago in Illinois cautions that color differences alone are "probably not sufficient for speciation," as other animals mate just fine with partners that look different from them.