Posted by: Dan | June 16, 2009

Speciation Caused by a Single Mutation

Credit: Robert Moyle

Credit: Robert Moyle

In ScienceNOW, On the Road to a New Species, a fantastic illumination of allopatric speciation being correlated to a single point mutation:

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.


Presumably, during the isolation of these flycatcher populations to different islands of the Solomon chain, the founder effect introduced different proportions of certain alleles. Competition for territory and for mates could have set up the conditions for sexual selection for or against alternative alleles. Or simple genetic drift in small island populations could have eliminated an allele on one island’s population where that allele was rare, but preserved it against alternative alleles in another population where it was common. And eventually, the populations adjacent islands cease to recognize each other as members of the same species – geographically isolated populations become reproductively isolated.

This study, finding putative speciation occurring through a single mutation, raises some questions about speciation itself. That is, it seems reminiscent of saltation or the ‘Hopeless Monster‘ hypothesis to me, and I’m not quite sure how to explain why the findings of this study fit better with allopatric speciation than with saltation. I’m not an evolutionary biologist, afterall. I suspect however that the difference lays in the assumption that this mutation would not have laid the groundwork for possible reproductive isolation for in a single generation, nor would it have done so in the absence of geographic isolation.

But the full article isn’t published yet, so I haven’t read the paper, only it’s synopsis. If the only way of observing whether this change in appearance made any difference to the birds themselves was their specific territorial responses to stuffed birds, and no indication of whether the color had any impact on mating preferences, then the researchers can’t (yet) claim that they’re reproductively isolated. Can they?

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