Saturday, February 19, 2011
Sexual selection is an aspect of evolution that has been extensively studied. I've blogged about it in stories about pipefish, insects, and fish. If you'll recall from the Fish-stache post I described sexual selection in this way:
"When it comes to sexual selection in the animal world, it is the usually the sex that puts more effort (and has more to lose) into the results that gets to be the choosy sex. Usually this means females get to be picky. After all, eggs are more expensive than sperm and females often end up contributing quite a bit with parental care. This choosiness means that males need to impress females through the evolution of secondary sexual traits. This could be, and usually is, just about anything: elaborate feathers, complicated dances or mating calls, gift giving, etc."
These secondary sexual traits have diversified more than nonsexual traits, with sexual selection driving phenotypic diversification and speciation. The traits are associated with high levels of additive genetic variation and evolve in response to a change in natural selection. Because of this it is difficult to find examples where traits diverge primarily to sexual selection, even strong sexual selection on a particular heritable characteristic is insufficient to cause contemporary evolutionary change. So perhaps there is some kind of evolutionary limit on sexually selected traits in nature. A new paper published online in the Proceedings of the National Academies of Science demonstrates the existence of this limit for male attractiveness in fruit flies.
You can commonly find fruit flies (Drosophila spp.) buzzing around trash cans and unripe or overripe fruit, and they have been used as a model organism by geneticists for decades. They are easy to obtain from the wild, small in size (~2.5mm), easy to rear in the lab, have fecund females, have a relatively small genome, are relatively inexpensive to work on, have the entire genome sequenced and many genes identified, and there are a variety of mutants available to purchase for study. Add all that together and you've got yourself one great experimental species.
This studies uses Drosophila serrata. D. serrata females are known to discriminate among potential male partners on the basis of a combination of eight male cuticular hydrocarbons (CHCs). These CHCs act as contact pheromones and the sexual selection exerted by female choice causes them to evolve. The researchers genetically engineered a group of male flies to release these highly attractive pheromones. Then they released the males into a colony. In this colony the engineered males greatly outnumbered the average/wildtype males. They found that, initially, the females preferred the sexy smelling males. Then, after about seven generations, they found that the ratio of sexy males to average males had become almost even. Interesting. If females pick attractive males to mate with and those genes are passed along to the next generation of flies then why didn't they see the ratio stay the same or increase? The researchers conclude that there is likely a substantial fitness cost to sexy smelling flies. Essentially, the males were too sexy to survive. They died before they could reproduce and pass on their sexy genes. And this is that evolutionary limit that I presented above. The males lack the genetic variation that would allow an increase in sexual fitness while simultaneously maintaining high nonsexual fitness, and because the divergence in these traits is constrained to occur in the direction of greatest genetic variance, rather than sexual selection, the combination of high attractiveness and high survivorship can't be reached.
That must explain the rare unicorn, and by that I mean the single, straight, and attractive human male.
Here's the paper:
Hine, Emma, Katrina McGuigan, and Mark W. Blows. (2011) Natural selection stops the evolution of male attractiveness. PNAS: published online. (DOI: 10.1073/pnas.1011876108)
Science did a short write-up on this paper too: http://news.sciencemag.org/sciencenow/2011/02/scienceshot-too-sexy-too-bad.html?ref=hp
(image from anu.edu.au and photo credit to A. Morin and the Jennions Lab)