Thursday, May 10, 2007

Why is there so much genetic variation?

One of the surprising discoveries when scientists first began looking at genetic variation was that there was a great deal more than expected by theory. In 1966, Richard Lewontin and Jack Hubby published a paper that revolutionized population genetics. They pioneered the use of protein gel electrophoresis to survey dozens of loci in the fruit fly, Drosophila pseudoobscura, and reported that a large fraction of the loci were polymorphic, and that at the average locus there was about a 15% chance that the individual was heterozygous. These results are surprising because it was expected that natural selection will reduce the amount of genetic variation in populations.

Forty years later evolutionary biologists still don't have many solid explanations for genetic variation in nature. However, a recent report in Nature by Fitzpatrick et al. shows that negative frequency dependent selection can maintain alternative feeding alleles in populations of fruit flies. Fruit fly larvae forage for food in two ways: by "roving" or "sitting". Rover larvae move around more than sitter larvae while feeding and they are also more likely to explore new food patches than sitters.

Fitzpatrick et al. discovered that each type was favored by natural selection when rare.

"If you're a rover surrounded by many sitters, then the sitters are going to use up that patch and you're going to do better by moving out into a new patch," says Marla Sokolowski, the PI on the research team. "So you'll have an advantage because you're not competing with the sitters who stay close to the initial resource. On the other hand, if you're a sitter and you're mostly with rovers, the rovers are going to move out and you'll be left on the patch to feed without competition."

Similar behaviors occur in C. elegans, which tend to clump or browse bacteria solitarily. I unsuccessfully looked for frequency and density dependent selection among these worms, but it is likely my assays weren't sensitive enough. The foraging gene is found in many animals, including honeybees, mice and humans. It is interesting to speculate on the roles that it plays in higher organisms, such as ourselves. Does it play any role in food-related behavioral disorders?


  1. Forty years later evolutionary biologists still don't have many solid explanations for genetic variation in nature.

    I'm a little puzzled by this comment. I thought the explanation was well known.

    Would you mind taking a look at Silent Mutations and Neutral Theory and let me know where I've gone wrong? :-)

  2. Larry-
    Nice post.

    A difficulty of writing short pieces for a blog is having to leave much information out.

    Our main theory of genetic variation is, as you say, the neutral theory. However, some would argue that the neutral theory is insufficient to explain all genetic diversity. Well the Fitzgerald paper gives evidence for another source, balancing selection. I merely meant to point out that this gives us another mechanism by which to explain genetic variation, not to imply that one theory is better than the other.

  3. I thought this work on flying away vs. playing dead (in response to prediation) was interesting.
    I wonder if it could be the same gene?

  4. Ford--
    That's an interesting paper. I'll have to have a look. It would be easy enough to test your hypothesis. It'd be a great project for a PhD student and I think it any evidence showing genes that are co-opted for new purposes is inherently interesting.