10 May 2007

Natural selection and stable equilibrium of alleles

Science Daily reports on an interesting new study of fruit fly population genetics. Fruit fly larvae have two different behavioral phenotypes, depending on which allele they have for a single foraging gene. Rovers will wander about for food, whereas sitters will stay put and nosh on whatever is in front of them. As it turns out, the two alleles are subject to negative frequency-dependent selection; that is, for certain characteristics, in a competing population, the minority has the advantage:

"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 Sokolowski. "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."

This particular case is interesting because the difference in behavior relies on a single gene, one that also happens to be homologous to genes in other species (including honeybees, mice, and humans).

It's a beautiful example of how natural selection can maintain diversity. It will be interesting to see whether they find a similar effect in the wild, where so many more environmental factors come to play in selection.

PaV at Uncommon Descent has problems understanding how this sort of system could arise via natural selection. The ignorance is hardly worth addressing, but since his post directed me to the study in the first place, it's only fair I address his concerns. He writes:

[H]ow would you explain NS being able to virtually decide that it is “best” to conserve both forms, rather than to single out one of the two forms?

And later, in the comments:

Isn’t it true that over time one trait is more advantageous than another? So, then, how does one define the “fittest”? And if there isn’t just one definition for the “fittest”, then how can evolution—the “survival of the fittest”, supposedly—make a choice? And, if it is then protested that in only certain situations this proves to be the case, then this only further obscures the equivocation that is Darwinism: Who survives? The “fittest”. Who are the “fittest”? Those who survive. Now we can add: Who survives? The “rarest”. Who are the “rarest”? Well, that all depends…..

As I initially stated, NS is virtually making a choice to “balance” these two forms. And we can see the reason why this is “good”, but we have foresight, and NS, as someone already posted, has none—it’s non-teleological, it can’t see purpose. So why does NS balance forms here, but not in the case of rhinoceroses?

Or, to put it another way, if only the “rovers” existed, would anybody be saying, “Oh, there ought to be a “sitter” population to balance them off”? I seriously don’t think so. Then how does NS ‘blindly’ come up with this ‘choice’?

First of all, natural selection doesn't "virtually decide" anything. Natural selection is not conscious, it's just a label for a process that nature undergoes.

As for how this process could result at this natural equilibrium without planning it out in advance, that should be obvious. Let's say we started with only rovers. The rovers are surviving just fine; there's no need to introduce any variation. But then variation happens, and you get a sitter. Given the current population, the sitter has the advantage being the rarer, and proliferates. It's not a choice anyone made. The sitter allele cropped up randomly, and was favored in the given environment.

So why doesn't natural selection get rid of all the rovers in favor of the new sitters? Because natural selection is a process determined by the environment. And as the sitter population changes, that changes the environment. If a sitter and a rover are surrounded by mostly rovers, the sitter will face less competition and be favored. If the same sitter and rover are instead surrounded by mostly sitters, then the rover will be favored, because the environment is different. Note that natural selection doesn't always have this effect; the increase in frequency of a certain allele doesn't always negatively impact that allele's propagation.

This particular case is a beautiful example of stable equilibrium in a biological population, and it's really interesting to think about what kind of other environmental factors (selective predation upon rovers or sitters, for example) could affect that equilibrium in the wild. It's a shame that some people have such a narrow and warped conception of natural selection that they can't appreciate how much we're learning about the workings of nature.

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