Limits of Adaptationism #2: Genetic constraints

Linked Genes: Pleitropy and Epistasis

Genetics is seldom a straight path from gene to protein to mustache. At every step, there can be modification and multiple effects. Pleitropy is when a single gene has an effect on multiple traits. For our mustache example, a gene regulating secondary sexual characteristics (the changes in men and women's bodies that occur during the advent of adolescence, such as facial hair) can have an effect on hair produced all over the body. Generally, the more hirsute a person is, the more likely they are to have a mustache. Since there is no actual mustache-gene, and since hairlessness isn't necessarily selected for, we will see mustaches as long as there is positive selection for hair.

Epistasis is when the expression of one gene effects the expression of another gene. For example say there is a gene at the "hipster-ness" locus that has two alleles, "hipster" and "non-hipster." When an individual has the "hipster" allele, the expression of this gene will turn on the expression of other genes such as the genes for mustaches, non-prescription thick-rimmed glasses, androgynous hair styles, and appreciation of terrible indie-rock. Any one of those downstream traits may be driving the selection for the hipster allele. Even if the mustache is selected against, it is regulated by another gene that may be positively selected for some inscrutable reason.

In both of these examples when we ask "what for?" we cannot give a satisfactory answer, because the expression of a mustache isn't the real unit of selection. The mustache is linked to other traits that are undergoing selection and influencing the expression of mustaches, despite their unpopularity.

Limits of Adaptationism #1: Founder Effects

This is the first in our series examining the limits of adaptationism, where we will examine why teleological questions (asking "what for?") don't necessarily get us the right answers in biology.

Take for example a man with an elaborate mustache. What is a mustache for? We could come up with all kinds of wild functional explanations for why mustaches exist, but none of them truly convincing. We know that men without mustaches live very normal, if not happier lives, than men with mustaches. In this era of human biology, mustaches are selected against, and the men who grow mustaches are cursed by the necessity of shaving them if they want women-folk to like them. In localized hipster populations, this trend can be reversed, but let's just say for the sake of argument, that all women feel more or less like I do.

So the reason for mustaches isn't really clear. Adaptationism would assert that since it takes energy to produce and maintain a sweet 'stache, it must serve some purpose for the adult male human, otherwise it would have been lost from the population and men would be largely mustache-less. But what if adaptationism has led us down a rabbit-trail looking for reasons where they may not exist?

An alternative explanation for why mustaches exist may be a genetic constraint on natural selection known as the founder effect or legacy effect. Basically, you can't throw out all your starting material and start over when starting a new population. The population always has founders. Perhaps the founders of the male human population had facial hair on their upper lip. Perhaps this was because they were bigfoot or wookie-like creatures with a lot of fur anyway. Selection against large amounts of body hair has occurred over the evolution of male humanity, but because of the founder effect, it's going to take a lot to get rid of those mustache genes.

Similarly, the legacy effect is a constraint placed on selection now due to selection in the past. A couple centuries ago, mustaches were the hotness. Women felt they were manly, and preferentially mated with men who had mustaches over those who did not. Now we have a gene pool full of mustaches, and we can't really get rid of them due to the legacy of the age of mustache-rage.

So, the "what for?" question doesn't help us understand what mustaches are for, unless we ask what mustaches might have been for back in the evolutionary history of hairy men.

If we go back to our insect antennae, we may ask why certain insect taxa seem to have a particular style of antenna. The question may not have a functional answer, because the reason for different styles of antennae is buried far back in the evolutionary history of these bugs. Perhaps the founders of modern butterflies had knobbed antennae, and the founders of modern moths straight or plumose (mustache-like) antennae. The reason why their antennae diverged back in that day may be as simple as genetic drift or it may have had a functional meaning in those days, but today it seems to be the capricious whim of a God who likes knobby antennae.

Photo by Flickr user a4gpa licensed for re-use by Creative Commons license.