Friday, November 04, 2005

A wrinkled landscape   posted by Razib @ 11/04/2005 12:13:00 PM

One of the most important things to remember about genetics and its effect on phenotype is the concept of correlated response. That is, you select for trait A, and you get a response from trait B, likely because A and B are both partly dependent on gene 1, which has pleiotropic effects. Antagonistic pleiotropy is this phenomenon in action in the world around us, selecting for long life tends to have correlated response in terms of reduced fecundity, so the two traits balance each other out at an equilibrium that maximizes fitness (obviously if you select for fecundity to the point that survivorship goes down too fast in the life history of an organism it might have fitness implications as the offspring might have benefited from parental protection). I only bring this up because of the hot or not post below, and musings about why there might even be variance on the metric of estrogenically induced beauty (i.e., if a trait is extremely beneficial, the population should shift toward reduced variation over time as positive alleles fix on a local at 99% frequencies, at which point it might become a genetic trait). If one assumes that estrogen levels have a causative influence on two traits, "beauty" and "fecundity," and if one of those traits (fecundity) has strong fitness implications, it makes sense to use the other as a proxy for fitness since the correlation between the two is strong due to a common variable. So why is there variation in beauty anyhow? If Geoffrey Miller has your ear, he would tell you it is a good proxy for individual mutational load, as he argues The Mating Mind. Mutations are simply a fact of the universe around us, not only do we inherit mutations from our ancestors, as individual humans we generate novel mutations de novo (I have seen estimates around potentially lethal 3 mutations per individual, the lethality being masked by a "good copy"). Because of variance around the expectation of a given number of mutations there will be a range of mutational load across the population, and even within a family. Even if a mutation doesn't kill you, the "masking" is likley to be imperfect, and the number of bad copies of a gene you carry might increase your predisposition to developmental problems which could result in asymmetry and other related traits. Some would argue that the sole reason males exist is to purge the genetic load out of a breeding population via the reproductive skew toward those individuals who are on the low tail of the mutational distribution (see Why Sex Matters for a human illustration of this model).

But back to point about correlated response. First, let me add the caveat that one must be cautious about connecting correlations together...but, bear with me and be generous with your credulity. Perhaps the variation within a population of females of relative testosterone levels (ergo, masculine features vs. feminine features) is partly a byproduct of correlated responses? Consider the fact that testosterone tends to correlate with sex drive. If women with higher testosterone levels do have higher sex drives, perhaps they are simply an "alternative strategy" that persists within the population? This might be because of frequency dependent effects, or because temporal variations in the fitness landscape (i.e., sometimes it pays to be promiscuous, and sometimes it doesn't). Additionally, I have read that women with higher testosterone levels tend to produce more sons (ratio). Again, in certain contexts this might be beneficial for fitness. And finally, perhaps women with more testosterone birth sons with higher basal levels of the hormone, and the sons are therefore far more fit than those of beautiful women? Ultimately, the coupled correlation between beautiful women and hypermasculine men might lead to modifier genes arising which would slowly dampen and eliminate any negative fitness implications of such matings (that is, the offspring being less fit than either parent because of additive effects resulting in population basal levels of testosterone or estrogen).

I don't offer any of these ideas seriously. They are speculations, but, the overall point is to be open to various explanations and remember that there are multiple paths to a given fitness peak. Over time I have become reluctant to post much about human psychology and its intersection with biology precisely because the popular press tends to take one research paper and take its model at face value, and then proceeds to blow out of proportion the importance of the factor which they pinpoint as explaining variation. I hope readers of this weblog have learned to be cautious and consider looking at the original paper that these stories are based on, that will allow one to take the step beyond entertainment and toward genuine analysis. Also, knowing a little genetic logic can add to both entertainment and analysis.

Update: Through the rugged roads of gene land.