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February 11, 2005

Genetic variation is a fact of life

There are many traits that humans display which are purely genetic in character, that is, they don't display phenotypic variation correlated to genotypic variation because there isn't genotypic variation, and there isn't an environmental component to muddy the waters (though developmental issues can result in some variation, and, importantly signal the "health" of the overall genome because of its correlation to developmental stability). Generally these are traits which have extremely high fitness costs when one deviates from the normal "plan," so mutations can never spread through a population, and the loci remain monomorphic (one genetic variant or allele is present nearly to fixation). On the other hand, there are many traits which are heritable and display a continuous range through populations. Their fitness implications are not as clear cut. For example, take height. There is some evidence that tall men and short women are more fertile than tall women and short men. So why do short men and tall women exist? Why is there is a normal distribution? One important point is that tall men and short women pass on both these traits to their children, so the next generation exhibits a range of heights. Additionally, one could concoct forms of balancing selection and correlated responses with act against extremes of height. Nevertheless, I think it is important to observe interpopulation differences as well. Why are Cambodians shorter than Pathans? Or "Pygmies" shorter than surrounding populations? The obvious response is adaptational constraints were at work,1 and and one height does not point in one fitness direction at all times, in other words, unlike an allele that results in only one finger developing on a hand, an allele that tends to produce a given height has a fitness differential from the mean which changes as a function of time, so it never reaches fixation or extinction because its change in frequency because of selection oscillates positively and negatively.

Humans are not all the same on some rather important traits. Evolutionary psychologists have emphasized the monomorphic nature of loci which shape human nature, but that neglects the reality that personalities tend to differ (a trivial example). This is all set up for this paper....

Heritability and genetic constraints of life-history trait evolution in preindustrial humans:

We found strong positive genetic correlations between female age at first reproduction and longevity, and between interbirth intervals and longevity, suggesting reduced life spans in females who either started to breed relatively early or who then bred frequently. Our results suggest that key female life-history traits in this premodern human population had high heritability and may have responded to natural selection. However genetic constraints between longevity and reproductive life-history traits may have constrained the evolution of life history and facilitated the maintenance of additive genetic variance in key life-history traits.
1 - Yes, nutrition does play a role, standard caveat, but let's be real for a moment....
Posted by razib at 11:15 AM