Why aren’t we all tall?

There’s a fair amount of social science and anecdata that tall males are more reproductively fit. More precisely, males one to two standard deviations above the norm in height seem to be at the “sweet spot” as an idealized partner (e.g., leading males). And, short men often have fewer children. Short women will pair up with tall men. Tall women will generally not pair up with shorter men. The question then has to be asked: why isn’t natural selection producing a situation where we’re all tall?

As it is, height is a highly heritable trait where there’s a lot of genetic variation present in the population. One hypothesis might be that short(er) people are simply individuals with a higher mutational load. In other words, there’s going to be variation in the load of deleterious alleles from person to person, and one’s value on quantitative traits (intelligence, height) is a reflection of one’s genetic fitness. There are problems with this model, starting with the fact that one you need to tease apart inter-population variation. Also, within families there doesn’t seem to be a correlation between height and intelligence, which you would expect to see if quantitative traits are reflections of variation in mutational load.

So naturally you have to move the possibility of balancing selection. I have suggested in the past that inter-population differences in height may be a function of expected levels of nutritional stress. Short people are smaller, and need to eat less. The same dynamic could produce variation in height within populations as well. But a new paper outlines what I think I think is the most elegant solution (though elegant does not mean right!), Intralocus sexual conflict over human height:

Intralocus sexual conflict (IASC) occurs when a trait under selection in one sex constrains the other sex from achieving its sex-specific fitness optimum. Selection pressures on body size often differ between the sexes across many species, including humans: among men individuals of average height enjoy the highest reproductive success, while shorter women have the highest reproductive success. Given its high heritability, IASC over human height is likely. Using data from sibling pairs from the Wisconsin Longitudinal Study, we present evidence for IASC over height: in shorter sibling pairs (relatively) more reproductive success (number of children) was obtained through the sister than through the brother of the sibling pair. By contrast, in average height sibling pairs most reproductive success was obtained through the brother relative to the sister. In conclusion, we show that IASC over a heritable, sexually dimorphic physical trait (human height) affects Darwinian fitness in a contemporary human population.

There isn’t much theoretical complexity in the paper. They’re looking at a huge data set of individuals from Wisconsin, and they observe that in families where siblings are short the sisters tend to be more fecund, and in families where the siblings are not short the brothers tend to be more fecund. The argument here is that antagonistic sexual selection maintains variation within the population. Some of the media reports suggest some sort of frequency dependent theory in the background; if the population gets too tall or short then males and females of the favored varieties may gain more fitness advantage.

As the authors note over time this sort of dimorphism should fix in a manner where the variation within the population diminishes as sex specific alleles emerge. But this takes a very long time, and may simply be impossible to attain toward equilibrium in the case of a trait with the genetic architecture of height. One would have to imagine modifier genes throwing out their net across the whole genome.

It’s easy to imagine why being tall might entail fitness gains for a male. What’s going on with females? I suspect that on the extreme margin very tall women probably have lower fertility for hormonal reasons. But that doesn’t explain to me why very short women seem to have such high fertility in relation to average height women. One explanation might be that they mature faster, and so enter their peak reproductive years rather early. This might extend their fertile period longer than average height or taller women. In contrast, this isn’t much of a gain for males, who have longer reproductive careers on the tail end.

Source: Discover Magazine – Gene Expression