A battle is raging in our bodies: genes for short stature are favoured in females, whereas genes for average stature are favoured in males. This tug of war means that neither men nor women reach their optimal height. It’s a real intralocus sexual conflict.
PhD student Gert Stulp is about two meters tall. A person’s height predicts, to some extent, his or her reproductive success (i.e. how many offspring he or she will have). Stulp explains, ‘Females of below average height tend to have more children than those of average height. But for men, those of average height have more children than short or tall guys.’
Height is regulated by many different genes and is also influenced by environmental conditions. The genetic component means that reproductively successful short females and average height males pass on their ‘short’ and ‘average height’ genes in relatively large numbers. What does this mean for the human population?
These two different sets of genes affect the height of males and females, which, says Stulp, ‘may cause what biologists call an intralocus sexual conflict. Take deer: the bucks need large antlers to ward off competitors. Big antlers provide them with a better chance of a large number of offspring. But a doe would have no advantage at all from large antlers.’ When there is a difference between the sexes in the natural selection of a genetic trait that both sexes express, this is called an intralocus sexual conflict.
In the case of deer, the conflict is prevented as the genes to produce big antlers are switched off in the does. But when it comes to human height, many genes are involved and they are not all regulated according to sex. This makes the situation rather complex, and Stulp is the right person to explain just how complex. He has just published a paper in the journal
Biology Letters (12 November), which describes how men and women are engaged in a genetic battle over body height.
‘Due to the genetic component, average-sized couples tend to have average-sized children, and siblings tend to be of a similar relative height,’ says Stulp. So average-height couples have average-height sons (who will have more reproductive success) and average-height daughters (who will have less reproductive success than short females). The result is a conflict in our genes.
In humans, the conflict over height would be prevented if small females produced mainly small daughters and average men mainly average-height sons. ‘But that isn’t the case. There really is a conflict,’ says Stulp. He is the first to have proven this and he did it by studying a large database of more than 10,000 inhabitants of Wisconsin born in 1937 and 1938. Their life history has been recorded and plenty of social and medical information is available.
‘We found that sibling height was indeed a predictor of the reproductive success of an individual.’ If you are below average height, your nieces and nephews are most likely to be your sister’s children, whereas if you are of average height, they are most likely to be your brother’s.’ Stulp and his colleagues are the first to describe the presence of this type of sexual conflict in humans.
Why small women have more children is unclear. ‘In many species there is a trade-off between growth and reproduction. So maybe these women invest more energy in reproduction than in growth,’ Stulp speculates. Furthermore, the earlier in life a women has her first child, the more children she will have. ‘And we know that women who are genetically disposed to have their first child at an earlier age also tend to be shorter than average. But whatever the cause, this is another example of how evolutionary processes are still active in modern humans. In this age of birth control and family planning, natural selection is still visibly at work in our species.’
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