A battle about the ideal height would appear to be raging in men and women’s genes. Gert Stulp, PhD candidate at the University of Groningen, has shown that this conflict is leading to a difference in reproductive success between men and women of varying height. His research will be published in the journal Biology Letters on 12 November.
‘Natural selection is still occurring in human beings, despite birth control and good medical facilities’, says Gert Stulp. Together with colleagues from Groningen, Amsterdam and Cambridge (UK), he managed to find evidence of an intralocus sexual conflict currently raging in the DNA of the human race.
‘A conflict like this arises because men and women are different and therefore subject to different selection pressures’, explains Stulp. A stag, for example, benefits from big antlers, but they would be impractical for a doe. ‘Nature has “switched off” antler development in does, so there is no conflict between the male and female animals. However, some of the traits that are helpful to one sex but a hindrance to the other cannot be quite so easily switched off.’
This is the case with height in human beings. Short parents tend to produce short daughters and short sons. This benefits the reproductive success of the daughters, but not that of the sons. ‘We know that shorter women have more children than women of an average height. With men, this is the other way round.’ As short women and men of average height have the most children, their genes are passed on the most.
This difference in selection pressures for human height between the sexes could mean that shorter families are more successful at reproducing via the women, while families of an average height produce more children via the men in the family. This is known as an intralocus sexual conflict: a particular trait (in this case: being short or of average height) is an advantage when it presents in one sex, but a disadvantage when it presents in the other.
The question being addressed is whether this conflict can be demonstrated in humans. Stulp studied the number of children born to brothers and sisters in a large-scale American database containing data on thousands of residents of Wisconsin born in 1937 or 1938.
‘It turned out that by taking the height of just one individual/person, we could predict whether his or her sibling would produce many or few children. Shorter individuals have a higher chance of becoming an uncle or aunt through their sister, while individuals of average height are more likely to have nephews and nieces via their brother.’ The sexual conflict relating to body height was clearly visible. ‘We are the first researchers to actually demonstrate this type of genetic conflict in humans.’
It is still unclear why shorter women have more children. ‘A conflict between growth and reproduction is common in some species of animals’, continues Stulp. Short women probably put more energy into reproducing. ‘In general, the earlier a woman has her first child, the more children she will have. Women with a genetic tendency to have children at a young age also appear to have a genetic tendency to be short. But whatever the reason, evolutionary processes still seem to be alive and kicking in modern society.’
Gert Stulp will be awarded a PhD in early 2013 by the Faculty of Behavioural and Social Sciences for research into human behaviour in an evolutionary perspective. His supervisors are evolutionary social psychologist Bram Buunk and behavioural biologist Simon Verhulst.
Gert Stulp: g.stulp rug.nl, +31 (0)50-363 6326
Gert Stulp, B. Kuijper, A.P. Buunk, T.V. Pollet and S. Verhulst: Intralocus sexual conflict over human height. Biology Letters, 12 November 2012. doi: 10.1098/rsbl.2012.05900
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