In actual fact, because it involves the fixation of deleterious alleles, menopause results in a net loss of fitness. This goes to show that all traits need not have evolved by positive natural selection.
Researchers at McMaster University propose a solution to the evolutionary mystery surrounding the origin of the menopause…
Dr Rama Singh
A study published in the open access journal PLOS Computational Biology
this week suggests that the menopause in human women evolved as a result of males preferring younger females as mates. The research, which was funded by the National Sciences and Engineering Research Council of Canada, was carried out at McMaster University in Ontario.
With existing theories failing to provide a fully satisfactory explanation in many eyes – the classic theory, for example, predicts the emergence of a ‘death barrier’, with women failing to live long beyond reproductive age – scientists have been searching for other potential explanations. Co-author of the paper Dr Rama Singh, from the Department of Biology at McMaster, spoke to ScienceOmega.com
about the new theory that he and his colleagues have developed.
One hypothesis which has received a lot of attention involves the so-called ‘grandmother effect’. This is the idea that the menopause is an adaptation whereby post-menopausal women – likely grandmothers – stop reproducing at a certain age in order to assist in the rearing of their grandchildren and provide continued support to their existing children.
Existing theories 'lacking'
Others have suggested that a combination of competition for resources among in-laws and the grandmothering principle could have been at work. While the grandmother hypothesis has found some favour in the past, its key failing is that it does not explain why men, on the other hand, live for so long and why they continue to produce sperm almost indefinitely.
"All previous evolutionary theories of menopause, including the grandmother hypothesis, try to explain menopause with the idea that the loss of fitness caused by menopause is compensated for by fitness gain through some sort of kin selection," Dr Singh told ScienceOmega.com
. "Researchers have investigated these theories and have found them lacking."
Hypotheses have often focussed on the evolution of beneficial traits – hence the idea that the menopause evolved because of the advantages conferred by the grandmother’s role in helping raise young – but there are many characteristics which, although they may once have been useful, have become obsolete over time. This phenomenon, which has resulted, for example, in cave-dwelling fish losing their eyesight and birds in predator-free environments losing the ability to fly, is known as relaxed selection.
Not necessarily positive natural selection
"By incorporating deleterious mutations and making them free from selection surveillance, our theory makes menopause a neutral trait and free to evolve without any gain of fitness," said Dr Singh. "In actual fact, because it involves the fixation of deleterious alleles, menopause results in a net loss of fitness. This goes to show that all traits need not have evolved by positive natural selection."
In other words, Dr Singh and his colleagues argue that rather than the menopause preventing older women from reproducing in order to benefit the species in other ways, a lack of reproduction in older women spurred the development of the menopause. So can we dismiss the grandmother theory completely?
"Our theory is simple and sufficient to explain the origin of menopause," was Dr Singh’s response. "We cannot say if other mechanisms may or may not have played a role. There is no need to deny a potential gain of fitness through grandmother's help, but neither do we need to link it to her own loss of fitness."
According to Dr Singh, the theory explains why menopause is unique to humans because it requires a unique mechanism – preferential mating. Other than in humans, survival past reproductive age has only been reported in certain whale species and in captive chimpanzees.
"I would like to see further investigation of the menopause in whales to find out if it resembles that in humans, and if so does the same mechanism hold," he went on. "Since in evolution similar outcomes do not mean similar causes; there is no need to assume that whale menopause, if it does exist, stems from a similar mechanism."
‘Sex being fun’ would have helped
But why would males have preferred younger females as mates when, presumably, older females were initially just as fertile, and more experienced at raising offspring to boot?
"Males mate because sex is fun," said Dr Singh. "Subject to ecological constraints, natural selection would select for early reproduction and sexual selection would select attractive sexual traits in young females. Continuous reception of human females and ‘sex being fun’ would have helped."
The team of researchers used computational models to simulate the effect on a human population of, among other things, a male preference for younger female mates. The model showed that this could account for the accumulation of mutations with a negative impact on female fertility.
"The simulation results were very important in not only showing that the theory works, but allowing investigation and testing of various aspects of the ‘simulation biology' in terms of varying parameters and constraints," Dr Singh related.
The findings, as well as being interesting from an evolutionary perspective, could also have implications for the consequences of menopause, such as the increased risk of osteoporosis. I concluded by asking whether Dr Singh and his colleagues have further research planned on the menopause.
"Absolutely, both in terms of simulation work as well as investigation of menopause and its consequences for population demography in view of the late onset of reproduction by career women," he responded. "It would be interesting to ask, ‘Can menopause be delayed, or even eliminated altogether? That’s assuming we would want to do that, of course."
Read the full text of the paper, 'Mate Choice and the Origin of Menopause'...