After decades of trying to shoehorn menopause into a variety of evolutionary contexts that never seemed to add up, a team of Canadian scientists from McMaster University has concluded that what really causes menopause in women is men.
Lead researcher on the new study, evolutionary geneticist Rama Singh, says that until now, no one has been able to offer a satisfactory explanation for why menopause occurs. The long-established “grandmother theory” holds that women have evolved to become infertile after a certain age to allow them to assist with rearing grandchildren, thus improving the survival of kin. But Singh says that does not add up from an evolutionary perspective.
“How do you evolve infertility? It is contrary to the whole notion of natural selection. Natural selection selects for fertility, for reproduction – not for stopping it,” he argues.
Central to Singh’s argument is that human males have long shown a preference for younger women when selecting mates. This, he contends, stacks the Darwinian deck against continued fertility in older women.
The new theory holds that, over time, competition among men of all ages for younger mates has left older females with much less chance of reproducing. The forces of natural selection, Singh says, are concerned only with the survival of the species through individual fitness, so they protect fertility in women while they are most likely to reproduce. After that period, natural selection ceases to quell the genetic mutations that ultimately bring on menopause, leaving women infertile.
While conventional thinking has held that menopause prevents older women from continuing to reproduce, Singh’s theory says it is the lack of reproduction that has given rise to menopause. “Natural selection doesn’t have to do anything,” he explains. “If women were reproducing all along, and there were no preference against older women, women would be reproducing like men are for their whole lives.”
Singh says that women continue to live past their fertile period because men remain fertile throughout their lives, and longevity is not inherited by gender. “If women had historically been the ones to select younger mates,” he adds, “the situation would have been reversed, with men losing fertility.”
The new work, by Singh and researchers Jonathan Stone and Richard Morton, appears in the journal PLOS Computational Biology.
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