A study published in this week's PLOS Computational Biology reports that menopause is an unintended outcome of natural selection caused by the preference of males for younger female mates. While conventional thinking has held that menopause prevents older women from continuing to reproduce, the researchers, from McMaster's University, concluded that it is the lack of reproduction that has given rise to menopause.
The researchers found that, over time, competition among men of all ages for younger mates has left older females with much less chance of reproducing. The pressures of natural selection focus on the survival of the species through individual fitness, so they protect fertility in women while they are most likely to reproduce.
"In a sense it is like aging, but it is different because it is an all-or-nothing process that has been accelerated because of preferential mating," says Rama Singh, one of the study's authors. "Menopause is believed to be unique to humans, but no one had yet been able to offer a satisfactory explanation for why it occurs,"
The researchers used computational models implemented by computer simulations to show how male mating preference for younger females could lead to the accumulation of mutations that were detrimental to female fertility and thus produce a menopausal period.
However, the prevailing "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. 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 says.
"This theory says 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."
The development of menopause, then, was not a change that improved the survival of the species, but one that merely recognized that fertility did not serve any ongoing purpose beyond a certain age.
The consequence of menopause, however, is not only lost fertility for women, but an increased risk of illness and death that arises with hormonal changes that occur with menopause. Singh says a benefit of the new research could be to suggest that if menopause developed over time, that ultimately it could also be reversed.
Source: Public Library of Science