The heart of the evolutionary process is the preferential selection of genes that help an organism to reproduce more often than other members of its species. These genes can act directly, by increasing reproduction, or indirectly – for example, by keeping the organism from dying. Whatever the case, the forces of selection operate solely on those of reproductive age.
Evolution has no further use for us once we lose the capacity to reproduce. Theory tells us, in fact, that we should expect to shuffle off quite promptly once we’re out of the parenting game.
So, how then do we understand the fact that women live for many decades beyond menopause?
Scientists have posited three explanations for this evolutionary puzzle. The first, known as the inter-age correlation model, suggests that genes selected for early-age functions also help prolong late-age.
Another theory, the inter-sex correlation model, argues for the existence of genes that permit men to sometimes defy old-age fertility decline and thereby father children late in life. In some cases, those genes are passed on to females, resulting not in late-life fertility but fitter longevity.
The third is known as the (grand)maternal model, and contends that older women contribute indirectly to the fitness of daughters and granddaughters by caring for them in various ways.
Now, surprising new research has put paid to all three models.
Jacob Moorad and Craig Walling from the Institute of Evolutionary Biology at the University of Edinburgh in the UK drew on one of the world’s most extensive sources of genealogical information: the Utah Population Database (UPDB), housed at the University of Utah, and drawn from the family records of Mormon settlers to the state. The database contains information on more than eight million people.
Moorad and Walling, using a subset of the UPDB for the first empirical test of the models, sought to discover if there is positive genetic correlation between “late-age lifespan and fitness” which is assumed in all three models. Such evidence would be, the authors write in the journal Nature Ecology & Evolution, “‘the smoking gun’ necessary to demonstrate the true efficacy of an evolutionary pathway to maintain post-reproductive survival.”
The problem is, they didn’t find it.
In fact, they found no indications at all of that supported the contentions of any of the three leading theories. The evolutionary reason why old women exist remains unknown.
In an editorial in the same issue, Alan Cohen of the University of Sherbrooke in the UK argues that evolutionary biologists should see the study as a cautionary tale. In seeking to explain their results, the authors speculate that one or more of these models might once have worked, but were disrupted over time.
Cohen takes a lesson from this, writing that “it is not realistic to think that a brief snapshot of any given population is likely to give a satisfying answer for how selection has produced stable traits of the species.”
Human evolutionary biologists, then, may well have to rely on mathematical modelling alone, and leave the empirical studies to sciences with firmer datasets.