I’ve talked about menopause before. One question in evolutionary anthropology is whether it is an adaptation, a derived trait in our species which emerged due to the force of natural selection, or simply a physiological byproduct of some other phenomenon. The key point is the peculiar asymmetry in male and female reproductive potentials; males decline gradually over time, while the general suite of female reproductive function simply shuts down at during middle age. Eric Michael Johnson reviews a new paper by the redoubtable Virpi Lummaa, Fitness benefits of prolonged post-reproductive lifespan in women. Eric concludes:
Dr. Lummaa has done just that in her study published in the journal Nature, demonstrating that children are 12% more likely to survive to adulthood when they have a grandmother’s support than when they don’t. That may not seem like a lot, but consider all of the descendants from that surviving 12%, each carrying the trait for reproductive senescence, and you can see how it wouldn’t take long for the trait to become fixed in a population. Furthermore, one of the key innovations of her study was her choice of sample set. By using Finnish records dating from the 18th and 19th centuries she could ensure that any modern health benefits wouldn’t influence the results and would therefore accurately pinpoint the grandmother’s role.
12% is a very big effect and would lead to rapid evolutionary change (on the order of thousands of years in the most simple population genetic model of a single locus of dominant effect). Seeing as to the fact that there are several models of Pygmy stature arguing that it is a product of life history changes, exploring this group’s physiological defaults might be interesting. A friend once told me that anthropologists report that Bantu populations regard Pygmy women as particularly fertile. Finally, one major consideration about menopause for me is that I was once told by a biological anthropologist who collaborated with some doctors in this area is that the process has many “moving parts” and seems to involve proactive shutting down a host of physiological processes. Whatever the fitness implications, the physiological tightness of menopause and the rarity of the phenomenon among animals suggests to me that the spandrel model should be viewed with skepticism.