The "Grandmother Hypothesis" says that menopause arose in primitive hominids because it prevented pregnancies in older women thereby freeing them to assist in the care of their grandchildren. This extra care had a significant effect on the survivability of the grandchildren thereby increasing the probability that the frequency of the menopause allele would increase in the population.
Over the course of thousands of generations, the allele for menopause became fixed in our ancestral populations because it conferred a significant adaptive advantage.
Razib Khan believes in this adaptationist explanation for menopause. He quotes from a recent study that seems. on the surface, to lend support to the idea. See the recent posting on Gene Expression: Menopause as an adaptation.
The study by Virpi Lummaa looked at the presumed benefit of grandmothers among Finnish families. She reports that children with the support of a grandmother are 12% more likely to survive than children without such support.
The prompts Razib to write.
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).I posted a comment on his blog. I'm reproducing it here in order to get more feedback.
12% is, indeed, a very large effect but what does it have to do with evolution?
We're looking at a study where every single woman underwent menopause so one of the things we certainly aren't doing is testing to see whether menopause has an effect on the survivability of grandchildren.
Let's think about reasons why some families have grandmothers to help out and some don't. First, there's the relative proximity of living grandmothers. Then there's the question of the relationship between parents and grandparents. Let's not forget possible financial help that has nothing to do with direct caregiving. Finally, there's the issue of whether a family even has a surviving grandmother.
None of theses things are affected in any way by the fertility or non-fertility of the grandmother, right? The 12% difference has nothing to do with menopause.
Now let's think about a time in the past when menopause was presumably evolving. You had two kinds of females in the population, those who underwent menopause and those who didn't. There will still be all kinds of families who experience the help of a grandmother irrespective of whether she can still have kids or not. That's the background that a presumed adaptation has to deal with.
If there are non-menopausal women who live into their fifties, are close to their grandchildren, and whose husband is dead, then they will presumably help raise thier grandchildren. Same thing applies to non-menopausal women who simply don't risk getting pregnant any more even though their spouse is alive. (Just say "no." It's probably more common than you think. Women are not stupid.)
In fact, the adaptationist just-so story only really applies to that small subset of women who have the following characteristics.
- They live past 50 years old.
- They have grandchildren who are young and still need care.
- They don't have too many grandchildren in different families so their caregiving can be effective.
- They live near their grandchildren and can help out.
- They have a good relationship with their children and their spouses.
- Their husbands are still living.
- They choose to get pregnant.
That's the only group that menopause affects. It eliminates #7 but has no effect on any of the other factors. It certainly doesn't have any effect on whether the grandmother was dead or alive at 50 years old.
Given that there were many families that received no help from grandmothers, whether they had the menopause allele or not, and given that there were many families who received help even if the grandmothers did not have the menopause allele, the question is "what is the adaptive value of menopause under those circumstances."
What does the Lummaa study have to say about that?
Since you are a supporter of this adaptationist explanation can you describe for me the kind of society where you think this allele became fixed in the population? Was it a hunter-gather society of small bands or a large agricultural society of small towns? Or something else?
I'd like to hear more details about how this grandmother hypothesis actually worked in Australopithicus or Homo erectus societies. Please include your estimate of how many grandmothers survived past the age where menopause could make a difference as you estimate the fitness coefficient.
[Image Credit: "Rudyard Kipling’s illustration for The Elephant’s Child from Just So Stories (1902)." From Encyclopedia Britannica