Here's an article on The Grandmother Hypothesis. Personally, I didn't take the idea seriously until a biological anthropologist told me that menopause was a tightly integrated proactive cascade of biochemical changes which shuts down female procreative capacity. In contrast, human males exhibit declining fertility in a gradual fashion due to a generalized breakdown of bodily function. I am generally suspicious of some sort of adaptation when something so precise in our physiology seems on the surface to reduce fitness.
Update: Here is an article from the originator of The Grandmother Hypothesis:
This article assesses effects of the modern world on hunter-gatherer adult mortality, with special reference to the Hadza. Evidence suggests that such effects are not sufficient to deny the existence of substantial life expectancy at the end of the childbearing career. Data from contemporary hunter-gatherers(Ache, !Kung, Hadza) match longevity extrapolated from regressions of lifespan on body and brain weight. Twenty or so vigorous years between the end of reproduction and the onset of significant senescence does require an explanation.
Note: I expect readers of this weblog to comprehend that small (e.g., less than 1%) selection coefficients over long periods of time are evolutionarily significant.
The Wiki link says that having kids in old age (or in general) would put the mother at risk for contracting infections -- not true for H-Gs, who don't have iatrogenic diseases.
The parental investment stuff sounds more plausible.
In order for the grandmother hypothesis to be valid there had to be a time when a substantial numbers of women were surviving beyond 50 years of age and having babies at that age. When was that time?
The selective benefit of menopause for an individual had to be significant enough to drive the allele to fixation. In order for this to happen there had to be young surviving grandchildren of a 50 year old and a 50 year old who was healthy (and alive) and capable of making a difference. The selective benefit had to be substantial enough to overcome the fact that many women died before 50 and of those who survived a significant number had no positive impact on their grandchildren.
It's extremely difficult to imagine that any indirect selective benefit of this sort could have swept to fixation in the face of all the physical problems associated with menopause. This is another adaptationist just-so story.
The selective benefit had to be substantial enough to overcome the fact that many women died before 50 and of those who survived a significant number had no positive impact on their grandchildren.
seriously dude, what the hell are you talking about? did you skip 6th grade math? the selective benefit had to be positive, that's it, for the likelihood of fixation to be greater than 0. if the s was of the same order as 1/(2Ne) then the mutation had to occur repeatedly so that stochastically driven extinction events could be overcome. in any case, repeated mutational events would make a positively beneficial trait highly likely to fix over evolutionary time.
there didn't have to be any fucking "substantial."
It's extremely difficult to imagine that any indirect selective benefit of this sort could have swept to fixation in the face of all the physical problems associated with menopause.
spare me your imagination. hopefully in the next generation we can map the physiology & biochemistry to the loci which control it and see if they are under selective constraint.
Oops! Sorry. I didn't realize how strongly you felt about adaptationist explanations. Dream on.
Isn't the default assumption of phylogenetic and/or developmental constraints as the sole explanation for a given feature just another "constraintist just-so story"?
wut colugo said. these 'just-so' vs. 'spandrel' arguments are so 1970s. genome surveys are detecting buttloads of regions subject to selection & constraint that we have no idea in regards to phenotype. in any case, a few points
1) the various vectors which compose the grandmother hypothesis are all subject to critique, and none of them are a 'smoking gun.' that being said, taken together they are considerably more than a 'just so.'
2) menopause is derived and rare. this isn't your typical garden-variety physiological process. it is possible that it emerges due to some pleiotropic interactions, but again, my own bias in favor of some sort of fitness benefit is derived from the impression of its integrated and proactive nature as communicated to me by a biological anthropologist who looked into subject in collaboration with some physicians.
3) i object to the tone of larry's objection because frankly he throws out a null hypothesis and uses imprecise language. what exactly does a 'significant' mean in terms of selective benefit? 10% selection coefficient? 1%? i would be skeptical about a 10% benefit, but definitely not a 1%.
i object to the tone of larry's objection because frankly he throws out a null hypothesis and uses imprecise language. what exactly does a 'significant' mean in terms of selective benefit? 10% selection coefficient? 1%? i would be skeptical about a 10% benefit, but definitely not a 1%.
Why not assume that I know what I'm talking about? "Significant" means a selection coefficient that's high enough to justify a claim that the allele was fixed by natural selection. I'm well aware of the fact that with a selection coefficient of 0.01 the probability of fixation is only about 2%. I'm well aware of the fact that you (and others) argue that the allele will still have a high probability of fixation because the mutations will occur many times.
What I'm questioning is whether there is any solid evidence that there is a selective advantage of menopause, even a very small one. So far, all I see are some studies showing that in today's society there's a correlation between a child's chance of survival and having a grandmother.
menopause is derived and rare
not so sure about that. it seems pretty common in mammals
Okay, but what if menopause occurred earlier in H/G times? Would almost have to be the case if it is true that most people died before 50 at that time. Yet if women bore children from say 15-25, went though menopause at around 30, it's quite possible they'd still have a few years to be useful.