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NancyLebovitz comments on When does heritable low fitness need to be explained? - Less Wrong
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I wonder why there hasn't been more selection against women having a difficult time giving birth. It's risky, but some women give birth more easily than others, so there's enough variation for evolution to work on.
Part of the reason is probably because of a conflict between the mother, father, and baby. The baby gets all of the benefit of, say, having a bigger head at birth but some of the cost is born by the mother and little by the father so from the baby and father's viewpoint the head size will be bigger than it would be for the mom.
That seems orthogonal to NancyLebovitz's point: some women have lower risk than other women when birthing infants with a given head size.
Also, if a woman dies in childbirth, or even if she's injured enough to not lactate well in the days immediately following birth, that strongly impacts the baby. Evolution may not care about how painful birth is, or even how long it takes (now that we don't need to fear predators), but it certainly cares about risk of injury or death to the mother.
Some gene increases the size of the baby's head at birth raising the baby's eventual IQ by X, but also increases the chance that the mom will die during childbirth by Y. There will exist (X,Y) such that the baby having the gene will increase the expected number of great grandchildren that the dad and baby will have, but decrease the expected number of great grandchildren that the mom will have.
How big a selection pressure does higher IQ command? Wikipedia on Fertility and Intelligence:
Many local times & places have had a negative correlation between IQ and fertility.
Of course, fertility isn't precisely the same as inclusive fitness, but it's strongly correlated.
On the other hand, many women die in childbirth or suffer significant complications in many parts of the world even today.
Enough that Homo Sapiens has a brain volume nearly twice as high as Homo Habilis. (~600 cc to ~1200 cc).
I'm asking about selection today, or in the last few millenia, not two million years ago in a different species.
Exactly.
Thanks. I was just going to make that point.
Pelvic structure is in play as well as head size.
Also note that there's more to raising children than lactating. Women typically needed to be in good enough shape to do hunting/gathering/food growing.
See Sarah Hrdy's Mother Nature for a reminder that motherhood takes place in the world, not just between the mother and child.
The real conflict is between the mother and the laws of nature that say that she can't have hips the size of barn doors and still be able to walk.
I'm not sure what you've got in mind there, but it would be worth looking at whether there are some tradeoffs between being able to give birth easily and how good a woman is at walking and/or running.
The larger the hip the easier it is to give birth but the less efficient the gait becomes for a bipedal animal, since at each step the projection of the center of mass on the ground is horizontally farther from the foot.
That's the reason why women swing their hips much more than men as they walk. While it looks sexy it is not very evolutionary advantageous if a saber-toothed cat is chasing you.
Birds and bipedal reptiles don't have that problem though, since their birth canal does not pass between the hip bones, thus they can get away with horizontally small hips and still be able to lay eggs which in some species can be as large as one fifth of the their body.
While in hunter gatherer times it would have been important to run fast, as civilisation arises you get chased by bears less often, and so I could guess that hips would start to become wider.
The hips already became a lot wider X-/ along with other body parts.
And given contemporary medicine, a narrow pelvis isn't really a big deal nowadays.