In some cases, fitness depends on an absolute standard, but in others it depends on a relative standard. Suppose someone throws you out of a fourth-story window: what matters is not whether you suffer less damage than others, but whether you are actually tough enough to survive the fall. Gravity doesn’t grade on a curve. On the other hand, there are many situations in which the world does grade on a curve. In a famine, one where there’s only enough food for half of the population, you only need to be in the top half to survive.
Or, are you fast enough to out-race the tsunami [absolute standard], or fast enough to outrun your rivals? [relative standard] .
Suppose that we ranked people by some kind of genetic merit and liquidated those that were more than a standard deviation below average. This turns out to be a very efficient way of reducing genetic load. Each liquidated individual carries significantly more deleterious genes than the population average, and so truncation selection reduces the number of deleterious genes passed on to the next generation – enough to keep up with a fairly high mutation rate. Of course all this also depends on how fitness varies with the number of deleterious mutation [is it really multiplicative?] , but that’s a different can of worms that I won’t address right now.
The question is, to what extent has natural selection in humans been truncation-like? And has it been equally truncation-like in all populations? Any variation would cause local differences in the efficiency of natural selection, which would (if long continued) cause local differences in the amount of genetic load, and possibly local differences in the flavor of genetic load, as well.
Case 1: just enough food is produced to feed the existing population, and production doesn’t increase with the population. Resource-limited. The average family has four kids. The half of the new generation with the most moxie [broadly defined, involving various forms of interpersonal competition] survive, the rest starve.
Case 2: There is plenty of food, but there is a virulent infectious disease that kills half the rising generation. This disease gets worse with increasing population density, so it always limits the population – food stays abundant. What matters is a quite narrow kind of moxie – the alleles that influence the course of that virulent disease. And if your defenses keep you from being infected, that’s as good as it gets.
Case 3: Half of each generation is killed by meteorites. Genes are irrelevant.
I don’t think that any human population is a pure case.
The overall intensity of selection can also vary – some populations may have three kids and lose one, some may have four and lose two.