those are a couple of graphs from wade and breden from when they did some mathematical modelling of the selection for “genes for altruism” under different circumstances (see previous post for more details).
the interesting graph is the bottom one which shows what the frequency of “genes for altruism” in a population would be IF selection was strong and IF the alleles in question were dominant. the gene frequencies are on the y-axis (at 1.0 the genes have reached fixation). the number of generations to get to the various gene frequencies is on the x-axis.
the interesting line on the graph for us is the solid line: those are individuals who are about two-times more related to one another than first-cousins in a randomnly mating population. that’s an exaggeration for most human populations, but it’s the most human-like of all the mating patterns they considered. the others are cloning, sib-mating (ewww!), and total outbreeding. so most human populations would be lower than that solid line, but not flatlining like the total outbreeding example. somewhere in between.
anyway. in my previous post i pointed out that after just 50 generations, there is already an increase in the frequency of “genes for altruism” in the solid line population. however, these models start at zero! in other words, the starting point they’re thinking of is if the populations start off with no “genes for altruism” at all. but that can hardly have ever been the case for any human population since altruistic behaviors are found in almost every living being on the planet!: plants, insects … even slime molds! not to mention our closest cousins, other primates.
so the baseline for the frequency of “genes for altruism” in any human population was probably never zero. who knows where it should be? 30%? 40%? 50%? 80%? i really don’t know. but not zero, anyway.
if we just say it was 50% — just picking an example right out of the hat — then the frequency of “genes for altruism” in the inbreeding solid line population increases much more sharply (i.e. the slope of the line is more slopey) over fifty generations than if we start the population off at zero. (look at the solid line between 0 and 50 generations versus 200 and 250 generations. there’s a much sharper increase in the latter group.)
i’m picking out 50 generation timespans, btw, because — at a very conservative estimate (1 generation=25 years) — arabs have been very closely inbreeding for ca. 56 generations (i.e. since at least mohammed’s days and most likely before).
anyway. that is all. (^_^)
update 05/30: see also inbreeding and the evolution of altruistic behavior ii
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