Group Selection posits that natural selection might not operate at the level of genes in individuals, and instead also operate at genes in groups of individuals, i.e. selecting for genes for the group even at the expense of the individual. For example, you might posit that human religion is an adaptation to make human groups more cohesive, since religious groups outfight nonreligious groups.
See also: Evolution, Alienness of evolution
Consider two groups on different sides of a mountain: in group A, each mother gives birth to 2 males and 2 females; in group B, each mother gives birth to 3 females and 1 male. Group A and group B will have the same number of children, but group B will have 50% more grandchildren and 125% more great-grandchildren.
But consider: The rarer males become, the more reproductively valuable they become—not to the group, but to the individual parent. If all the females are doing what’s good for the group and birthing 1 male per 10 females, then you can make a genetic killing by birthing all males, each of whom will have (on average) ten times as many grandchildren as their female cousins.
So while group selection ought to favor more girls, individual selection favors equal investment in male and female offspring. Just by looking at the statistics of a maternity ward, you can see that the quantitative balance between group selection forces and individual selection forces is overwhelmingly tilted in favor of individual selection in Homo sapiens.
Group selection is extremely hard to make work mathematically. In this simulation, for example, the cost to altruists is 3% of fitness, pure altruist groups have a fitness twice as great as pure selfish groups, the subpopulation size is 25, and 20% of all deaths are replaced with messengers from another group. The result is polymorphic for selfishness and altruism. If the subpopulation size is doubled to 50, selfishness is fixed. If the cost to altruists is increased to 6%, selfishness is fixed. If the altruistic benefit is decreased by half, selfishness is fixed or in large majority. Neighborhood-groups must be very small, with only around 5 members, for group selection to operate when the cost of altruism exceeds 10%.
To the best of this editor’s knowledge, no definite example of a group-level adaptation has ever been observed in a mammalian species. Ever.
Hence, postulating group selection in any species—let alone in humans—is guaranteed to make professional evolutionary biologists roll their eyes.
It seems to be extremely popular among a certain sort of amateur evolutionary theorist, though—there’s a certain sort of person who, if they don’t know about the incredible mathematical difficulty, will find it very satisfying to speculate about adaptations for the good of the group.
The historical fiasco of group selectionism is relied on as a (clear-cut) case in point of the dangers of anthropomorphism.
From the old discussion page:
I’d love to see lw posts that discuss/critique the group selection ideas of E.O. Wilson, David Sloan Wilson, and Herbert Gintis. Should the wiki cite outside non-LW sources on such issues? --MichaelBishop 19:48, 11 December 2009 (UTC)
The wiki is primarily for systematization and clarification of the topics already discussed on the blog. New material should go through the blog. --Vladimir Nesov 15:37, 12 December 2009 (UTC)
The reference to the model on the Gene Expression webpage should be removed, as that model does not include group selection. Group selection means the effect of selection at the level of the group. That model does not incorporate selection at the level of the group. PhilGoetz (talk) 16:44, 14 December 2014 (AEDT)
The discussion of birth ratios makes no sense. If group selection predicts higher female/male birth ratios, individual selection by the same token predicts higher male/female ratios. The fact that the ratio is nearly 1:1 then indicates a compromise between group selection and individual selection. Since individual selection exists, this would seem to be a proof rather than a disproof of the counterbalancing existence of group selection. PhilGoetz (talk) 18:06, 14 December 2014 (AEDT)