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 s...