You can imagine situations in which natural selection would favor an increase in frequency for a trait that aided group survival while hurting individual reproductive success – but it’s not all that easy. Here’s the problem: imagine a situation in which some individuals in the group have an allele that causes them to fight in a way that saves the collective – the catch is that some get killed in the process. Members of the tribe that don’t have this allele are saved as well, but they don’t pay the price. At the end of this fight, the frequency of the self-sacrificing allele has gone down, not up. So how can the altruistic allele hang around? How would it ever have become common in the first place?
If the altruistic act (defined as one that increases the fitness of another individuals while reducing personal fitness) is aimed at close relatives, an altruistic allele can succeed. As Haldane once said, “I would lay down my life for two brothers or eight cousins”. It’s called kin selection. Close relatives are more likely to carry a copy of that same altruistic allele than the average bear, so altruistic acts focused on close relatives can pay off – can cause the causal alleles to increase in frequency. This is particularly so if circumstances allow very big payoff from altruistic acts, for example, species that nest in cavities. Successful defense of a breach in the nest is tactically easy, rather like Horatius at the bridge, and greatly increases the fitness of many relatives.
You also see a kind of altruism among some infectious organisms. Some bacteria make a toxin that furthers the infection process. Each individual bacterium would be better off if he stopped making that toxin and relied on all the other bacteria to do it – it would save energy – but if the infection starts with a single organism, the descendants are all closely related and kin selection can favor expensive cooperation. In some cases, like cholera or diphtheria, phages carry genes that code for the production of toxins. You can think of this as a method of forcing high relatedness.
Some people have suggested that human homosexuality is an adaptation produced by group selection. I can’t see how this could possibly work. They would have to do stuff for close relatives – lots of stuff. This is a quantitative question: if they concentrated on the closest possible relatives, nephews and nieces, they’d have to cause four more to survive than would otherwise. We’re talking a behavior stronger and more effective than mother love. It doesn’t exist. And how would being homosexual help?
In some other loony scenarios, homosexuality was favored by benefits to the group as a whole. Disregarding the fact that this kind of selection is almost impossible in the first place, and that we don’t even find homosexuality in most hunter-gatherer populations, what is it they are supposed to have done to save or aid the tribe?