On the selfish gene:
Having seen nature documentaries, you may believe that wolves weed out the old and weak deer to keep the herd healthy, that lemmings commit suicide to prevent the population form starving, or that stags ram into each other for the right to breed so that the fittest individuals may perpetuate the species. The underlying assumption—that animals act for the good of the ecosystem, the population, or the species—seems to follow from Darwin’s theory. If in the past there were ten populations of lemmings, nine with selfish lemmings who ate their groups into starvation and one in which some died so that others might live, the tenth group would survive and today’s lemmings should be willing to make the ultimate sacrifice. The belief is widespread. Every psychologist who has written about the function of the social emotions has talked about their benefit to the group.
When people say that animals act for the good of the group, they seem not to realize that the assumption is in fact a radical departure from Darwinism and almost certainly wrong. Darwin wrote “Natural selection will never produce in a being any structure more injurious than beneficial to that being, for natural selection acts solely by and for the good of each.” Natural selection could select groups with selfless members only if each group could enforce a pact guaranteeing that all their members stayed selfless. But without enforcement, nothing could prevent a mutant or immigrant lemming from thinking, in effect, “To heck with this! I’ll let everyone else jump off the cliff, and then enjoy the food they leave behind.” The selfish lemming would reap the rewards of the others’ selflessness without paying any costs himself. With that advantage, his descendants would quickly take over the population, even if the population as a whole was worse off. And that is the fate of any tendency toward sacrifice. Natural selection is the cumulative effect of the relative successes of different replicators. That means that it selects for the replicators that replicate best, namely, the selfish ones.
The inescapable fact that adaptations benefit the replicator was first articulated by the biologist George Williams and later amplified by Richard Dawkins in The Selfish Gene. Almost all evolutionary biologists now accept the point, though there are debates over other issues. Selection among groups is possible on paper, but most biologists doubt that the special circumstances that let it happen are ever found in the real world. […] Animals just don’t care what happens to their groups, species, or ecosystems. Wolves catch the old and weak deer because they are the easiest to catch. Hungry lemmings set out for better feeding grounds and sometimes fall or drown by accident, not suicide. Stags fight because each wants to breeds, and one concedes when defeat is inevitable, or as part of a strategy that works on average against others playing the same strategy. Males who fight are wasteful to the group—ndeed, males in general are wasteful to the group when they make up half of it, because a few studs could sire the next generation without eating half the food.