In the struggle for existence, according to Darwin, some organisms win and others lose. Those that win are called “fit” in the sense that they have a higher probability of surviving and reproducing than the losers. In the struggle to exist and reproduce, life involves competition, and the winners in the contest are those that can adapt to their environments long enough to bear offspring. Still, no species can survive on competition alone. If life is to last for many generations there must also be cooperation among the members, and even among separate species. Evolution entails at least as much cooperation as competition. It even requires self-sacrifice. When an organism forfeits its own reproductive opportunities for those of its family, group or species, biologists call it “altruism.”

In the human sphere of life, altruism and self-sacrifice are generally considered the highest expressions of “morality.” But according to much contemporary evolutionary thought, human “virtue” has its origin in the cooperation and altruism that already show up in pre-human forms of life. Some biologists and social scientists now locate the birth of morality in the fascinating process by which genes are passed on from one generation to the next. Darwin himself knew nothing about genes, and he thought of evolutionary selection as taking place primarily at the level of individual organisms. But prominent evolutionists have concluded that selection applies more precisely to arrays of genes shared by many members of a species than to individual organisms alone.