Individuals are not always the unit of selection, however. Individuals carry genes, and it is, after all, the genes that get transmitted into the next generation. So sometimes, behaviours can evolve that while bad for individuals, benefit their genes. For example, close relatives have many of the same genes. Sometimes individuals will contribute more genes to the next generation not by reproducing themselves, but by helping close relatives to reproduce, a phenomenon known as kin selection. This explains why, for example, some birds will help at their parentsΓÇÖ nests instead of having their own offspring. It explains why so many social insects help to rear their sisters instead of starting up colonies of their own. And it explains why humans are more likely to help out their relatives than strangers, or even friends. It even explains why some organisms live beyond the end of their reproductive years: after the menopause, females in many species are important in helping to rear their childrenΓÇÖs children.
