Final answer:
Altruistic behaviors can be explained by kin selection, where individuals help close relatives who share many of the same genes. Additionally, seemingly altruistic behavior can benefit less-related individuals, such as social insects, by indirectly passing on genes. Unrelated individuals may also act altruistically through reciprocal altruism, where cooperation is based on the expectation of future benefits. Evolutionary game theory suggests that some behaviors labeled as altruistic may actually have selfish components. Overall, heritable behaviors that improve fitness are favored by natural selection.
Step-by-step explanation:
One possible answer is that helping others often means helping close relatives. Close relatives share many of the same genes that they inherited from their common ancestor. In this way, a behavior that puts oneself at risk could actually increase through natural selection. This form of natural selection is called kin selection.
Even less-related individuals, those with less genetic identity than that shared by parent and offspring, benefit from seemingly altruistic behavior. The activities of social insects such as bees, wasps, ants, and termites are good examples. Sterile workers in these societies take care of the queen because they are closely related to it, and as the queen has offspring, she is passing on genes from the workers indirectly. This phenomenon can explain many superficially altruistic behaviors seen in animals.
Unrelated individuals may also act altruistically to each other, and this seems to defy the "selfish gene" explanation. An example of this observed in many monkey species where a monkey will present its back to an unrelated monkey to have that individual pick the parasites from its fur. This behavior is termed reciprocal altruism and requires that individuals repeatedly encounter each other, often the result of living in the same social group, and that cheaters (those that never "give back") are punished.
Evolutionary game theory has shown that many of these so-called "altruistic behaviors” are not altruistic at all. Game theorists are good at finding "selfish" components in them. "Selfish" and "altruistic" should be dropped completely when discussing animal behavior, as they describe human behavior and may not be directly applicable to instinctual animal activity. Heritable behaviors that improve the chances of passing on one's genes or a portion of one's genes are favored by natural selection and will be retained in future generations as long as those behaviors convey a fitness advantage.