As humans, we readily understand the concept of extended family—brothers, sisters, aunts, uncles, nieces, nephews. Extended families are a common form of social organization in other species as well, and members of these families may cooperate in territory defense, predator avoidance, foraging, and rearing of young. If behavior is favored when it increases the fitness of the individual performing it, then how can we explain the evolution of social behaviors that do not lead to the performer having more offspring and that may even appear to be altruistic—benefiting another individual at a cost to the performer?

An individual’s fitness is increased by having offspring because those offspring carry the parent’s genes into the next generation. Fitness gained by producing offspring is referred to as direct fitness. However, an individual’s genes are carried into the next generation by more than his or her own offspring. In diploid organisms, two offspring of the same parents share, on average, 50 percent of the same alleles, and an individual is likely to share 25 percent of its alleles with its siblings’ offspring (nieces or nephews). Therefore, by helping parents and other relatives raise their offspring, an individual increases the transmission of those shared alleles to the next generation. Inclusive fitness is the individual’s direct fitness plus its indirect fitness: the reproductive success of the individual’s relatives, to the extent that those relatives share the individual’s alleles.

The maximization of inclusive fitness is the mechanism driving kin selection, selection for behaviors that increase the reproductive success of relatives even when they come at a cost to the performer. One example is “helping at the nest” behavior, which was studied extensively in Florida scrub-jays (Aphelocoma coerulescens). Scrub-jay pairs mate for life and establish large territories, which they defend aggressively. The mating pair may be assisted in rearing their young by three to five helpers (Figure 52.18). The helpers guard against predators, feed the young, clean the nest, and fly with fledglings. Why are these birds helping others rather than rearing their own young? A long-term study established a number of important facts:

These results support the conclusion that helper scrub-jays are maximizing their inclusive fitness by helping their parents raise siblings until they are mature enough to have a reasonable probability of successfully raising their own offspring.

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The concept of kin selection was formalized by W. D. Hamilton in what has become known as Hamilton’s rule. He argued that, for an apparent altruistic behavior to be adaptive, the fitness benefit of that act to the recipient times the degree of relatedness between the performer and the recipient has to be greater than the cost to the performer. This relationship was clearly stated years before by the eminent geneticist J. B. S. Haldane, who said during an argument about altruism that he would not be willing to risk his life to save his brother, but for two brothers or eight cousins, he would consider it.