4.4: Inclusive fitness, kin and group selection, and social evolution

The question that troubled Darwin and others was, how can evolutionary processes produce this type of social, self-sacrificing behavior? Consider the behavior of bees. Worker bees, who are sterile females, “sacrificed themselves to protect their hives” even though they themselves cannot reproduce, they are sterile.^[187] Another example, taken from the work of R.A. Fisher (1890-1962), involved the evolution of noxious taste as a defense against predators. We can assume that the organisms eaten by predators do not directly benefit from this trait, after all, they have been eaten. So how can the trait of “distastefulness” arise in the first place? If evolution via natural selection is about an individual’s differential reproductive success, how are such traits possible? W.D. Hamilton (1936-2000) provided a formal answer, expressed in the equation rb > c. As in our consideration of costs and benefits, “b” stands for the trait’s benefit to the organism and others, “c” stands for the cost of the trait to the individual, while “r” indicates the extent to which two organisms within the population are related and genetically similarity.

Let us think more about what this means. How might active cell death in bacterial cells be beneficial evolutionarily? In this case, reproduction is asexual. The organism’s (cell’s) offspring, and its likely neighbors, are clonally-related to one another, much as the cells of a multicellular organism are related. Aside from occasional mutations, the cells in a clone and within an organism are genetically identical, with DNA molecules that are identical in sequence.^[188] We can characterize the degree of relationship through their r value, the coefficient of relationship. In two genetically identical organisms r is close to 1. Two totally unrelated organisms, with minimum possible genotypic similarity would have an r very close to 0.^[189] Now let us return to our cost-benefit analysis of a trait’s effect on reproductive success. Each trait has a cost c to the organism that produces it, as well as a potential benefit b in terms of reproductive success. Selection leads to a trait becoming prevalent (frequent or fixed) within a population if b >> c. But this equation ignores the effects of a trait on other related and neighboring organisms. Let us call the benefits to the individual that result from their cooperative/altruistic behavior b_i and the benefits to others/neighbors b_o. To generate our social equation, known as Hamilton’s rule, we consider what is known as the inclusive fitness, which changes b > c into b_i + r x b_o > c. This allows for the selection of social (potentially detrimental) traits provided that their impact on the reproductive success of closely related individuals is increased sufficiently. Given the clonal nature of many microbes, inclusive fitness can be powerful in these organisms. It can also be significant in small populations of sexually reproducing, genetically related organisms.

Inclusive fitness requires a close relationship to the recipient of the beneficial act. How does one individual “know” that it is making a sacrifice for a relative and not an unrelated strangers? As social groups get larger, identifying relatives becomes more and more difficult. One approach is to genetically link the social, altruistic behavior to a visible trait, called a “green beard” trait. The likelihood that an organism will behave socially is linked to the visible trait. The presence of the green beard trait indicates that an organism is "prepared" to "sacrifice" itself for you in the same way you are prepared to sacrifice for it. In some cases, a trait may evolve to such a degree that it becomes part of an interconnected set of behaviors, a type of biosocial moral system.^[190]

Once humans developed a brain sufficiently complex to do what it was originally selected for, this complexity may have produced various unintended byproducts. Empathy, self-consciousness, or a tendency to anxiety may not be directly selected for but could be side effects of processes that were. As a completely unsupported but plausible example, the development of a good memory as an aid to hunting might leave us susceptible to nightmares. If empathy is an “unintended” by-product of selective processes, once present they could alter future selection pressures. They might not be easy to evolve away from, particularly if they are mechanistically linked to a trait that is highly valued, that is, selected for. The effects of various genetic mutations on personality and behavior support the idea that such traits may be influenced by one’s genotype.

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