9.9: Selfish genes win out over group selection.

Kin selection and reciprocal altruism can evolve in a population of animals, as we have seen, and examples of kin selection abound in nature. As a consequence, casual observers of nature frequently see individuals acting in ways that appear altruistic, even though these individuals are truly acting—from the perspective of evolutionary fitness—in their own selfish interests. This nearly universal selfishness raises the question of whether evolution ever leads to behaviors that are good for the species or population but detrimental to the individual exhibiting the behavior, a process called group selection.

It might seem that evolution would favor individuals that behave in a manner that benefits the group, even if it comes at a cost to the individual’s own inclusive fitness. But this does not happen. Behaviors that reduce an individual’s fitness (relative to that of other individuals in the population) are not likely to evolve.

Let’s look at an example. Imagine that a new allele appears in a population (perhaps through mutation) that causes the individual carrying the allele to double its reproductive output, even though this might spell doom for the species as individuals overuse their resources. An individual carrying this “selfish” allele will pass on more copies of the allele to its offspring than an individual carrying the alternative allele, coding for the production of fewer offspring, will pass on to its offspring—and this will lead to excessive, “selfish” consumption of the species’ resources. The selfish offspring, in turn, will pass on the selfish allele at a higher rate than the alternative allele is passed on. This scenario may lead to extinction of the species, yet it still occurs. With its market share perpetually increasing, the selfish allele eventually is favored by evolution and will predominate (FIGURE 9-15).

Now consider what happens when a new allele appears that causes an individual to reduce its reproductive output below what is best for that individual’s fitness. Just because a behavior (determined by an “unselfish” allele) leads to a better outcome for the group, this doesn’t mean that natural selection will favor that behavior. Even if it reduces the likelihood that, in the long run, the population goes extinct, such an allele does not increase its market share relative to the alternative “normal” or “selfish” allele. Instead, natural selection generally causes increases in the frequencies of alleles that benefit the individual carrying them, even when this comes at the expense of the group. In some special situations, it is possible for natural selection to lead to group selection. But the stringent conditions necessary for this to occur are so rarely found in nature that we almost never see it.