Alleles Can Be Shared for a Variety of Reasons

Kin selection, as its name implies, is usually thought of as involving interactions between relatives. Correspondingly, the coefficient r in Hamilton’s rule is thought of as describing the chance that genes are identical by descent (e.g., r = 1/2 for autosomal genes in sisters). However, as we explained at the outset, alleles gain an advantage by increasing the fitness of other individuals if those others share the allele, for whatever reason. Here, we summarize the several ways in which organisms that interact with each other come to share genes.

The most straightforward examples of kin selection involve close relatives, such as competition between siblings in the same nest or matings between brother and sister wasps in the same fig. However, whenever individuals mate within a small group, they will come to be related through inbreeding and random genetic drift (Chapter 15). Selection can then favor traits that increase group survival and proliferation, even if these traits are slightly deleterious to individuals. Historically, this kind of group selection has been considered separately from selection on close relatives, but is fundamentally equivalent. We discuss it further on page 607. In fact, we have already seen several such cases. When a bird eats a distasteful and warningly colored butterfly, this aids others that carry the same pattern, because that bird will avoid them thereafter (Fig. WN21.8). Bacteria that lyse to produce bacteriocins aid others that carry the same plasmid, because they are resistant whereas their competitors are not.

Alleles need not be shared solely because interacting individuals are related through common ancestors. W.D. Hamilton proposed a simple thought experiment, in which an allele causes its carrier to display some arbitrary signal (e.g., a green beard). (The term “green beard” was coined by Dawkins, but the idea was laid out in Hamilton [1964b]. See Hamilton [1996a] for the history and Queller et al. [2003] for an example involving social amoebae.) If individuals assist others that carry the same inherited mark, then the allele will increase through a form of kin selection. (One can, of course, wonder whether the term “kin selection” is appropriate in such cases.) In fact, any case of positive frequency dependence (p. 470), where an allele’s fitness increases with its frequency, involves interactions that can be interpreted using Hamilton’s metaphor. One can argue that the terminology of social evolution makes such examples seem more complicated than they really are; but the key is to understand how different genotypes interact with each other.