Microscale kin discrimination in a famous soil bacterium.

Hamilton (1) argued that prior to his theory of inclusive fitness, nobody had sought evidence for kin discrimination in animals because nepotism was an embarrassment to human society. Why did it take so long after Hamilton’s theory was proposed, however, to search for kin discrimination in simpler organisms, like bacteria? Perhaps we were prideful that we, as animals, have the unique ability to recognize kin and use the kin-unit as the basic building block of our advanced societies. This pride has since been diminished by the discoveries of kin discrimination in plants (2), social amoebae (3), and Gram-negative bacteria (4, 5). Now the discovery of kin discrimination in the Gram-positive soil bacterium Bacillus subtilis (6) may cause us to rethink what it means to be kin and why we exist as organisms at all. Discriminating Nepotism Hamilton started the modern interest in kinship with a simple idea that organisms will take advantage of available cues to preferentially help or avoid harming kin (1, 7). The basic idea was so commonsensical that Hamilton remarked, “. . .we have known that our species is inclined to nepotism for as long as we have known anything; we are steeped in its existence and known of it since, as infants, we began to have any ideas at all” (1). Like the recognition of obvious truths more generally, Hamilton’s insight had much deeper implications. The first was that altruism would evolve under less restrictive population structures than Haldane assumed (8), and thus that the mathematical condition for altruism was worth explicating as part of a general kinship theory (7). The second was that individuals within apparently cooperative groups would often take advantage of cues to selfishly exploit distant kin or nonkin (1, 7).