With microorganisms, it’s possible to demonstrate that social behaviors such as cooperation, discrimination, and warfare have evolutionary origins and consequences. That’s because microbes have very short generation times, and so scientists like me can observe evolution in real time. In this post, I’ll briefly discuss three papers that illustrate the complexity (and brutality) of social behaviors in microbes. In very colloquial terms, the first describes “suicide bomber” E. coli; the second describes Vibrio bacteria that practice “ethnic cleansing” (that is, genocide that results in spatially segregated types); and the third describes the rapid but undirected evolution of kin discrimination in the social bacteria Myxococcus in a process analogous to the divergence of human languages over time.
The first paper, by Lin Chao and Bruce Levin, is a classic. In my first year of graduate school, Rich Lenski used this paper in his introductory lectures on social evolution in bacteria. Rich loves this paper, and he’s written an excellent blog post about it. The basic question is, why would a bacterium evolve to blow itself up to kill others? This paper finds that in a structured environment, the cost of blowing oneself up is repaid by the benefit of making more resources available to the bacteria’s kin. In contrast, this is not an effective strategy in a well-mixed environment where the benefit of blowing oneself up is as likely to go to competitors as kin.
The second paper, written by Luke McNally and co-authors, describes how populations of Vibrio bacteria use the Type VI secretion system to stab (“contact-mediated killing” ) the "wrong" kinds of Vibrio. This rapidly drives spatial segregation between different Vibrio types, essentially due to ethnic cleansing on the microbe scale. The most interesting finding in the paper is evidence that the number of secreted proteins in Vibrio genomes is related to the number and diversity of Type VI secretion system proteins found in that genome. Secreted proteins are costly to make, but their benefits accrue to neighboring bacteria. By killing dissimilar Vibrio, these gangster bacteria ensure that the production of public goods largely benefits kin.
Discrimination against others is not a trait that requires the direct action of natural selection to evolve. Rather, kin discrimination can evolve indirectly due to general divergence, in much the same way that human languages evolve. The Tower of Babel myth explains that God cursed humans to have a diversity of languages and cultures to ensure that they would not be able to cooperate in building a tower to heaven. However, languages can diverge from each other simply because people who live far away from each other have no need to speak a common language. Korean differs from Telugu, but not because they evolved specifically to be different—speakers of those languages simply weren’t in contact. Over hundreds and thousands of years, languages can easily diverge into mutual unintelligibility. Shakespeare’s 16th century Early Modern English is difficult but still poetic to English speakers in the 21th century. Old English, spoken by Anglo-Saxons in the 11th century, is completely unintelligible to the average English speaker today.
In a similar fashion, isolated populations of Myxococcus (which hunt other bacteria in packs) rapidly evolve to stop mixing in the lab. The experiments done by Olaya Rendueles and co-authors beautifully show that kin discrimination evolves in the absence of any selection pressure to discriminate against different populations of Myxococcus. Rather, they basically lose the ability to ‘speak’ with each other; colony incompatibilities rapidly evolve that prevent two different types from communicate the right signals for mixing with each other. Misunderstanding between types of individuals can easily arise in the absence of incentives for mutual understanding.