In recent years the social life of microbes has received a lot of attention from biologists interested in social evolution. Traits for helping and harming, as well as for social signalling, are commonly found. For instance, bacteria attacking a host organism can cooperate by secreting substances that break down the host's defences. Scientists have concluded that the best way of understanding this is to see microbes as helping relatives. There are often genetic variants of microorganisms, for instance so-called cheater-cooperator polymorphisms, and these variants could be adaptations to different environments. Bacteria can, for instance, be found in soil or in water, but also in the animals or plants they attack. Although genetic polymorphism in social traits of microbes has attracted much interest, social evolution theory has not dealt with it so far.

A group of researchers led by Olof Leimar at Stockholm University solved the problem by developing a new theory on how genetic variants can coexist in heterogeneous environments. Their idea is that the genotype at a polymorphic locus can function as a "genetic cue", by providing a microbe with statistical information about its environment, and thus about which traits, such as investment in helping, are likely to be advantageous.

An important prediction from this theory is that different parts of the genome can have different evolutionary interests. Genes that are genetically tightly linked to a variant suited to giving help will evolve to strengthen this tendency, which can for instance give rise to a particularly virulent variant that specialises in attacking host organisms. Other, loosely linked or unlinked, genes will tend to trade off the benefits of their traits in one environment with possible costs in other environments, in this way promoting a sort of compromise phenotype, for instance between doing well in soil or in water and being good at attacking host organisms. This distinction, between specialised and more generalistic variants, could be crucial to understand the evolution of genetic polymorphism in cooperation between microbes.

The article "Genes as Cues of Relatedness and Social Evolution in Heterogeneous Environments", by Olof Leimar, Sasha Dall, Peter Hammerstein, and John McNamara, was published in the journal "PLOS Computational Biology".