Sunday, 23 August 2015

Symbiont kin selection

Cultural kin selection proposes that cooperation can arise due to shared memes - as well as due to shared DNA genes.

However, cultural kin selection can be thought of as a special case of symbiont kin selection - an idea that may apply to many types of close living organisms.

Colony life - as found in ants, bees and mole rats - leads to increased levels of transfer of symbionts between the organisms involved (often due to sheer close proximity). The mole rats eat each others feces - and so come to share the bacteria they need to digest their tubers. Ants frequently cultivate fungi - and their nests are heavily dominated by fungi digesting rotting wood. They have many other symbionts too - there are special bacteria that they use to suppress the growth of competing strains of fungi, for example.

As well as acting on the host genes, kin selection acts on the genes of the symbionts too. If the symbionts in different organisms are related, then - to the extent that these can manipulate the behaviour of their hosts in favour of cooperation - they will tend to do so.

Probably ant fungus is the most extreme example of this form of kin selection. Though distributed the fungus is closely related - more so than the ants themselves are. It forms something like a massive multi-cellular organism in each ant colony - a superorganism. It may benefit from that ants acting as a coordinated whole more than the ants themselves do. The ants snack on the fungus - so it probably has a variety of ways of manipulating ant behaviour - through taste, smell and direct chemical action.

Seen from the crude perspective of Hamilton's rule, shared symbiont genes may elevate relatedness further. For example, intra-colony relatedness in naked mole rats has been estimated to be 0.81 - but this relatedness figure is based on the host genes. As with other mammals, most of the genes involved are not mole rat nuclear DNA, but are genes in gut microorganisms. The bacterial cells outnumber those of their hosts by a factor of ten. What happens to that relatedness figure once "horizontal" sharing of bacteria is accounted for? It probably goes up: a lot of those bacteria will be asexual clones.

For an example relevant to humans but still involving DNA genes, consider yeasts - as found in bread, wine and beer. Many yeasts have become human-transmitted symbionts. The main way they spread their genes around in the world involves human social contact. If they can somehow make their human hosts socialize more with other humans, they are likely to directly benefit. Kombucha may be one of the best examples of a socially-transmitted yeast - since it often spreads directly through peer-to-peer contact. Are Kombucha enthusiasts more sociable than other humans? Probably. But are they more sociable as a result of being manipulated by the Kombucha? It is an interesting question that deserves further study.

Symbiont kin selection is a bit different from the symbiont hypothesis of social evolution - but it is fair to say that these ideas are related.

Symbiont kin selection should illuminate cultural kin selection, which can be accurately modelled as a special case of it involving cultural symbionts - rather than DNA-based ones.

Symbiont kin selection is a neglected idea in social evolution. Because of lack of study, it is not easy to assess its overall significance - but it could easily be a big deal. If you look at humans, a lot of their cooperation is based on shared memes - rather than shared genes. In the workplace, for example, shared memes are ubiquitous - and shared genes are rare. Even within family life, shared memes are ubiquitous. Cultural kin selection could easily explain more cooperation than genetic kin selection does. This example illustrates the potential power of symbiont kin selection - but maybe it is equally powerful in other eusocial creatures. More powerful, maybe - since they are further along in the road to colony life than we are. Symbiont kin selection could easily be stronger in them than it is in us.

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