Sunday, 6 July 2014

Tim Tyler: Eusociality: the symbiont hypothesis


Hi. I'm Tim Tyler, and this is a video about the origin of eusociality - and the role that symbiology might play in that. Most modern ideas about of the origin of eusociality are fairly simple. They invoke kin selection. The ecological advantages of forming large cooperative groups are obvious - and those groups that have managed it dominate the planet - but it isn't easy for evolution to get individual creatures to cooperate in large groups. Kin selection helps to explains how such cooperative groups can be stable. Almost all cases of close group living involve close relatives. The conventional wisdom is that kin selection did it.

Some have invoked group selection as an alternative idea about the origin of eusociality. However, this explanation has turned out to be a dud - in the sense that it doesn't really add anything to the kin selection explanation - because modern forms of group selection and kin selection have turned out to make equivalent predictions.

There are other forces that promote cooperation and group living. Byproduct mutualism is the idea that individuals enjoy straightforward fitness benefits to living in groups - for example, by sharing defense, foraging and lookout duties with others.

This video is about another explanation how eusociality can be stable - one involving symbiosis. The idea is that living in close groups promotes the spread of symbionts between group members. In turn, the symbionts manipulate their hosts into close living arrangements in order to allow the symbionts to reproduce. These two effects create a synergetic spiral that leads towards ultrasociality and eusociality.

This is not a new explanation for eusociality. Indeed it was the most widely accepted explanation for the origin of eusociality in termites before kin selection came on the scene. In this context, it is known as "the symbiont hypothesis". Termites depend on interactions with other colony members to obtain gut bacteria which are essential of their survival. It was thought that this regular need for bacteria from other colony members forced the termites into living a social life.

Multiple symbonts may be involved in creating pro-social forces. Both mutualists and parasites may be present. Each additional symbiont increases the ecological forces that pull the hosts together.

I think that the symbiont hypothesis is an unfairly neglected explanation for the origin of eusociality. It is interesting to me partly because of its relevance to human culture. There, it suggests that memes manipulate humans into coming into close contact - in order to facilitate their transmission between humans. In turn, humans living in close proximity to each other makes them more vulnerable to horizontal transmission of pathogens and symbionts - including more memes. This leads to a spiral of increased sociality - and to more and more memetic transmission.

Eusocial species are well known for their mutualisms. Termites have their cultivated fungi. Mole rats have their gut bacteria. Bees have their flowering plants while ants have their domatia. The idea of social groups being drawn together by a web of ecological interactions between multiple symbionts makes a lot of sense.

The symbiont explanation differs significantly from the conventional kin selection one. Under kin selection the genes of organisms involved benefit from the cooperation. Under the symbiology explanation, benefit to the host genes is not necessary - the hosts could be being manipulated into engaging in social behaviour - for the benefit of mutualist and parasitic symbionts. You could still classify this as kin selection - kin selection between the symbionts involved and their own offspring, but that's a bit different from the orthodox idea that kin selection between host genes is responsible.

While the conventional kin selection explanation is obviously an important factor, I think that more attention should be given to the symbiont hypothesis of the evolution of eusociality. We need to quantify the effect to see how important it is - relative to kin selection at the level of host genes. The effect seems to be especially clearly applicable to humans - where the cooperating humans involved are often not themselves closely related.



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