Sunday, 29 September 2013

Kin selection, population bottlenecks, founder effect

Kin selection theory raises the possibility that organisms may use variation between them as clues to relatedness - and direct cooperative behaviour preferentially towards perceived relatives. This effect is sometimes referred to as "kin recognition".

However, this method of identifying relatives depends on the existence of population-scale variation. Population bottlenecks can destroy such variation - and may promote cooperation.

Similarly, the founder effect might also produce local regions with little variation.

These effects have been demonstrated experimentally:

In theory, kin selection should mostly produce adaptations that work on relatedness cues that dynamically take overall population similarity into account. However, kin selection effects must work by manipulating development. Cruder measures of detecting similarity and relatedness will often be employed in practice.

Humans are among those species that have experienced a relatively recent population bottleneck - in the form of the Toba catastrophe. It is intriguing to consider the scale of the resulting increased levels of cooperation between humans that might be the result of this.

Saturday, 28 September 2013

The problem with cultural group seletion

What do you get when you put two new theories together?

In the case of cultural evolution and group selection, what you seem to get is a big scientific muddle. Let me explain:

Many modern cultural theorists treat culture as a second inheritance channel that affects and modifies human phenotypes (extended phenotypes in the case of artifacts). This is instead of modeling cultural symbionts as separate lineages with distinct phenotypes of their own. For more on this see Against the extended genotype

This perspective has led to multiple claims that group selection is responsible for various features of human culture:

For example monogomous marriage customs and human ultrasociality have both been recently attributed to group selection.

There are two separate problems with this sort of literature:

The first is that typically no evidence is presented to show that these memes are deleterious within groups. If you count the spread of memes that are simply advantageous as a form of "group selection", then all widespread cultural phenomena qualify and the term becomes meaningless. Group selection theorists should try harder to distinguish between group selection and byproduct mutualism - which can both produce "groupish behaviour", but by independent mechanisms. It is true that some individuals benefit from polygamous marriage and anti-social selfishness, but other individuals are harmed, and if the balance is positive, group selection must compete with an obvious explanation: that some individuals are manipulating other ones using memes which have high average fitness and spread by perfectly ordinary natural selection.

The second problem is that - just because you can model a phenomena using multi-level selection and the Price equation, it doesn't mean that it makes sense to do so. Rather than observing that selection on human hosts and selection on memes operates on different scales - and reaching for the multi-level selection toolkit - what should happen is that the fact that humans and their memes are not geneaologically related should be noted, and they should be modeled as independent species using natural selection on humans and natural selection on memes. Multi-level selection is an unnecessary and confusing complication in such cases.

To illustrate, compare with the case of the smallpox virus. Smallpox germs wiped out many native Americans. The smallpox scabs are an aspect of individual human phenotypes. They are deleterious to individual humans (they kill people). However has historically been advantageous to human *groups* to have the smallpox phenotype - since this trait obliterated many competing tribes of humans (e.g. see American history). I think that few would advocate multi-level selection modeling in this case. There are two distinct types of entity involved here: humans and the smallpox virus. Natural selection operates on them more-or-less independently. Group selection brings no enlightenment and much confusion to this situation. The idea that smallpox scabs are traits which spread because they are deleterious to individuals and advantageous to groups is just a misleading and bad way of looking at the situation. It could equally well have been true that smallpox scabs were deleterious to human individuals and human groups - in which case it could still spread for its own "selfish" reasons. Group selection would then have been a red herring in explaining the spread of smallpox.

What holds for smallpox virus scabs holds for many kinds of memes. "Gun" memes result in people dying just as surely as in the case of the smallpox virus. Like smallpox, guns killed many native Americans - and gun memes provide broadly-similar group level advantages. You can model the spread of gun memes using multi-level selection and the Price equation. It is an unenlightening and confusing thing to do, but you can do it.

Its much the same with monogamy memes and cooperative memes. Memes don't interbreed with humans - they are more like separate species. So, you can forget about multi-level selection and just apply ordinary natural selection models to the humans and the memes - and have a much cleaner, neater model. Since human genes and memes are more like different species, it is an unnecessary source of complication and confusion to muddle them together in a unified multi-level selection model - just because they influence the same phenotypic traits sometimes.

It's worth sorting this muddle out - partly because memes exhibit their own kin/group selection dynamics, which can be important and significant. This is where genuine kin sleection/group selection operate in the cultural realm. If "cultural group selection" gets turned into a meaningless catch-all term for cultural phenomena which spread despite being deleterious to human hosts, it risks losing the possibility of performing useful work in other contexts.