Saturday, 29 September 2012

Similarity selection

Both Price's formalism and tag-based cooperation strongly suggest that it is trait similarity that is important to evolving cooperation - and that relatedness isn't the sole cause of similar identifying markers leading to cooperation.

This suggests that kin selection's name is dubious - and that the actual effect might be better described as a form of "similarity selection".

Reviewing the causes of similarity in biology, we find:

  • Similarity based on selection
    • Similarity based on mimicry
    • Similarity based on convergent evolution
  • Similarity based on chance
  • Similarity based on inheritance
    • Similarity based on organic inheritance
    • Similarity based on cultural inheritance
    • Similarity based on environmental inheritance
However, looking at where cooperation arises in nature, we see it between relatives, but there's relatively little sign of cooperation based on mimicry, convergent evolution or chance resemblances.

There are some examples of mimicry leading to cooperation. For example, cuckoos generate cooperative behaviour in their hosts by employing egg mimicry. Some orchids mimic the abdomens of their pollinators, to encourage insects to attempt to have sex with their flowers. These examples are instructive: mimicry seems to be associated with producing cooperative behaviour via deception and manipulation.

Chance and convergent evolution seem to result in cooperation much less frequently. The resulting resemblances are rarely close enough.

Convergent evolution may explain the similarity between the wings of bats and birds - however, neither the bats not the birds regard each other as kin, and they don't cooperate with each other especially frequently. Much the same goes for sharks and whales - which are another case of convergent evolution - but not much cooperation arises from the resulting resemblance.

The best example I can think of for similarity which is not caused by some kind of relatedness resulting in cooperation arises in a variant of the prisoner's dilemma. Imagine a group of unrelated agents of various types playing an iterated prisoner's dilemma game with each other - under circumstances where both agents can choose whether to continue with their current partner or pick another one at random from the pool of agents without partners. No agents reproduce, but the worst agents may die. In such a game, the "nice" agents will gradually find "nice" partners, and stick with them - gaining higher payoffs than other agents. They will do this regardless of whether they are related by kinship, or not. These kind of dynamics might help to explain some forms of mutualism. However, there's a problem with describing the cooperation in such models in terms of kin selection. An alternative explanation for the resulting cooperation is reciprocity.

Whether similarity selection really adds anything to the idea of selection based on relatedness is a somewhat contentious point.

The moral of this story is that in practice most, but not all forms of similarity selection take place as a result of relatedness or kinship.

Note that we are not talking about blood relatendess here. Organisms which are memetically related may also come to cooperate - as a result of the related memes manipulating their hosts.

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