Cultural divergent selection

Divergent selection is selection which pushes traits in different directions in different sub-populations. Selection on gamete size is a classic example - if large gametes (eggs) do well and small gametes (sperm) also do well, then divergent selection will force these traits apart and eliminate gametes of intermediate size.

Divergent selection is often invoked in cases of speciation. If two incipient species come into contact under circumstances where they can interbreed, but the hybrid offspring have reduced viability (due to crossover recombining incompatible genes) then divergent selection will favour traits that allow the individuals to avoid interbreeding.

The most obvious case of divergent selection in cultural evolution involves divergent selection for tribal markers. If individuals in groups benefit from interactions with ingroup members, and are harmed by their interactions with outgroup members then traits that help to distinguish ingroup from outgroup can be favoured. This results in divergent selection on cultural variation that distinguishes the groups - and this works to drive the groups apart - into different tribes with different cultural tribal markers.

It is interesting that the logic of this is very similar to that given for the case of two incipient species. In both cases, ingroup interactions are favoured, while outgroup interactions are punished. Divergent selection on human tribes can be reasonably expected to sharpen tribal boundaries, and to increase the frequency of tribe formation.

It has previously been widely speculated that memes increase speciation rates. There's evidence for this in songbirds, for example. Cultural divergent selection illustrates one part of the mechanisms that are probably involved.