KindFriends

There are two main evolutionary theories of altruism: Fisher, Haldane and Hamilton's idea of kin selection, and Trivers' idea of reciprocal altruism.

Neither of these theories is mutually exclusive and both may operate together.  I would like to propose a third that may also be operating.

It is this.  Your fitness is increased if you associate with altruistic people, whether you yourself are altruistic or selfish.   In the latter case the others may well find you irritating, but even then - all things being equal - the others are less likely to act against you than more selfish people would.  Thus we would expect all individuals to seek out altruistic company.

People are inclined to have children with those with whom they associate, simply because of opportunity.   When altruistic people have children with other altruistic people that will tend to reinforce impulses towards altruism in their children (though we should be cognizant of the regression to the mean).  And when selfish people have children with altruistic people, that will tend to dilute selfish impulses.

Thus we should expect altruistic behaviour in the population as a whole to rise in response to the statistical effect that everyone is more likely to have children with altruistic people than they are with selfish people, simply because of the breeding opportunities provided by the ubiquitous preference for association with the altruistic.

This principle does not just apply to altruism and selfishness.  For example everyone - whether well or ill - will have a preference for associating with people who are well because the associators will then be less likely to catch something nasty from the associatees.  Thus we should expect disease resistance to rise, even above the rise that would be expected anyway simply because disease resistance is in itself intrinsically evolutionarily fitter.

Thus, let X be a characteristic possessed by animal A.  If, by associating with A, animal B increases its fitness regardless of whether B possesses X or not, then we would expect the proportion of X in the population to increase. 

Note that I said "animal" - animals are motile and associate voluntarily.  This principle should apply to any organism that can move about and decide who its friends are, and it will apply particularly strongly in social species (us, say, or bats).  But plants, for example, and solitary animals (leopards, say, or polar bears) will be much less likely to exhibit the principle.

I have decided to call this selection by associative opportunity.