NEPOTISM VS TIT-FOR-TAT, OR, WHY SHOULD YOU BE NICE TO YOUR ROTTEN BROTHER

It is well known that interactions among relatives facilitate the evolution of altruistic behaviours. Game theoretic models show, however, that guarded altruism (such as Tit-For-Tat) can evolve among non-relatives when individuals interact many times and 'cheating' behaviours can be punished. Strangely, no one has yet asked whether the guarded altruism that evolves among non-relatives might also evolve among close relatives, supplanting unconditional altriusm. We present a series of one-locus sexual haploid models in which Tit-For-Tat, unconditional altruists and selfish individuals interact in groups of full siblings. Tit-For-Tat frequently (but not always) replaced unguarded altruism, in which case the strategic 'rules' for interacting with kin vs non-kin are identical. Even when Tit-For-Tat is selected at a single locus, however, by withholding altruism for non-reciprocating relatives it may qualify as an 'outlaw' from the standpoint of modifier genes at other loci.