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The fish population that founded the 500 to 1,000 species in Lake Malawi some two to three million years ago was highly polymorphic at its major histocompatibility complex (MHC) loci. MHC genes seem to do for fish what they do for mammals: aid the immune response by telling self from non-self peptides Extant cichlid species exhibit large variation at their MHC loci. During adaptive species radiation different species kept different MHC alleles, perhaps helping us estimate the size of the initial population.

A study exploring the sexual preferences of female three-spined sticklebacks on males with nest eggs was conducted. Results indicate that females are more likely to choose males with an average clutch of eggs over those males with no eggs or a with an above average size clutch. A nest with noeggs may signify a male who steals eggs and one with an above average size clutch may pose some danger to newly deposited eggs since they may be eaten by the male.

Sneaking is an alternative mating behavior shown by male three-spined sticklebacks and it is influenced by the distance between nests. Sticklebacks' sneaking behavior in eight natural habitats was studied. Results indicated that sneaking behavior is shown when sexually active males are close to each other and when the distance between nests is small.