Molecular trans-species polymorphism

Trans-species polymorphism (TSP) is the occurrence of similar alleles in re lated species. Excluding instances in which the similarity arose by converg ent evolution, TSP is generated by the passage of alleles from ancestral to descendant species. Closely related, recently diverged species, such as th ose of the Lake Victoria cichlid flock, may share neutral alleles, but long -lasting TSPs occur only in genetic systems evolving under balancing select ion. Two such systems have been studied extensively, the major histocompati bility complex (Mhc) of jawed vertebrates and the self-incompatibility (SI) system of flowering plants. Allelic lineages that diverged many millions o f years ago and passed through numerous speciation events have been describ ed in both systems. The lineages may differ at up to 50% of their coding si tes, both synonymous and nonsynonymous. The differences arise by the proces s of incorporation of mutations, which is different from the process of fix ation. TSP, on the one hand, complicates phylogenetic analysis, but on the other, it is a useful tool for the study of speciation.