THE PROSPECTS FOR POLYMORPHISMS SHARED BETWEEN SPECIES

Recent molecular evidence has shown that many MHC polymorphisms are not only shared between species but are in fact identical at the molecular level. Species that share these polymorphisms may be very distantly related and often diverged millions of generations ago. It is now known that this phenomenon is very unlikely to occur if the alleles are selectively neutral. A large number of other studies suggest, however, that this phenomenon of shared polymorphisms is very common and extends to many other loci beyond just the MHC/HLA complexes. These studies also suggest that some polymorphisms may be older than the MHC/HLA polymorphisms. The maintenance of these polymorphisms via overdominant and frequency-dependent selection is discussed. Strong levels of selection are required for overdominance to maintain shared polymorphisms but most studies of effective population size produce long-term estimates that are very small and would not permit the level of overdominant selection required to maintain these polymorphisms. Frequency-dependent selection can maintain them for longer with less 'apparent' equilibrium selection and might permit smaller effective sizes. The variance of allele frequencies is suggested to be one way to distinguish between these two selection models.