We've now seen one good example of natural selection acting to maintain diversity at the molecular level, but that example involves only a pair of alleles. Let's examine how selection operates on a more complex polymorphism involving many alleles and several loci, specifically the polymorphsims at the major histocompatibility complex (MHC) loci of vertebrates.
MHC molecules are responsible for cellular immune responses in
vertebrates. They are expressed on all nucleated cells in vertebrates
and present intracellularly processed ``foreign'' antigens to T cell
receptor lymphocytes. When the MHC 
antigen complex is recognized,
a cytotoxic reaction is triggered killing cells presenting the
antigen. It's been known for many years that the genes are highly
polymorphic.1 Although plausible adaptive scenarios for that variation
existed, a competing hypothesis had been that MHC loci were
``hypervariable'' not because of selection for diversity, but because
of an unusually high mutation rate.