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 they 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.