From "The Long Game"

 A Race With No Finish Line Some nuance to the discussion of selection, of a principle that was discovered fifty years ago and resisted at first. Parasites and hosts co-evolve in a dynamic and asymetric situation.

 Imagine you have a rare MHC variant. Circulating parasites have not yet adapted to evade this particular variant, so you are going to benefit from that relative rarity. This fitness advantage enables the variant to become more common - those with the variant will have more offspring - so then the parasites adapt to avoid this variant. The variant is now less useful and the advantage moves to whatever variants are rare. No variant remains useful for long, and the set of effective variants keeps changing. This is called frequency dependent selection.