Polymorphism of attack and defense.

Coevolution of attack and defense occurs in host-parasite systems and various forms of genomic conflict. Attackers benefit when their specific molecules allow entry past host defenses. Defenders gain when their matching biochemical specificities aid recognition. Selection continually favors new attack specificities that avoid matching defense and, in turn, new defense specificities that match novel attackers. The introduction of novel specificities strongly influences the spatial and temporal dynamics of conflict. Lack of reciprocally matching diversity in a particular system suggests biochemical constraints that prevent diversification. New work on cytoplasmic male sterility, B chromosomes and meiotic drive suggests that varying biochemical constraints on recognition cause varying patterns of diversity and spatiotemporal dynamics