Antagonistic coevolution is particularly likely to take place between parasites and their hosts. It is easy to imagine how a change in a parasite, which improves its ability to penetrate its hosts, will reciprocally set up selection for a change in the host.

Antagonism can cause cyclical or escalatory coevolution:

• If the range of genetic variants in parasite and host is limited, coevolution can be cyclic.

• But if new mutants continually arise, the parasite and host may undergo unending coupled changes in a particular direction.

Many properties of the biology of parasites and hosts have been attributed to antagonistic coevolution, such as parasitic virulence, and the simultaneous phylogenetic branching of parasites and hosts. Antagonisms are thought to be the biological factor most likely to cause extinction.

The shells of these molluscs are an example of escalatory coevolution: the fossil record shows that the thickness of the shells increases in response to the evolution of more sophisticated predators. Some biologists such as Richard Dawkins often use the metaphor of an arms race to describe antagonistic coevolution.