Blackwell Publishing

Coevolution - Are parasites and their hosts coevolving?



Coupled coevolution is particularly likely to take place between parasites and their hosts. They can have specific and close relations, and 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.

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. Coevolution in parasites and hosts is antagonistic, unlike the mutualistic coevolution of ants and caterpillars: coevolution can be antagonistic or mutualistic according to the circumstances of the species.

Many properties of the biology of parasites and hosts have been attributed to coevolution.

Two examples:

• parasitic virulence;

• the phylogenies of parasites and their hosts.

Cyclic coevolution in parasites and hosts is illustrated by the following animation.

Figure: frequency changes of host and parasite genotypes. (a) As H2 becomes more common, there is selection to increase the frequency of P2, which in turn selects against H2, and H1 increases in frequency. (b) plotted against time, the frequency of each genotype oscillates cyclically.

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