Coevolution
Coevolution is an evolutionary change in two or more species resulting from reciprocal selective pressures.
In parasite-host systems, the demonstrations of
genetic variability in parasites for transmission to hosts and survival in hosts on the one hand,
genetic variability in hosts for avoidance of parasites and resistance mechanisms on the other hand
constitute what Hochberg and Crompton call “a cornerstone prediction of active coevolution”.
When reciprocal selective pressures occur, parasites and hosts are engaged in arms races. Coevolution between parasites and hosts might thus be a universal phenomenon, although it has been rarely demonstrated.
Optimal pathogenicity is at the cross-roads of two arms-races: In a first arms race (the encounter arms race), the parasite selects genes to improve the probability of contact with the host whilst the host selects genes to avoid meeting infective stages of the parasites. In a second arms race (the compatibility arms race), the host selects...