Blackwell Publishing

Coevolution - What happens if a species falls behind in an arms race?


Pattern of extinction rates

As ecological competitors, or parasites and hosts, evolve against each other, if one competitor fails to evolve an adaptive improvement to keep up with its antagonists, it may go extinct. If a host species evolves a new kind of immunity, then if the parasite does not soon evolve a way of penetrating the defence, it will go extinct. What is the pattern of extinction rates likely to be for this process?

Lag load

Stenseth and John Maynard Smith (opposite) have discussed the question theoretically in terms of what they call lag load. The lag load has the standard form for loads. We imagine that at any time there is an optimum state that the members of a species could have; a genotype with that optimum has fitness wopt; it is defined such that w opt = 1. But the genotype that is needed to have that optimum fitness keeps changing, because of the changes in competing species. Each species will evolve towards its optimum (i.e. to have all its members with the optimum genotype); but will lag a certain distance behind it. The actual mean fitness of the population is mean w, a number usually less than one; v = 1 should all members of the population have the optimum genotype. The lag load L is then defined as

L = (wopt - v) / wopt

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