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 w_opt; 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 w_m.

The lag load L is then defined as

L = (w_opt - w_m) / w_opt

As the lag load L increases, the rate of evolution of the species will increase, because it is subject to stronger selection pressure. The lag load also controls the chance that the species will go extinct. As L increases, the species lags further and further behind its competitors and its chance of extinction goes up. As L decreases toward zero, its chance of extinction also decreases.