Lag load can determine chance of extinction

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.

Coevolution, in this model, can have four forms:

• Contractionary

If a species lags behind its competitors and does not evolve fast enough to keep up with them, it will fall further behind until it goes extinct.

• Expansionary

A species could be ahead of its competitors and out-evolving them; it could theoretically expand until it had an infinite number of descendants.

Clearly, the expansionary mode could not continue for long. In real systems, the main possibility is that a species could alternate between periods of expansionary and contractionary evolution as the fortunes of natural selection favored one species and then another among a group of competitors.