Measuring the Change of Population Fitness by Natural Selection

Abstract

Haldane and Van Valen^1,2 measured the intensity of natural selection by the difference of the mean fitness from the optimum. Haldane used the formula I_H = log_ew₀ – log_ew̄ where w₀ is the fitness of the optimum phenotype and w̄ is the mean fitness. Van Valen's formula is I_V=(W₀ − W)/W₀ which gives values similar to Haldane's. But w₀ may be unknown. The optimum phenotype with the highest fitness may not have occurred in the population. If, for example, among existing phenotypes fitness increased as a linear function of a character, there would be no way of obtaining a value of the optimum phenotype. Values of I_H and I_v would then be arbitrary and depend on an arbitrary value chosen for W₀.