Substitutional Genetic Load
in Biston moths

    Substitutional genetic load is the 'cost' of replacing a deleterious allele in a population by directional selection. This can be measured as cumulative loss of fitness Wbar. The graph shows the load from two instances of directional selection on color morph in Biston betularia. In generations 1-10, the environment is light and the population is mostly light phase moths with a small number of dark phase moths due to a dominant allele. Population fitness (Wbar) rapidly increases 0.95 1.0: genetic load is the area above the curve in this interval. At t=10, the environment changes (darkens) and the dark phase is favored. The population becomes "less fit": fitness increases again as the light morph is replaced by the dark morph.