Results confirm qualitative prediction of equation for gene frequency change. Industrial melanism is seen in more than 70 species of British moths, and all show patterns similar to that seen in B. betularia . As a result, many of the formerly polluted woodlands have returned to their original condition, in which the color of the tree trunks is light and mottled. As would be expected from Kettlewell's work, the melanic forms that were once so prevalent in these areas have almost disappeared. Properties of evolution by natural selection illustrated by this example.
Natural selection is directional.
Given a particular population, we can predict almost with certainty that the allele favored by selection will eliminate the other allele. This property is illustrated by the fact that of the 70 British moth species that exhibit melanic variants, all species exhibit fixation of the melanic form in areas of heavy industrial pollution. Which allele is favored depends on the environment. Although in any one environment one allele may be favored over another by natural seleciton, a change in the environment may reverse the relative survival values of the two alleles. Thus, we may speak of the relative advantage of one allele over another only with respect to a particular environment.