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.
Natural selection can be quite rapid.
Although one commonly thinks of evolutionary changes occurring over times measured by geological eras, the Pepper moth example demonstrates that evolutionary change can occur quite quickly, in this case over periods of approximately 100 years. It is conceivable that if male mating success, survival during the larval stages, etc., had been measured, the picture obtained would turn out not to be consistent with theory. B. Other similar examples 1. C. Sickle-Cell anemia and malarial resistance 1. Sickle cell anemia affects primarily people from Africa and is caused by a mutant allele of the normal gene that codes for the production of hemoglobin.