Industrial melanism illustrates a type of selection called directional selection. In directional selection an extreme phenotype is favored. The extreme phenotype in this case is the melanic form. This phenotype is the result of a particular allele at a single gene locus. The endpoint of the process is a population of dark colored moths. The outcome of directional selection is different for a trait determined by many genes (polygenic), such as body size, which has a continuous variation of phenotypes. In the example shown, the largest individuals in the population are selected for. After many generations, the body size of the average individual in the population is actually larger than that of any individuals in the original population.