While the genotype represents the genetic constitution of an individual, the gene pool represents the genetic constitution of an entire population, being composed of the genes of all the individuals in a population. While the genotype of an individual can have only two alleles for each type of gene, a gene pool of a population can contain many different alleles of the same gene. The allelic frequencies of each gene define the gene pool with regard to any given gene. Changes, over time, in allelic frequencies within the gene pool of a population result in the change, over time, of the genetic makeup of the population: This change is known as evolution.
The factors that cause shifts in allelic frequencies are those factors that determine evolution. In 1908, G. H. Hardy, an Englishman, and W. Weinberg, a German, both independently recognized that shifts in allelic frequencies and, thus, evolutionary changes only occur when something disturbs the genetic equilibrium in which the gene pool ordinarily exists. Indeed, the Hardy-Weinberg Principle states that allelic frequencies and genotypic ratios will remain constant from generation to generation in sexually reproducing populations if four conditions of stability hold true. First, the population must be large enough so that chance alone would be unlikely to alter allelic frequencies. In populations with more than 10,000 breeding age members, chance elimination of an allele from the gene pool is unlikely. Thus, chance plays an However, in small breeding populations of less than 100, chance elimination of an allele, even an allele of adaptive superiority, is very likely. Evolutionary change in small
