 Natural selection occurs when some variants in a population enjoy a survival or reproductive advantage over others. The simplest population-genetic model of natural selection focuses on a single autosomal locus with two alleles, \(A_1\) and \(A_2\), in a large population. Random mating is assumed. The three diploid genotypes \(A_1 A_1,\) \(A_1 A_2\) and \(A_2 A_2\) have different fitnesses, denoted by \(w_{11},\) \(w_{12}\) and \(w_{22}\) respectively. These fitnesses are assumed to be constant across generations. A genotype’s fitness may be defined, in this context, as the average number of successful gametes that an organism of that genotype contributes to the next generation—which depends on how well the organism survives, how many matings it achieves, and how fertile it is. Unless \(w_{11},\) \(w_{12}\) and \(w_{22}\) are all equal, then natural selection will occur, which may lead the genetic composition of the population to change.