Population of individuals. Each individual has two copies of the A gene. The big A allele or the little a allele. An individual in a diploid population which reproduces sexually is the result of one male gamete plus one female gamete being combined to form a zygote. If the mating is random, the proportions of genes in the gametes will determine the proportion of genotypes in the population.
Natural selection is one of the main factors, resulting in changes in gene proportions.
Natural selection may take many different degrees from completely lethal to very slight. Ordinarily, however, natural selection is probably rather subtle in its action, being recognized as one population having a very slight reproductive advantage over another. The effects of selection are often illustrated by using an extreme example such as a completely lethal recessive. This situation represents complete selection against a certain genotype. The following exercise, if carried out and understood, will provide you with an understanding of the basic Hardy-Weinberg equilibrium and at the same time illustrate the effects of several kinds of selection on this equilibrium.