The unit of evolution is the reproductive population. The genotype of a population is referred to as the gene pool, which is all the alleles carried by the members of a population. As the frequencies of alleles within the gene pool change, the population evolves. If the allele frequencies remain constant, the population does not evolve; it is said to be in a state of genetic equilibrium. Genetic equilibrium, however, can only be a hypothetical state because various evolutionary forces are always present and lead to change. By using the Hardy-Weinberg formula, we can measure the strength of evolutionary forces by making comparisons between the hypothetical situation of no change and observed situations of change.

Natural populations are not in equilibrium because five mechanisms bring about change in allele frequency. Mutations, the ultimate source of genetic variability, provide one example. Genetic drift is another factor in which, by chance alone, not all alleles in a population will be represented proportionally in the next generation. The smaller the population, the more pronounced the effect. According to founder principle, a new population based on a small sample of the original population may show distinctive gene frequencies. Gene flow can bring new alleles into a population where they may be adaptive and increase in frequency.

The genetic-equilibrium model assumes random mating, but individuals consciously choose mates for myriad reasons. Nonrandom mating leads to changes in gene frequencies from generation to generation. Differential fertility, or natural selection, is a powerful force of evolutionary change. This topic will be the major focus of the next chapter.

It should be emphasized that the mechanisms of evolution work together to create net change. For example, natural selection would have nothing to "select" for or against if the variability provided by mutation were not present.

I. Students should understand the difference between evolution and genetic equilibrium.

II. Students should know the Hardy-Weinberg equilibrium equation and be able to apply it to specific examples.

III. Students should be able to list and distinguish between the various evolutionary forces that disrupt equilibrium.