Evolution Versus Genetic Equilibrium

What conditions are required to maintain genetic equilibrium?

One way to understand how and why populations evolve is to imagine a model of a hypothetical population that does not evolve. If a population is not evolving, allele frequencies in its gene pool do not change, which means that the population is in genetic equilibrium.

Sexual Reproduction and Allele Frequency Gene shuffling during sexual reproduction produces many gene combinations. But a century ago, researchers realized that meiosis and fertilization, by themselves, do not change allele frequencies. So hypothetically, a population of sexually reproducing organisms could remain in genetic equilibrium.

FIGURE 17–9 A Large Population Large populations are unlikely to remain in genetic equilibrium.

The Hardy-Weinberg Principle The Hardy-Weinberg principle states that allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change. The Hardy-Weinberg principle makes predictions like Punnett squares—but for populations, not individuals. Here's how it works. Suppose that there are two alleles for a gene: A (dominant) and a (recessive). A cross of these alleles can produce three possible genotypes: AA, Aa, and aa. The frequencies of genotypes in the population can be predicted by these equations, where p and q are the frequencies of the dominant and recessive alleles:

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Suppose that, in one generation, the frequency of the A allele is 40 percent (p = 0.40) and the frequency of the a allele is 60 percent (q = 0.60).