It is clear from the example of the four o'clock flowers that pheno-type frequencies can change dramatically from generation to generation. But what happens to allele frequencies over generations? A German physician, Wilhelm Weinberg (1862-1937), and a British mathematician, Godfrey Hardy (1877-1947), independently showed that genotype frequencies in a population tend to remain the same from generation to generation unless acted on by outside influences. This principle is referred to as Hardy-Weinberg genetic equilibrium, and it is based on a set of assumptions about an ideal hypothetical population that is not evolving:
1. No net mutations occur; that is, the alleles remain the same.
2. Individuals neither enter nor leave the population.
3. The population is large (ideally, infinitely large).
4. Individuals mate randomly.
Bear in mind that true genetic equilibrium is a theoretical state. Real populations, such as the flock of mallards in Figure 16-4, may not meet all of the conditions necessary for genetic equilibrium. By providing a model of how genetic equilibrium is maintained, the Hardy-Weinberg principle allows us to consider what forces disrupt genetic equilibrium.
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