Figure 1. Changes in the frequency of an advantageous allele A₂ under (a) codominant selection, (b) dominant selection where A₂ is dominant over A₁ and (c) dominant selection where A₁ is dominant over A₂. In all cases, the frequency of A₂ at time 0 is q=0.04, and the selective advantage is s=0.01.

Figure 2. (a) Changes in the frequency of an allele subject to overdominant selection. Initial frequencies from top to bottom curves: 0.99, 0.75, 0.50, 0.25 and 0.01; s=0.04 and t=0.02. Since the s and t values are exceptionally large, the change in allele frequency is rapid. Note that there is a stable equilibrium at q=0.667. (b) Changes in the frequency of an allele subject to underdominant selection. Initial frequencies from top to bottom curves: 0.75, 0.668, 0.667, 0.666, 0.50 and 0.20; s=–0.02 and t=–0.01. Again, because of the large values of s and t, the change in allele frequency is rapid. Note that there is an equilibrium at q=0.667. This equilibrium, however, is unstable since even the slightest deviation from it will cause one of the alleles to be eliminated from the population.

Figure 3. Random sampling of gametes. Allele frequencies in the gamete pools (large boxes) in each generation are assumed to reflect exactly the allele frequencies in the adults of the parental generation (small boxes). Since the population size is finite, allele frequencies fluctuate up and down. (Bodmer WF and Cavalli-Sforza LL (1976) Genetics, Evolution, and Man. San Francisco: Freeman.)

Figure 4. Changes in frequencies of alleles subject to random genetic drift in populations of different sizes. The smallest population reached fixation after 42 generations. The other two populations were still polymorphic after 150 generations but will ultimately reach fixation (allele frequency=0 or 100%) if the experiment is continued long enough. (Bodmer WF and Cavalli-Sforza LL (1976) Genetics, Evolution, and Man. San Francisco: Freeman.)

Figure 5. Two possible outcomes of random genetic drift in populations of size 25 and p₀=0.5.

Figure 6. Schematic representation of the dynamics of gene substitution for (a) advantageous and (b) neutral mutations. Advantageous mutations are either quickly lost from the population or quickly fixed, so that their contribution to genetic polymorphism is small. The frequency of neutral alleles, however, changes very slowly in comparison, so that a large amount of transient polymorphism is generated, is the conditional fixation time, and 1/K is the mean time between two consecutive fixation events. (Nei M (1987) Molecular Evolutionary Genetics. New York: Columbia University Press.)