8.5–8.10: Four mechanisms can give rise to evolution.

Close-up of the forewing (elytron) of the green tiger beetle, (Cicindela campestris). The coloration may help deter predators by resembling a species of wasp with a harmful sting.

Suppose you were put in charge of a large population of tigers in a zoo. Almost all of them are orange and brown with black stripes. Occasionally, though, unusual all-white tigers are born. This white phenotype is the result of a rare pair of alleles that suppresses the tiger’s production of most fur pigment (FIGURE 8-9). (Recall from Section 7-1 that alleles are variants of a gene and that most mammals inherit one allele from their mother and one from their father for each gene.) Because visitors flock to zoos to see white tigers, you want to increase the proportion of your tiger population that is white. How would you do so?

Figure 8.9: Evolution defined. Evolution is a change in allele frequencies in a population.

One possibility is that you could alter the population by trying to produce more animals with the white phenotype. In this case, your best strategy would be to try to breed the white tigers with each other. Over time, this would lead to more and more white tigers. And as the generations go by (this will take a while, though, since the generation time for tigers is about eight years), your population will include a higher proportion of white tigers. When this happens, you will have witnessed evolution, a change in the proportion of alleles for the pigment-suppression gene in the population.

Another way you might increase the proportion of white tigers in your population would be to acquire some white tigers from another zoo. By directly adding white tigers to your population, the population would include a higher proportion of white tigers (you might even trade some of your orange tigers, which would further increase the proportion of your population that is white). By adding white tigers or removing orange tigers, you will again have witnessed evolution in your population.

As this example illustrates, evolution doesn’t involve changing the genetics or physical features of individuals. Individuals do not evolve. Rather, you change the proportions of the alleles in the population. An allele’s frequency is the proportion in which it is present in a population relative to the other alleles of the same gene, much like its “market share.” In the tiger example, the white-fur alleles initially had little market share, perhaps as small as 1%. Over time, though, they came to make up a larger and larger proportion of the total fur-pigment alleles in the population. And as this happened, evolution occurred.

Any time an allele’s market share changes, evolution is taking place. Perhaps the most dramatic way we see this is in acts of predation. When one organism kills another, the dead animal’s alleles will no longer be passed on in the population. And if certain alleles make it more likely that one animal will be killed by another—maybe they alter its coloration, or speed, or some other defense mechanism—those alleles are likely to lose market share. The converse is equally true: evolution occurs when alleles increase in frequency.

Darwin demonstrated that natural selection could be an efficient mechanism of evolution and a powerful process that results in adapting populations to their environments. Evolution and natural selection, however, are not the same thing. Natural selection is one way that evolution can occur, but it is not the only mechanism of evolutionary change. It is simply one of them, and the only one that leads to adaptation (FIGURE 8-10). The four evolutionary mechanisms are:

• 1. Mutation
• 2. Genetic drift
• 3. Migration
• 4. Natural selection

Figure 8.10: Four ways in which evolution can occur.

Keeping in mind that evolution is genetic change in a population, we’ll now explore each of these four processes that lead to such genetic changes.