Populations in nature are rarely at equilibrium. We’ll discuss four evolutionary pressures that cause populations to change. They are genetic drift, gene flow, nonrandom mating, and natural selection. Genetic drift can occur when the population under study is very small. In small populations, chance events in reproduction can permanently change the population’s gene pool, or allele frequencies.

To illustrate one important cause of genetic drift, let’s look at blue and brown marbles in a bottle. The marbles represent the alleles for eye color. A chance event greatly reduces the size of the population. The proportion of brown and blue marbles after the spill is different from the original population. This phenomenon is called the bottleneck effect. In the bottleneck effect, a reduction in population size causes genetic drift.

Another important cause of genetic drift is the founder effect. Rather than a reduction in population size, some of the population breaks away. This may happen when a few individuals leave to populate a new habitat. When the founders reproduce, the resulting population may look quite different from the old population. The founder effect is a cause of genetic drift resulting from the migration of a few individuals.

A second cause of population change is gene flow. Gene flow is the change in a population's allele frequencies resulting from migration. Let’s go back to our example with the marbles. Say we toss a handful of brown marbles into our bottle. What happens to the frequencies of the brown and blue marbles? Look at the numbers. If we start out with 45 brown marbles and 55 blue marbles, what happens when we add 30 brown marbles? The frequency of the brown marbles increases. If the frequency of the brown marbles increases, what must happen to the frequency of the blue marbles? It must decrease. Remember, the frequencies always add up to one.

No population is completely isolated. Whenever individuals enter or leave a population, allele distributions change. A third way a population changes is through nonrandom mating. Nonrandom mating occurs when mates within a species are selected not by chance, but on the basis of a trait or group of traits. One type of nonrandom mating is assortative mating.

In assortative mating, mating partners resemble each other. What effect does assortative mating have on allele frequency? What effect does it have on genotype frequency? Once again, let’s go back to our marbles. If we pour the marbles onto a board with slots to simulate breeding, the marbles randomly pair.

If we preferentially pair brown marbles with brown marbles, the number of brown marbles doesn’t change and the number of blue marbles doesn’t change. From this we can conclude that assortative mating doesn’t change allele frequency. What does change?

Look carefully at the marbles.

The nonrandom pairing reduces the number of heterozygotes while increasing the number of homozygotes. Regardless of the number of generations in which breeding is nonrandom, it only takes one generation of random pairing to restore a population to the allele distributions predicted by the Hardy-Weinberg equation.

Nonrandom mating can lead to inbreeding. Inbreeding is mating between genetically related individuals. Inbreeding increases the risk that an individual in a population will be born homozygous with a damaging recessive trait. Some recessive disorders that are more prevalent in certain ethnic populations including Tay-Sachs disease, cystic fibrosis, and thalassemia.

Another type of nonrandom mating involves sexual selection.

Sexual selection is selection based on the evolution of traits unique to one sex of a species. It results from either selection by the opposite sex or competition among individuals of the same sex. One example of sexual selection is the difference in coloration between male and female mallard ducks.

The fourth cause of population change that we’ll discuss is perhaps the most challenging to understand. It’s natural selection. Natural selection adapts organisms to their environments. It facilitates evolution by either increasing or decreasing the odds that the organism successfully reproduces. The reproductive success of an organism is a measure of its Darwinian fitness. Since this is difficult to quantify, population geneticists measure relative fitness by comparing genotypes rather than individuals. The reproductive success of an organism ultimately depends on its genotype.

Differences in genotypes arise from mutations. Mutations occur randomly.

Most mutations have little effect. Some mutations are harmful, and some are beneficial. When a beneficial mutation occurs, the individual has a reproductive advantage. This disrupts the Hardy-Weinberg equilibrium because all individuals no longer reproduce equally. The population evolves. Natural selection causes favorable mutations in a population to accumulate and persist.

Natural selection changes allele frequency, and therefore the accumulation of favorable traits, in three ways: stabilizing selection, directional selection, and diversifying selection. The process of stabilizing selection tones down, or modifies, extreme phenotypes. This occurs whenever the modified phenotype has more reproductive success than either extreme. For example, stabilizing selection keeps birth weights at a normal average. Higher mortality rates are observed in babies that are either too small or too big.

Directional selection shifts the phenotypes toward one extreme. Directional selection is often caused by environmental shifts. An example of directional selection is moth color. During the Industrial Revolution, the soot on trees favored the selection of dark- colored moths over white moths.

Diversifying selection favors both extremes. Diversifying selection occurs whenever individuals on the fringes have a reproductive advantage over those in the middle. An example of diversifying selection is beak size in finches feeding in different habitats. Diversifying selection is the least common of the three types of natural selection.

We can easily illustrate the three types of selection using graphs. Natural selection promotes traits in a population by increasing the reproductive success of the selected individuals. Natural selection also discourages traits by decreasing the reproductive success of certain individuals. Now that we’ve seen how natural selection causes some populations to become more diverse, let’s see how the diversity is maintained.

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