questions that i had on a test that i did not know the answer too

The discovery of large amounts of genetic variation in nearly all populations led to the formulation of a different question: How is genetic variation maintained? In many cases, after all, natural selection removes genetic variation by eliminating genotypes that are less fit.

Many factors act to increase or maintain the amount of genetic variation in a population. One of these is mutation, which is in fact the ultimate source of all variation. However, mutations do not occur very frequently, only at a rate of approximately one mutation per 100,000 to 1,000,000 genetic loci per generation. This rate is too slow to account for most of the polymorphisms seen in natural populations. However, mutation probably does explain some of the very rare phenotypes seen occasionally, such as albinism in humans and other mammals.

A second factor contributing to genetic variation in natural populations is selective neutrality. Selective neutrality describes situations in which alternate alleles for a gene differ little in fitness. Because small fitness differences result in only weak natural selection, selection may be overpowered by the random force of genetic drift. Alleles whose frequencies are governed by genetic drift rather than by natural selection are said to be selectively neutral. Under neutrality, allele frequencies vary over time, increasing or decreasing randomly. Over long periods of time, random fluctuations in the relative frequencies of different alleles may result in some being eliminated from the population. However, genetic polymorphisms are long-lived, and novel neutral alleles may arise continually through mutation.

Finally, several forms of natural selection act to maintain genetic variation rather than to eliminate it. These include balancing selection, frequency-dependent selection, and changing patterns of natural selection over time and space.

Balancing selection occurs when there is heterozygote advantage at a locus, a situation in which the heterozygous genotype (one including two different alleles) has greater fitness than either of the two homozygous geno-types (one including two of the same allele). Under heterozygote advantage, both alleles involved will be maintained in a population.

A classic example of heterozygote advantage concerns the allele for sickle-cell anemia. Individuals who are homozygous for the sickle-cell allele have sickle-cell anemia, which causes the red blood cells to become sickle-shaped when they release oxygen. These sickle-shaped cells become caught in narrow blood vessels, blocking blood flow. Prior to the development of modern treatments, the disease was associated with very low fitness, since individuals usually died before reproductive age.

Heterozygotes, however, have normal, donut-shaped blood cells and do not suffer from sickle-cell anemia. In addition, they enjoy a benefit of the sickle-cell allele, which offers protection from malaria. Consequently, heterozygous individuals have greater fitness than individuals who have two copies of the normal allele. Heterozygote advantage in this system is believed to have played a critical role in allowing a disease as harmful as sickle-cell anemia to persist in human populations. Evidence for this comes from an examination of the distribution of the sickle-cell allele, which is only found in places where malaria is a danger.

Another form of natural selection that maintains genetic variation in populations is frequency-dependent selection. Under frequency-dependent selection, the fitness of a genotype depends on its relative frequency within the population, with less-common genotypes being more fit than genotypes that occur at high frequency.

Frequency-dependent selection is believed to be fairly common in natural populations. For example, in situations where there is competition for resources, individuals with rare preferences may enjoy greater fitness than those who have more common preferences. Frequency-dependent selection may also play a role in predation: if predators form a search image for more common prey types, focusing on capturing those, less common phenotypes may enjoy better survival.

Finally, changing patterns of selection over time or space can help to maintain genetic variation in a population. If selection patterns fluctuate over time, different alleles or genotypes may enjoy greater fitness at different times. The overall effect may be that both alleles persist in a population. Changing selection pressures over time are encountered by a species of grasshopper characterized by two color morphs, a brown morph and a green morph. Earlier in the year, when the habitat is more brown, the better-camouflaged brown grasshoppers enjoy greater protection from predators. Later in the season, however, the environment is greener and the green grasshoppers have higher fitness.

Another possibility is that selection patterns vary from one place to another as a result of differences in habitat and environment. The prevalence of different genotypes in different habitats, combined with gene flow between habitats, can result in the maintenance of multiple alleles in a population.

One example comes from the allele for resistance to copper toxicity in species of grass. Copper-tolerant alleles are common in areas adjacent to copper mines, where the soil is contaminated. They are not expected in un-contaminated areas, however, where they are less fit than normal alleles. However, because grass species are wind pollinated, gametes can travel considerable distances, and copper-tolerant alleles are often found in areas where they are at a selective disadvantage.