Genetic Variation in a Population
Genetic Variation in a Population
Genetic variation in a population describes the existence in that population of different alleles, or alternative forms, for a given gene. The presence of genetic variation implies that individuals of the population vary in the alleles they possess, meaning that individuals differ in genotype. Genetic loci for which there are multiple alleles are described as polymorphic. Humans, for example, are polymorphic for traits such as eye color and blood type.
Genetic variation is one facet of the more general concept of phenotypic variation. Phenotypic variation describes differences in the characteristics of individuals of a population. Phenotypic variation is of interest to biologists because it is what natural selection acts upon: different pheno-types may have different fitnesses, and selection results in fitter phenotypes leaving more descendants.
Phenotypic variation arises from either of two sources: genetic variation and environmental variation. However, only differences that arise from genetic variation can be passed on to future generations. Furthermore, only a fraction of the genetic component of variation, the additive genetic variation, is actually heritable. The additive genetic variation divided by the total phenotypic variation yields the heritability, which describes how much offspring resemble their parents.
The Amount of Genetic Variation
In the 1960s there was considerable debate regarding how much genetic variation actually exists in populations. The common view was that polymorphic loci are fairly rare. Then, the development of the technique of gel electrophoresis allowed biologists to examine patterns of protein variation across populations and to quantify genetic variation.
Biologists detected surprisingly large amounts of genetic variation. In most vertebrate species, for example, approximately 30 percent of genes were found to be polymorphic. Studies in the 1970s in humans showed that genetic variation occurs at approximately the same levels as in other animal species. The studies in humans also revealed, famously, that so-called human races are not real biological groupings. It was found that there is considerably more genetic variation within races than between them.
Since then it has been the absence of genetic variation that is considered anomalous. Absence of genetic variation in populations generally suggests that there was a population bottleneck in the recent history of the group, a time when the population size became very small. The result of a population bottleneck is that all members of the current population are descended from a small number of individuals, and therefore have only limited genetic variation. Genetic variation is expected to build up over time in these populations as new mutations appear.
How Genetic Variation Is Maintained
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.
see also Genes; Genetics; Peppered Moth; Selective Breeding.
Curtis, Helena. Biology. New York: Worth Publishers, 1989.
Futuyma, Douglas J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, 1998.
Gould, James L., and William T. Keeton, with Carol Grant Gould. Biological Science,6th ed. New York: W. W. Norton & Company, 1996.
Patterson, Colin. Evolution, 2nd ed. Ithaca, NY: Comstock, 1999.
Ridley, Mark. Evolution. Boston: Blackwell Scientific, 1993.
One well-studied example of genetic variation in populations is that of Biston betularia, the peppered moth.
There are three color morphs in the peppered moth: a light morph, a dark or melanistic morph, and an intermediate morph. Before the Industrial Revolution, the light morph was the most common form, although melanistic moths were also seen occasionally. However, by the end of the nineteenth century, the melanistic morph had become much more common, and had practically replaced the light morph in certain areas.
Biologists traced this shift to industrial pollution in urban areas. Without camouflaged resting places, the light moths became easy targets for bird predators. This explained both the prevalence of melanistic moths in polluted urban environments, and of light moths in comparatively pristine country habitats.
The puzzling aspect of the peppered moth story is that genetic variation was not entirely eliminated in populations. In urban areas, for example, melanistic moths make up only from 90 to 100 percent of the total population, despite very strong selection. Apparently there are forces other than predation pressure at work. It was hypothesized briefly that heterozygote advantage might be the explanation, but that theory was ultimately rejected. It is now believed that gene flow between country and urban areas, and frequency-dependent selection are viable alternatives. However, much work remains to be done on this historic system.