Skip to main content
Select Source:

Population Genetics

Population Genetics

The field of population genetics examines the amount of genetic variation within populations and the processes that influence this variation. A population is defined as a group of interbreeding individuals that exist together at the same time. Genetic variation refers to the degree of difference found among individuals, for instance in height, coat color, or other less observable traits. The particular set of genes carried by an individual is known as his or her genotype, while all the genes in a population together comprise the "gene pool."

Foundations

The foundation for population genetics was laid in 1908, when Godfrey Hardy and Wilhelm Weinberg independently published what is now known as the Hardy-Weinberg equilibrium. The "equilibrium" is a simple prediction of genotype frequencies in any given generation, and the observation that the genotype frequencies are expected to remain constant from generation to generation as long as several simple assumptions are met. This description of stasis provides a counterpoint to studies of how populations change over time.

The 1920s and 1930s witnessed the real development of population genetics, with important contributions by Ronald Fisher, Sewall Wright, and John B. S. Haldane. They, with many others, clearly established the basic processes which caused populations to change over time: selection, genetic drift, migration, and mutation. The change in the genetic makeup of a population over time, usually measured in terms of allele frequencies, is equivalent to evolutionary change. For this reason, population genetics provides the groundwork for scientists' understanding of evolution, in particular microevolution, or changes within one or several populations over a limited time span.

The questions addressed by population genetics are quite varied, but many fall within several broad categories. How much genetic variation is found in populations, and what processes govern this? How will a population change over time, and can a stable endpoint be determined? How much and why do populations of the same species differ? The answer is always cast in terms of selection, drift, mutation, migration, and the complex interplay among them. Of the four, selection and genetic drift are usually given credit as the major forces.

Selection

Simply put, selection occurs when some genotypes in the population are on average more successful in reproduction. These genotypes may survive better, produce more offspring, or be more successful in attracting mates; the alleles responsible for these traits are then passed on to offspring. There is broad theoretical consensus and abundant empirical data to suggest that selection can change populations radically and quickly. If one genetic variant, or allele, increases survivorship or fertility, selection will increase the frequency of the favored allele, and concurrently eliminate other alleles. This type of selection, called directional selection, decreases the amount of genetic variation in populations.

Alternatively, an individual carrying two different alleles for the same gene (a heterozygote) may have advantages, as exemplified by the well-known example of the sickle-cell allele in Africa, in which heterozygotes are more resistant to malaria. In this case, called overdominant selection, genetic variation is preserved in the population. Although a number of similar examples are known, directional selection is much more common than overdominant selection; this implies that the common action of selection is to decrease genetic variation within populations. It is equally clear that if different (initally similar) populations occupy different habitats, selection can create differences among populations by favoring different alleles in different areas.

Genetic Drift

Often overlooked by the layperson, genetic drift is given a place of importance in population genetics. While some analyses of genetic drift quickly become complicated, the basic process of drift is simple and involves random changes in allele frequency. In sexual species, the frequency of alleles contained in the progeny may not perfectly match the frequency of the alleles contained in the parents. As an analogy, consider flipping a coin twenty times. Although one might expect ten heads and ten tails, the actual outcome may be slightly different; in this example, the outcome (progeny) does not perfectly represent the relative frequency of heads and tails (the parents).

What does this mean for populations? Start by considering neutral alleles, which have no impact on survival or reproduction. (An example is the presence or absence of a widow's peak hairline.) The frequency of a neutral allele may shift slightly between generations, sometimes increasing and sometimes decreasing. What outcomes are expected from this process? Suppose that a particular allele shifts frequency at random for a number of generations, eventually becoming very rare, with perhaps only one copy in the population. If the individual carrying this allele does not pass it on to any offspring or fails to have any offspring, the allele will be lost to the population. Once lost, the allele is gone from the population forever. In this light, drift causes the loss of genetic variation over time. All populations are subject to this process, with smaller populations more strongly affected than larger ones.

Perhaps better known than the pervasive, general effects of genetic drift are special examples of drift associated with unusually small populations. Genetic bottlenecks occur when a small number of individuals from a much larger population are the sole contributors to future generations; this occurs when a catastrophe kills most of the population, or when a few individuals start a new population in different area. Genetic bottlenecks reduce the genetic variation in the new or subsequent population relative to the old. Cheetahs, which have very little genetic variation, are presumed to have gone through several genetic bottlenecks. Occasionally, these new populations may have particular alleles that are much more common than in the original population, by chance alone. This is usually called the founder effect.


HALDANE, J. B. S. (18621964)

British biologist and author who immigrated to India. Haldane was famous for both his flamboyant personality and his influence on genetics and evolutionary biology. Haldane, along with Ronald Fisher, showed that evolution is the change in frequency of individual genes over time.


Migration and Mutation

Migration may also be important in shaping the genetic variation within populations and the differences among them. To geneticists, the word "migration" is synonymous with the term "gene flow." Immigration may change allele frequencies within a population if the immigrants differ genetically. The general effect of gene flow among populations is to make all of the populations of a species more similar. It can also restore alleles lost through genetic drift, or introduce new alleles formed by mutation in another population. Migration is often seen as the "glue" that binds the subpopulation of a species together. Emigration is not expected to change populations unless the migrants are genetically different from those that remain; this is rarely observed, so emigration is often ignored.

The last important process is mutation. Mutation is now understood in great detail at the molecular level, and consists of any change in the deoxyribonucleic acid (DNA) sequence of an organism. These mutations range from single base substitutions to the deletion or addition of tens or hundreds of bases to the duplication or reorganization of entire chromosomes . Mutation is most important as the sole source of all new genetic variation, which can then be spread from the population of origin by migration. This importance should not be undervalued, although the impact of mutation on most populations is negligible at any given time. This is because mutation rates are typically very low.

Questions and Contributions

The real challenge of population genetics has been in understanding how the four processes work together to produce the observable patterns. For instance, genetic drift eliminates variation from populations, as do the most common modes of natural selection. How then can the abundance of genetic variation in the world be explained?

This question has many complicated answers, but some cases, such as the observation of deleterious alleles in humans (for example, alleles for phenylketonuria, a genetic disease), might be explained in terms of mutation and selection. Mutation adds these alleles to a population, and selection removes them; although the rate of mutation is likely to be nearly constant, the rate at which selection removes them increases as the abundance of the allele increases. This is certainly true for recessive alleles, which are only expressed when an individual has two copies. With only one, the allele remains unexpressed and therefore not selected. At some point, predictable from the mutation rate and physical consequences of the disease, the two opposing forces balance, producing the stable persistence of the disease allele at low frequency.

As a discipline, population genetics has contributed greatly to scientists' understanding of many disparate topics, including the development of resistance of insects to insecticides and of pathogenic bacteria to antibiotics, an explanation of human genetic variation like the alleles for sickle-cell anemia and blood groups, the evolutionary relationships among species, and many others. Of particular interest is the use of genetic data in conservation biology.

By definition, endangered and threatened species have reduced population sizes, making them subject to the vagaries of genetic drift and also to inbreeding. Inbreeding is mating between genetically related individuals, and often leads to inbreeding depression, a reduction of health, vigor, and fertility. Genetic drift leads to a loss of genetic variation, which limits what selection can do to produce adaptations if the environment changes. Keeping these two issues in mind, greatly reduced populations may be at increasingly greater risk for genetic reasons, leading to further declines.

see also Conservation; Endangered Species; Evolution; Extinction; Hardy-Weinberg Equilibrium; Natural Selection; Sexual Reproduction

Paul R. Cabe

Bibliography

Gillespie, John H. Population Genetics: A Concise Guide. Baltimore, MD: Johns Hopkins University Press, 1998.

Hardy, Godfrey. "Mendelian Proportions in Mixed Populations." Science 28 (1908): 4950.

Hartl, Daniel. A Primer of Population Genetics. Sunderland, MA: Sinauer Associates, 1999.

Hedrick, Philip W. Genetics of Populations. Boston, MA: Jones and Bartlett, 2000.

Smith, John Maynard. Evolutionary Genetics, 2nd ed. Oxford, England: Oxford University Press, 1998.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Population Genetics." Biology. . Encyclopedia.com. 27 Jun. 2017 <http://www.encyclopedia.com>.

"Population Genetics." Biology. . Encyclopedia.com. (June 27, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/population-genetics

"Population Genetics." Biology. . Retrieved June 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/population-genetics

Population Genetics

Population Genetics

Population genetics is the study of the genetic structure of populations, the frequencies of alleles and genotypes . A population is a local group of organisms of the same species that normally interbreed. Defining the limits of a population can be somewhat arbitrary if neighboring populations regularly interbreed. All the humans in a small town in the rural United States could be defined as a population, but what about the humans in a suburb of Los Angeles? They can interbreed directly with nearby populations, and, indirectly, with populations extending continuously north and south for a hundred or more miles. In addition, a large human population often consists of subpopulations that do not readily interbreed because of differences in education, income, and ethnicity. Despite these complexities, one can make some simple definitions.

Gene Pool and Genetic Structure

All of the alleles shared by all of the individuals in a population make up the population's gene pool. In diploid organisms such as humans, every gene is represented by two alleles. The pair of alleles may differ from one another, in which case it is said that the individual is "heterozygous" for that gene. If the two alleles are identical, it is said that the individual is "homozygous" for that gene. If every member of a population is homozygous for the same allele, the allele is said to be fixed. Most human genes are fixed and help define humans as a species.

The most interesting genes to geneticists are those represented by more than one allele. Population genetics looks at how common an allele is in the whole population and how it is distributed. Imagine, for example, an allele "b " that when homozygous, "bb," produces blue-eyed individuals. Allele b might have an overall frequency in the population of 20 percent; that is, 20 percent of all the eye-color alleles are b.

However, not everyone who has the b allele will be homozygous for b. Some people will have b combined with another allele, "B," which gives them brown eyes (because B is dominant and b is recessive). Others won't have the b allele at all and instead will be homozygous for B.

The frequency of each genotypewhether bb, Bb, or BB in the population is also of interest to population geneticists. The frequency of alleles and genotypes is called a population's genetic structure. Populations vary in their genetic structure. For example, the same allele may have a frequency of 3 percent among Europeans, 10 percent among Asians, and 94 percent among Africans. Blood types vary across different ethnic groups in this way. The frequency of genotypes depends partly on the overall allele frequencies, but also on other factors.

Hardy-Weinberg Theorem

Large, isolated populations whose members mate randomly and do not experience any selection pressure will tend to maintain a frequency of genotypes predicted by a simple equation called the Hardy-Weinberg Theorem. For example, if b has a frequency of 20 percent and B has a frequency of 80 percent, we can predict the frequency of the three genotypes (bb, Bb, and BB ). The total of all the genotype frequencies is 100 percent (b + B ), and the frequencies of each are given by (b + B )2 100 percent. This can be restated as the following equation:

100% = b 2 + 2(bB + B2).

And we can calculate the genotype frequencies as:

100% = (20%)2 + 2(20% × 80%) + (80%)2 = 4% + 32% + 64%.

So even though 20 percent of all the genes in this imaginary population are b alleles, only 4 percent of the population is homozygous for b and actually has blue eyes. Furthermore, this same distribution will be maintained over time, as long as the conditions of the Hardy-Weinberg Theorem are met.

However, few, if any, natural populations (including human ones) actually conform to the assumptions of Hardy-Weinberg, so both genotype frequencies and allele frequencies can and do change from generation to generation. For example, humans do not mate randomly. Instead, they tend to take partners of similar height and intelligence. And even in modern human populations, genetic diseases such as Tay-Sachs kill children long before they grow up and reproduce. A difference in survival and reproduction due to differences in genotype is called selection. Even subtle selection can change gene frequencies over long periods of time.

Another assumption of the Hardy-Weinberg theorem is that individuals from different populations do not mate, so that gene flow, the passage of new genetic information from one gene pool into another, is zero. Such isolation does characterize many animal and plant populations, but almost no modern human populations are isolated from all other populations. Instead, humans travel to different countries, intermarrying and producing children who reflect the novel intermingling of unusual alleles.

Genetic Drift

In very small populations, rare alleles can become common or disappear because of genetic driftrandom changes in gene frequency that are not due to selection, gene mutation, or immigration. We can explain this as follows. When flipping a coin 1,000 times, it is likely to get 50 percent heads and 50 percent tails (if it's a fair coin). But flip it only five or ten times, and it is unlikely to get exactly half heads and half tails. Chances are good that the results will be something quite different. In the same way, if 10,000 people mate and produce children, the bb genotype will pretty much conform to the Hardy-Weinberg equation described above (provided the other assumptions are approximately true). For example, in a sample of just twenty people, instead of getting a group of children of whom 4 percent have blue eyes, the result could end up none with blue eyes, or maybe half having blue eyes. It all depends on how the alleles happen to combine when eggs meet sperm.

Because of genetic drift, small, isolated populations often have unusual frequencies of a few alleles. Although similar to other people in most important respects, such isolated populations may harbor high frequencies of one or more alleles that are rare in most other populations. For example, in 1814, fifteen people founded a British colony on a group of small islands in the mid-Atlantic, called Tristan de Cunha. They brought with them a rare recessive allele that causes progressive blindness, and the disease, extraordinarily rare in most places, is common on Tristan de Cunha. Such "inbreeding" produces more homozygotes than usual and increases the probability of children born with genetic diseases. The Old Order Amish have a high frequency of Ellis-van Creveld syndrome, and Ashkenazi Jews were, until a few years ago, susceptible to Tay-Sachs disease. Fortunately, genetic testing has greatly reduced the incidence of Tay-Sachs and many other such genetic diseases.

Population genetics also provides information about evolution. It is known, for example, that populations that have unusual allele frequencies must have been isolated from other populations. And we can surmise that populations that share similar frequencies of certain rare alleles may have interbred at some point in the past. Human populations in sub-Sarahan Africa show the greatest diversity of all human populations. On the basis, in part, of this diversity, one theory of human evolution suggests that all humans originated in Africa, and then emigrated to Asia, Europe, and the rest of the world.

see also Founder Effect; Genetic Drift; Hardy-Weinberg Equilibrium; Molecular Anthropology; Population Bottleneck; Selection; Tay-Sachs Disease.

Jennie Dusheck

Bibliography

Jones, J. S. "How Different Are Human Races?" Nature 293 (1981): 188-190.

Klug, W. S., and M. R. Cummings. Concepts of Genetics, 6th ed. Upper Saddle River, NJ: Prentice Hall, 2000.

Lewontin, R. Human Diversity. Redding, CT: W. H. Freeman, 1982.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Population Genetics." Genetics. . Encyclopedia.com. 27 Jun. 2017 <http://www.encyclopedia.com>.

"Population Genetics." Genetics. . Encyclopedia.com. (June 27, 2017). http://www.encyclopedia.com/medicine/medical-magazines/population-genetics

"Population Genetics." Genetics. . Retrieved June 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/medical-magazines/population-genetics

population genetics

population genetics The study of inherited variation in populations of organisms, and its modulation in time and space. Population genetics relates the heritable changes in populations to the underlying individual processes of inheritance and development. Such studies generally involve the estimation of gene frequencies and the influences of selection, mutation, and migration upon these frequencies in natural (and experimental) populations.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"population genetics." A Dictionary of Ecology. . Encyclopedia.com. 27 Jun. 2017 <http://www.encyclopedia.com>.

"population genetics." A Dictionary of Ecology. . Encyclopedia.com. (June 27, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/population-genetics

"population genetics." A Dictionary of Ecology. . Retrieved June 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/population-genetics

population genetics

population genetics The study of inherited variation in populations of organisms, and its modulation in time and space. Population genetics relates the heritable changes in populations to the underlying individual processes of inheritance and development. Such studies generally involve the estimation of gene frequencies and the influences of selection, mutation, and migration upon these frequencies in natural (and experimental) populations.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"population genetics." A Dictionary of Plant Sciences. . Encyclopedia.com. 27 Jun. 2017 <http://www.encyclopedia.com>.

"population genetics." A Dictionary of Plant Sciences. . Encyclopedia.com. (June 27, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/population-genetics-0

"population genetics." A Dictionary of Plant Sciences. . Retrieved June 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/population-genetics-0

population genetics

population genetics The study of inherited variation in populations of organisms, and its modulation in time and space. Population genetics relates the heritable changes in populations to the underlying individual processes of inheritance and development. Such studies generally involve the estimation of gene frequencies and the influences of selection, mutation, and migration upon these frequencies in natural (and experimental) populations.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"population genetics." A Dictionary of Zoology. . Encyclopedia.com. 27 Jun. 2017 <http://www.encyclopedia.com>.

"population genetics." A Dictionary of Zoology. . Encyclopedia.com. (June 27, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/population-genetics-1

"population genetics." A Dictionary of Zoology. . Retrieved June 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/population-genetics-1

population genetics

population genetics The study of the distribution of inherited variation among a group of organisms of the same species. The potential for change depends on the sum total of alleles that are available to the organisms (the gene pool), and estimates of changes in allele frequency in a population give an indication of its response to a changing condition.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"population genetics." A Dictionary of Biology. . Encyclopedia.com. 27 Jun. 2017 <http://www.encyclopedia.com>.

"population genetics." A Dictionary of Biology. . Encyclopedia.com. (June 27, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/population-genetics-2

"population genetics." A Dictionary of Biology. . Retrieved June 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/population-genetics-2