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."
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.
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.
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. (1862–1964)
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
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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.
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 bmight 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 genotype—whether 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.
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.
In very small populations, rare alleles can become common or disappear because of genetic drift—random 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.
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Population genetics is the statistical study of the natural differences found within a group of the same organisms. Instead of examining the genes of individuals, it looks at the dominant (the trait that first appears or is visibly expressed in the organism) and recessive (the trait that is present at the gene level but is masked and does not show itself in the organism) genes found within an entire population. Population genetics seeks to understand the factors that control which genes are expressed. It also creates mathematical models to try to predict which differences will be expressed and with what frequency.
In the life sciences, a population consists of all the individuals of the same species (all of the same kinds of organisms, like all the tigers) that live in a particular habitat at the same time. Scientists know that in any population, whether it be tigers or people, the individuals that make it up are all different. They may all be tigers, but each has individual and very recognizable traits.
Although some might think that all animals of the same species look exactly alike, it is known that once someone becomes familiar with a certain group of the same species, he or she can usually tell one from another. At first all black labrador retrievers look alike. After a closer look, it can be seen that there are very obvious and easily recognizable differences among them. It is known that it is mostly the individual's genetic inheritance that accounts for these minor differences. This means that the unique combination of dominant and recessive genes that the individual has inherited is responsible for all of its individual traits (color, size, abilities, and tendencies, to name only a few).
WHAT IS POPULATION GENETICS?
Population genetics is a tool used to study the genetic basis of evolution (the process by which gradual genetic change occurs over time to a group of living things), and it is helpful in allowing scientists to understand the relative importance of the many factors that influence evolution. It studies a given population's gene pool (which is the total of all of the genes available to a generation). Knowing what the gene pool consists of enables scientists to establish a sort of genetic base out of which future offspring will be composed. This assumes that over time, the population is made up of individuals that breed only with others of their species that live in the same habitat.
Once the gene pool is established, scientists are able to use Mendel's laws of inheritance (concerning patterns of dominant and recessive genes) and predict what differences there will be among individuals in that population. Scientists are able to establish what are called gene frequencies, or percentages at which certain genes will be expressed. Scientists also have been able to establish a law that actually measures what changes will take place. Called the Hardy-Weinberg law, since it was proposed independently in 1906 by the English mathematician Godfrey H. Hardy (1877–1947), and the German physician Wilhelm Weinberg (1862–1937), this is a mathematical formula that has become the basis of population genetics. Using this formula (which only works perfectly when certain ideal conditions are met), scientists are able to describe a steady state called genetic equilibrium. In this state, gene frequencies stay the same and nothing changes unless some outside force intervenes.
Naturally, the real world, especially that involving human beings, is not perfect and there are many factors always at work that make conditions less than ideal. Chance events happen all the time. Reproduction does not always work and individuals leave populations while others may wander in. These are only a few potential variables. However, the Hardy-Weinberg formula is still useful and helps in being able to arrive at some relative frequencies, so it is still applies in some way to the real world.
WHY STUDY POPLUATION GENETICS?
By studying what makes individuals in the same population different, science is able to learn more about evolutionary change. Population genetics can also draw very useful conclusions. For example, when populations of interbreeding individuals are very small, they are highly susceptible to extinction by any number of chance events. This is because their interbreeding has not given them much genetic variation (differences). When something in their habitat changes, they may be unable to adapt quickly enough. Population genetics, therefore, is a valuable, if not always statistically perfect, tool for life scientists.