Background Informaion

For some individuals, evolution is a controversial topic. For most biologists, evolution is a central, unifying theme that incorporates many fields of biology including genetics, molecular biology, anatomy, and ecology among other disciplines. You may have already used EvolutionLab to study principles of evolution by natural selection in simulated populations of finches. Even if you have not studied evolution in detail, you are likely to know that changes in the genetic material of an individual in a population can and do occur through mutations. But remember that individual organisms do not live long enough to undergo significant genetic change whereby biologists consider an individual to have evolved. Genetic changes that occur in generations of populations over long periods of time are the basis for evolution of a species.

In a population of any species, there are typically individuals that show differences in phenotype for a particular trait. These differences represent genetic variation in the population. For example, the finches Charles Darwin studied on the Galapagos Islands showed variations in beak size. Of course, the underlying basis for phenotypic differences is the genotype, or genetic composition of an organism. Mutations, both random and induced, are the sources of new genes and new alleles that cause the heritable variation that is essential for evolution of a species and its population. The total range of genetic variation, due to alleles for all genes in a population, is known as the gene pool for that population. Geneticists interested in population genetics study allele frequencies as a way to predict whether a population is evolving. When there are multiple alleles for a particular gene, it is sometimes possible to determine the relative proportion of each allele in a gene pool. If all individuals in a population are homozygous for an allele being studied, that allele is known as a fixed allele.

What is the relationship between the frequency of alleles and the frequency of genotypes in a population? In 1908, two geneticists, G. H. Hardy and W. Weinberg, independently proposed an equation to relate allele frequency to genotype frequency. If one is studying two alleles called A and a for a particular gene at a known locus, the genotype frequencies of the possible combinations of these alleles--AA, Aa, and aa--can be determined from the equation p2 + 2pq + q2 = 1.0, where p can be designated as the frequency of the A allele, and q can be designated as the frequency of the a allele. Consider a population of 100 individuals, each of which has two alleles for this gene. The sum of p and q represents 100% of the alleles for this gene in the population. Therefore, if 80 percent of the alleles are A, then p would be 0.8. Twenty percent of the alleles would then be a; therefore q would be 0.2. The genotype frequencies would be p2 = (0.8)2 = 0.64 for the frequency of AA homozygotes, 2pq = 2(0.8)(0.2) = 0.32 for the frequency of Aa heterozygotes, and q2 = (0.2)2 = 0.04 for the frequency of aa homozygotes. We would predict 64 AA, 32 Aa, and 4 aa genotypes in this population of 100 individuals.

How does an understanding of the Hardy-Weinberg theorem help us understand population genetics? An understanding of allele frequency in a population can tell us whether evolution is occurring in a population or if that population is in a state of genetic equilibrium. If a population is not evolving, then allele frequency and genotype frequency will remain the same from generation to generation and the genotype frequencies will correspond to the Hardy-Weinberg prediction. The allele and genotype frequencies of a parental generation will match those of their offspring. Such nonevolving populations are said to be in a state of Hardy-Weinberg equilibrium. However, to truly understand population genetics, we need to consider the factors that influence allele and genotype frequencies. Hardy and Weinberg proposed that there are a number of conditions that must be held constant in a population if allele frequencies and genotype frequencies are to remain constant over several generations. These conditions are as follows:

  1. Large breeding population. The effect of random changes in allele frequencies (genetic drift) is greatly reduced in a population with large number of individuals.
  2. Random mating. Individuals in a population show no preference for mating with other individuals of a particular phenotype.
  3. The alleles under study do not mutate. Alleles are not being mutated to create new alleles, which would change the gene pool and alter genotype frequencies in a population.
  4. There is no migration of individuals into or out of the population; therefore, the gene pool will not change due to migration.
  5. There is no selective advantage for any individual. If all individuals in a population have an equal chance of surviving and reproducing, then all of the genotypes represented in the population are equally viable and all alleles should be inherited equally.

These five conditions are required to maintain Hardy-Weinberg equilibrium. If all conditions are met in a population, then no change in allele or genotype frequency will occur in that population. Therefore, we can use these criteria to determine if changes in a gene pool are occurring in a population--a process known as microevolution. It is important to understand how changes in Hardy-Weinberg criteria can result in microevolution. Many of these criteria will be further described in the assignments that accompany this lab as you design experiments that will help you learn how these criteria can influence population genetics.

It is possible to predict the proportions of individuals in different generations that will show certain phenotypes when one is studying very large populations and the population size is effectively infinite. It can be difficult to accurately study population genetics in small populations. Small populations may arise from larger populations due to events such as a natural disaster--for example, loss of habitat in an extreme weather condition such as a fire may isolate populations of organisms that were once part of a larger population. However, in a small population, the proportions of individuals with a certain phenotype can be strongly influenced by random events in the gene pool. These random events are called genetic drift. Genetic drift can make it very difficult to accurately predict phenotypic frequencies in a small population. For example, random variation in survival and number of offspring among genotypes could, over time, change the proportions of genotypes in the population. The effects of chance events such as these are greatly minimized in large populations.

One particularly well-characterized example of natural selection in a population involves British moths called peppered moths (Biston betularia). These moths are found in two different colors or morphs. One morph has light, almost white-colored wings with small flecks of brown, while the other morph is predominantly black and brown in color. Because birds are predators of peppered moths, wing color is an important camouflage for moths. White morphs can blend well into the light, peppered appearance of lichen-colored trees.

In the early 1800s the black morph began to appear in greater frequencies in cities throughout England, particularly highly industrialized cities, while the white morph dominated populations in rural areas. The increased frequency of the black morph coincided with the industrial revolution occurring in England. It was determined that the increase in soot released from coal-burning plants killed many species of lichen growing on trees in industrialized areas of England and blackened tree bark to a much darker color than lichen-covered trees in rural areas. As a result of this pollution-induced change in the environment, white morphs became much more visible and more vulnerable to predation by birds. Black morphs began to dominate populations in industrialized areas because they were less likely to be seen and eaten by birds. In some industrialized areas, genotype frequencies for black morphs approximated 100% of the population. The selective advantage by black morphs disrupted Hardy-Weinberg equilibrium conditions for moths in industrialized areas. The story of Biston betularia is a classic case of natural selection in a population and an example of how evolution can proceed via strong selection pressures.

You will use PopGenLab to learn how changes in important parameters of population genetics can influence evolution in simulated populations of moths that resemble peppered moths. You are provided with moths living in tree stands. A single gene with two alleles controls wing color of these moths, and each genotype produces a different color pattern. Moth survival depends on the insect's ability to blend against the bark of the trees they are living in. Different tree types are provided with bark colors that match the colors of the moths in the simulation. The experiments that you set up and analyze in PopGenLab will provide you with an important understanding of the factors influencing Hardy-Weinberg equilibrium and natural selection. By varying parameters such as allele frequencies and survival rates of each genotype, population numbers, population carrying capacity, mating patterns, and the frequency of population crashes due to natural disasters, you will design experiments to help you understand how each parameter can affect evolution within the population of moths.

References

  1. Atherly, A. G., Girton, J. R., and McDonald, J. F. The Science of Genetics, 1st ed. New York: Saunders, 1999.

  2. Freeman, S., and Herron, J. C. Evolutionary Analysis, 2nd ed. Upper Saddle River, NJ: Prentice-Hall, 2000.

  3. Smith, J. M. Evolutionary Genetics, 2nd ed. New York: Oxford University Press, 1999.

  4. Stearns, S. C., and Hoekstra, R. F. Evolution: An Introduction, 1st ed. New York: Oxford University Press, 2000.