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Population Genetics and Ecology Philip Hedrick Copyrighted Material I.15 Population Genetics and Ecology Philip Hedrick OUTLINE can be represented as a binomial function of the al­ lele frequencies. 1. Introduction neutral theory. Genetic change is primarily the result of 2. Genetic drift and effective population size mutation and genetic drift, and different molecular 3. Neutral theory genotypes are neutral with respect to each other. 4. Gene flow and population structure population. A group of interbreeding individuals that 5. Selection exist together in time and space. 6. Future directions selective sweep. Favorable directional selection that re­ sults in a region of low genetic variation closely About 40 years ago, scientists first strongly advocated the linked to the selected region. integration of population ecology and population genetics into population biology (Singh and Uyenoyama, 2004). Even today these two disciplines are not really integrated, but 1. INTRODUCTION there is a general appreciation of population genetic con­ The primary goals of population genetics are to un­ cepts in population ecology and vice versa. For example, the derstand the factors determining evolutionary change new subdiscipline molecular ecology, and many articles and stasis and the amount and pattern of genetic var­ in the journal Molecular Ecology, use genetic markers and iation within and between populations (Hedrick, 2005; principles to examine both ecological and evolutionary Hartl and Clark, 2007). In the 1920s and 1930s, shortly questions. Although some aspects of population genet­ after widespread acceptance of Mendelian genetics, the ics have changed quickly in recent years, many of its fun­ theoretical basis of population genetics was developed damentals are still important for aspects of ecological by Ronald A. Fisher, J.B.S. Haldane, and Sewall Wright. study. Population genetics may be unique among biological sciences because it was first developed as a theoretical discipline by these men before experimental research GLOSSARY had a significant impact. coalescence. The point at which common ancestry for The advent of molecular genetic data of populations two alleles at a gene occurs in the past. in the late 1960s and DNA sequence data in the 1980s effective population size. An ideal population that in­ revolutionized population genetics and produced many corporates such factors as variation in the sex ratio new questions. Population genetics and its evolution­ of breeding individuals, the offspring number per ary interpretations provided a fundamental context in individual, and numbers of breeding individuals in which to interpret these new molecular genetic data. different generations. Further, population genetic approaches have made gene flow. Movement between groups that results in fundamental contributions to understanding the role genetic exchange. of molecular variation in adaptive differences in mor­ genetic bottleneck. A period during which only a few phology, behavior, and physiology. A primary goal in individuals survive and become the only ancestors determining the extent and pattern of genetic variation of the future generations of the population. is to document the variation that results in selective genetic drift. Chance changes in allele frequencies that differences among individuals, the ‘‘stuff of evolution.’’ result from small population size. The amount and kind of genetic variation in popu­ Hardy-Weinberg principle. After one generation of lations are potentially affected by a number of factors, random mating, single-locus genotype frequencies but primarily by selection, inbreeding, genetic drift, Copyrighted Material 110 Autecology gene flow, mutation, and recombination. These factors Weinberg equilibrium [HWE]). It states that after one may have general or particular effects; for example, generation of random mating, single-locus genotype genetic drift and inbreeding can be considered to al­ frequencies can be represented by a binomial (with two ways reduce the amount of variation, and mutation to alleles) or multinomial (with multiple alleles) function always increase the amount of variation. Other factors, of the allele frequencies. This principle allows great such as selection and gene flow, may either increase or simplification of the description of a population’s ge­ reduce genetic variation, depending on the particular netic content by reducing the number of parameters situation. Combinations of two or more of these fac­ that must be considered. Furthermore, in the absence tors can generate many different levels and patterns of of factors that change allele frequency (selection, ge­ genetic variation. In 1968, Motoo Kimura introduced netic drift, gene flow, and mutation), and in the contin­ the important ‘‘neutral theory’’ of molecular evolution ued presence of random mating, the Hardy-Weinberg that assumes that genetic variation results from a com­ genotype proportions will not change over time. bination of mutation generating variation and genetic drift eliminating it (Kimura, 1983). This theory is 2. GENETIC DRIFT AND EFFECTIVE called neutral because allele and genotype differences POPULATION SIZE at a gene are selectively neutral with respect to each other. This theory is consistent with many observations Since the beginning of population genetics, there has of molecular genetic variation (see below). been controversy concerning the importance of chance To understand the influence of these evolutionary changes in allele frequencies because of small popula­ factors, one must first be able to describe and quantify tion size, termed genetic drift. Part of this controversy the amount of genetic variation in a population and the has resulted from the large numbers of individuals pattern of genetic variation among populations. In re­ observed in many natural populations, large enough to cent years, new laboratory techniques have made it think that chance effects would be small in comparison possible to obtain molecular genetic data in any spe­ to the effects of other factors, such as selection and cies, and a number of software packages have become gene flow. However, if the selective effects or amount available to estimate the important parameters in of gene flow are small relative to genetic drift, then population genetics and related topics. In addition, the long-term genetic change caused by genetic drift may online Evolution Directory (EvolDir) is a source of in­ be important. formation about different molecular techniques, esti­ Under certain conditions, a finite population may be mation procedures, and other current evolutionary so small that genetic drift is significant even for loci genetic information. with sizable selective effects, or when there is gene Let us first define the evolutionary or genetic con­ flow. For example, some populations may be contin­ notation of the term population. As a simple ideal, uously small for relatively long periods of time because a population is group of interbreeding individuals that of limited resources in the populated area, low ten­ exist together in time and space. Often it is assumed dency or capacity to disperse between suitable habitats, that a population is geographically well defined, al­ or territoriality among individuals. In addition, some though this may not always be true. Below we discuss populations may have intermittent small population the concept of effective population size, which provides sizes. Examples of such episodes are the overwintering a more explicit definition of population in evolutionary loss of population numbers in many invertebrates and terms. epidemics that periodically decimate populations of Many of the theoretical developments in population both plants and animals. Such population fluctuations genetics assume a large, random-mating population generate genetic bottlenecks, or periods during which that forms the gene pool from which the female and only a few individuals survive and become the only male gametes are drawn. In some real-life situations, ancestors of the future generations of the population. such as dense populations of insects or outcrossing Small population size is also important when a pop­ plants, this ideal may be nearly correct, but in many ulation grows from a few founder individuals, a phe­ natural situations, it is not closely approximated. For nomenon termed founder effect. For example, many example, there may not be random mating, as in self- island populations appear to have started from a very fertilizing plants, or there may be small or isolated small number of individuals. If a single female who was populations as in rare or endangered species. In these fertilized by a single male founds a population, then cases, modifications of the theoretical ideal must be only four genomes (assuming a diploid organism), two made. from the female and two from the male, may start a One of the basic concepts of population genetics new population. In plants, a whole population may is the Hardy-Weinberg principle (often called Hardy- be initiated from a single seed—only two genomes, if Copyrighted Material Population Genetics and Ecology 111 self-fertilization occurs. As a result, populations des­ the parents of any progeny individual. In other words, cended from a small founder group may have low ge­ the gametes are drawn randomly from all breeding netic variation or by chance have a high or low fre­ individuals, and the probability of each adult pro­ quency of particular alleles. ducing a particular gamete equals 1/N, where N is the Another situation in which small population
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