restricted access I.15 Population Genetics and Ecology
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I.15 Population Genetics and Ecology Philip Hedrick OUTLINE 1. Introduction 2. Genetic drift and effective population size 3. Neutral theory 4. Gene flow and population structure 5. Selection 6. Future directions About 40 years ago, scientists first strongly advocated the integration of population ecology and population genetics into population biology (Singh and Uyenoyama, 2004). Even today these two disciplines are not really integrated, but there is a general appreciation of population genetic concepts in population ecology and vice versa. For example, the new subdiscipline molecular ecology, and many articles in the journal Molecular Ecology, use genetic markers and principles to examine both ecological and evolutionary questions. Although some aspects of population genetics have changed quickly in recent years, many of its fundamentals are still important for aspects of ecological study. GLOSSARY coalescence. The point at which common ancestry for two alleles at a gene occurs in the past. effective population size. An ideal population that incorporates such factors as variation in the sex ratio of breeding individuals, the offspring number per individual, and numbers of breeding individuals in different generations. gene flow. Movement between groups that results in genetic exchange. genetic bottleneck. A period during which only a few individuals survive and become the only ancestors of the future generations of the population. genetic drift. Chance changes in allele frequencies that result from small population size. Hardy-Weinberg principle. After one generation of random mating, single-locus genotype frequencies can be represented as a binomial function of the allele frequencies. neutral theory. Genetic change is primarily the result of mutation and genetic drift, and different molecular genotypes are neutral with respect to each other. population. A group of interbreeding individuals that exist together in time and space. selective sweep. Favorable directional selection that results in a region of low genetic variation closely linked to the selected region. 1. INTRODUCTION The primary goals of population genetics are to understand the factors determining evolutionary change and stasis and the amount and pattern of genetic variation within and between populations (Hedrick, 2005; Hartl and Clark, 2007). In the 1920s and 1930s, shortly after widespread acceptance of Mendelian genetics, the theoretical basis of population genetics was developed by Ronald A. Fisher, J.B.S. Haldane, and Sewall Wright. Population genetics may be unique among biological sciences because it was first developed as a theoretical discipline by these men before experimental research had a significant impact. The advent of molecular genetic data of populations in the late 1960s and DNA sequence data in the 1980s revolutionized population genetics and produced many new questions. Population genetics and its evolutionary interpretations provided a fundamental context in which to interpret these new molecular genetic data. Further, population genetic approaches have made fundamental contributions to understanding the role of molecular variation in adaptive differences in morphology , behavior, and physiology. A primary goal in determining the extent and pattern of genetic variation is to document the variation that results in selective differences among individuals, the ‘‘stuff of evolution.’’ The amount and kind of genetic variation in populations are potentially affected by a number of factors, but primarily by selection, inbreeding, genetic drift, gene flow, mutation, and recombination. These factors may have general or particular effects; for example, genetic drift and inbreeding can be considered to always reduce the amount of variation, and mutation to always increase the amount of variation. Other factors, such as selection and gene flow, may either increase or reduce genetic variation, depending on the particular situation. Combinations of two or more of these factors can generate many different levels and patterns of genetic variation. In 1968, Motoo Kimura introduced the important ‘‘neutral theory’’ of molecular evolution that assumes that genetic variation results from a combination of mutation generating variation and genetic drift eliminating it (Kimura, 1983). This theory is called neutral because allele and genotype differences at a gene are selectively neutral with respect to each other. This theory is consistent with many observations of molecular genetic variation (see below). To understand the influence of these evolutionary factors, one must first be able to describe and quantify the amount of genetic variation in a population and the pattern of genetic variation among populations. In recent years, new laboratory techniques have made it possible to obtain molecular genetic data in any species , and a number of software packages have become available to estimate the important parameters in population genetics and related topics. In addition, the online Evolution Directory (EvolDir) is a source of information...


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