Chapter 8: Quantitative GeneticsOverview |
Many interesting traits are determined by the combined influence of alleles at many loci. When studying such traits, we often do not know the identities of the particular loci involved. Chapter 7 introduces quantitative genetics, the branch of evolutionary biology that provides tools for analyzing evolution at multilocus traits.
8.1 The Nature of Quantitative Traits Throughout our coverage of evolutionary genetics we have been discussing traits for which the phenotypes come in discrete categories. A flour beetle is either alive or dead; a person either has cystic fibrosis or does not. We might call characters such as these qualitative traits, because we can assign individuals to categories just by looking at them, or perhaps by conducting a simple genetic test. Traits with discrete phenotypes are special examples; most traits in most organisms show continuous variation. For traits with continuous variation we cannot assign individuals to discrete phenotypic categories by simple inspection. Instead, we have to take measurements. For this reason, characters with continuously distributed phenotypes are called quantitative traits. Quantitative traits are determined by the combined influence of (1) the genotype at many different loci, and (2) the environment.
Refer to section 8.1 to learn how quantitative traits are determined and measured.
8.2 Identifying Loci that Contribute to Quantitative Traits The loci that influence quantitative traits are called, appropriately enough, quantitative trait loci, or QTLs. Often we would like to identify the QTLs behind an interesting quantitative trait. Modern genetic and statistical methods make it possible to do so. We will review two such methods: QTL mapping, and investigation of candidate loci.
In section 8.2 you will learn about examples of these methods through the study of adaptation of various traits in the plant and animal kingdom.
8.3 Measuring Heritable Variation Recall the basic tenets of Darwin’s theory of evolution by natural selection: If there is heritable variation among the individuals in a population, and if there are differences in survival and/or reproductive success among the variants, then the population will evolve. Quantitative genetics includes tools for measuring heritable variation, tools for measuring differences in survival and/or reproductive success, and tools for predicting the evolutionary response to selection.
In section 8.3 of your textbook you will learn how to measure heritable variation.
8.4 Measuring Differences in Survival and Reproductive Success In the preceding section we developed techniques for measuring the heritable variation in quantitative traits, the first tenet of Darwin’s theory of evolution by natural selection. The next tenet of Darwin’s theory is that there are differences in survival and/or reproductive success among individuals.
Refer to section 8.4 to explore the techniques for measuring differences in success—that is, for measuring the strength of selection. Once we can measure both heritable variation and the strength of selection, we will be able to predict evolutionary change in response to selection!
8.5 Predicting the Evolutionary Response to Selection Once we know the heritability and the selection differential, we can predict the evolutionary response to selection. Using a mathematical models you can develop a set of tools for studying the evolution of multilocus traits under natural selection.
Refer to section 8.5 to learn about estimating how much of the variation in a trait is due to variation in genes, quantifying the strength of selection that results from differences in survival or reproduction, and finally predicting how much the population will change from one generation to the next.
8.6 Modes of Selection and the Maintenance of Genetic Variation In our discussions of selection on quantitative traits, we have assumed that the relationship between phenotype and fitness is simple. In our mice, short tails were better than long tails; in sky pilots, bigger flowers were better than smaller flowers.
In section 8.6 of you textbook you will explore the different modes of selection such as directional selection, stabilizing selection and disruptive selection and how they influence genetic variation.
8.7 The Bell-Curve Fallacy and Other Misinterpretations of Heritability Studies of heritability are often misinterpreted as implying that differences between populations are due to differences in genes. The mistaken notion that heritability can tell us something about the causes of differences between populations has been particularly persistent in studies of human intelligence.
In section 8.7 of your textbook you will explore analyses of one famous book, The Bell Curve, that claimed that the difference in average IQ scores between African Americans and European Americans is due to genetic differences between these groups.