I.14 Phenotypic Selection
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I.14 Phenotypic Selection David W. Pfennig and Joel G. Kingsolver OUTLINE 1. Introduction 2. How phenotypic selection works 3. Measuring phenotypic selection 4. Phenotypic selection in the wild 5. Misunderstandings about phenotypic selection 6. Future directions In this chapter, we describe the strength and patterns of natural selection in the wild. We focus on phenotypic selection because natural selection acts on the phenotypes of individual organisms. We begin by explaining what phenotypic selection is and how it works. We then explore how scientists study phenotypic selection in natural populations and discuss general patterns that have emerged from such investigations. Finally, we address common misunderstandings about selection and identify profitable avenues for future research. GLOSSARY fitness. The extent to which an individual contributes its genes to future generations relative to other individuals in the same population; a good operational definition of fitness is an individual’s relative reproductive success. heritability. In the broad sense, the fraction of the total phenotypic variation in a population that can be attributed to genetic differences among individuals; in the narrow sense, that fraction of the total phenotypic variation that results from the additive effects of genes. natural (phenotypic) selection. A difference, on average, between the survival or fecundity of individuals with certain phenotypes compared with individuals with other phenotypes. phenotype. The outward characteristics of organisms, such as their form, physiology, and behavior. quantitative trait. A trait that shows continuous rather than discrete variation; such traits are determined by the combined influence of many different genes and the environment. selection gradient. A measure of the strength of selection acting on quantitative traits: for selection on a single trait, it is equal to the slope of the best-fit regression line in a scatterplot showing relative fitness as a function of phenotype; for selection acting on multiple traits, it is equal to the slope of the partial regression in a scatterplot showing relative fitness as a function of all phenotypes. sexual selection. A difference, among members of the same sex, between the average mating success of individuals with a particular phenotype and that of individuals with other phenotypes. 1. INTRODUCTION In the introduction to On the Origin of Species, Darwin wrote, ‘‘a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that each species had not been independently created, but had descended . . . from other species . Nevertheless, such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modified. . .’’ (emphasis added). Thus, Darwin recognized that no theory of evolution would be complete if it failed to provide a plausible mechanism that could explain how living things change over evolutionary time. Darwin’s theory of evolution by natural selection provided such a mechanism. Yet, Darwin’s theory goes beyond explaining how living things change over time; it also explains the important concept of adaptation: the tendency for living things to evolve traits that make them so apparently well designed for survival and reproduction. Because of this broad explanatory power, Darwin’s theory ranks among the most important ideas in the history of human thought. Although the central concept of Darwin’s theory is natural selection, Darwin never attempted to measure selection in nature. Moreover, in the century following the publication of On the Origin of Species, selection was generally regarded as too weak to be observed directly in natural populations. Partly for these reasons, some early evolutionists even questioned selection’s ef- ficacy in driving evolutionary change. This view that selection is weak and cannot be measured has changed dramatically. Beginning in the 1930s, evolutionists demonstrated mathematically that natural selection alone could power evolutionary change and adaptation. Moreover, in the past three decades, selection has been detected and quantified in hundreds of populations in nature. These data demonstrate that not only does selection occur routinely in nature, but that it is often sufficiently potent to bring about substantial evolutionary change in a relatively short time period. Indeed, selection is now viewed as the cause of adaptive evolution within natural populations. 2. HOW PHENOTYPIC SELECTION WORKS Phenotypic selection takes place when individuals with particular phenotypes survive to reproductive age at higher rates than do individuals with other phenotypes, or when individuals with particular phenotypes produce more offspring than do individuals with other phenotypes. In either case, selection results in differential...