Field Studies of Natural Selection

by

Jeffrey K. Conner

• LAST REVIEWED: 10 May 2023
• LAST MODIFIED: 22 February 2018
• DOI: 10.1093/obo/9780199941728-0097

Introduction

Natural selection can be defined as a non-random relationship between a phenotypic trait and fitness. Therefore, both traits and fitness are central to the study of natural selection, but both can be difficult to define and measure. Complex organisms can have enormous numbers of traits; it is most fruitful to define traits as some characteristic that performs a function for the organism. Examples of traits include morphological traits like wings, fins, and legs that function for movement; behavioral displays that function in mate attraction; and physiological traits that underlie photosynthesis in plants and energy use in all organisms. Fitness can be even more difficult to define; in general, fitness refers to the ability of an organism to survive and reproduce and, thus, have descendants in future generations. An excellent estimate of fitness for field studies of selection is the lifetime number of offspring produced, but most of these studies to date have been limited to less comprehensive estimates of fitness. Natural selection is fundamental to all life on earth because it is the only process that produces adaptations, that is, phenotypic traits that increase fitness by solving a problem the organism faces in its environment, such as avoiding predators and disease or finding food, shelter, and mates; these are the functions of the adaptive traits. Adaptations are what prevent extinction, at least temporarily. Field studies of natural selection are crucial to our understanding of evolution because they enable us to estimate the strength of ongoing selection as well as understand the underlying mechanisms of selection. This article focuses on field studies of selection on phenotypes and the mechanisms underlying this selection, not on adaptation itself, which is the result of past selection; thus, studies of local adaptation and the molecular signals of past selection are not included here. See the separate Oxford Bibliographies articles “Adaptation” and “Phenotypic Selection” for more details. However, showing that there is direct selection on a trait in the field, that is, that there is likely a causal relationship between a trait and fitness, is some of our best evidence that the trait is adaptive.

Major Books

The study of natural selection began with Darwin’s Origin of Species (Darwin 2010, originally published in 1859), but fell out of favor again until the “Modern Synthesis”; the main books from the synthesis that dealt with field studies of natural selection were Dobzhansky’s Genetics and the Origin of Species (Dobzhansky 1937), Mayr’s Systematics and the Origin of Species (Mayr 1942), and Stebbins’s Variation and Evolution in Plants (Stebbins 1950). The next major books studies of selection in the field came with Ford’s Ecological Genetics (Ford 1964) and Endler’s Natural Selection in the Wild (Endler 1986). The most recent books on natural selection with some focus on field studies is Bell’s Selection: The Mechanism of Evolution (Bell 2008). For more on the early work on natural selection see the separate Oxford Bibliographies in Evolutionary Biology article “Natural Selection.”

• Bell, G. 2008. Selection: The mechanism of evolution. 2d ed. Oxford: Oxford Univ. Press.

  In some ways an update of Endler 1986, but quite different in its approach and broader in its scope—this book reviews field studies of selection as well as studies of natural selection in the laboratory and artificial selection by humans.

• Darwin, C. 2010. On the origin of species by means of natural selection. New York: Modern Library.

  Originally published in 1859 (London: John Murray). Laid out abundant evidence for Darwin’s mechanism of adaptive evolution, that is, natural selection.

• Dobzhansky, Th. 1937. Genetics and the origin of species. New York: Columbia Univ. Press.

  Argues that population genetics, including selection, is consistent with genetic differences between species.

• Endler, J. A. 1986. Natural selection in the wild. Princeton, NJ: Princeton Univ. Press.

  Reviews studies of selection in the field to that point in time, which now included more complex traits; the book includes a quantitative analysis of the strength of selection from estimates available, foreshadowing many later syntheses (see Syntheses).

• Ford, E. B. 1964. Ecological genetics. London: Methuen.

  This book presents better evidence for natural selection actually occuring in the field than had been previously available, focusing on simple polymorphic traits. Four editions were published, the last in 1975.

• Mayr, E. 1942. Systematics and the origin of species. New York: Columbia Univ. Press.

  Takes Dobzhansky’s argument down one level to show that differences among populations within animal species could arise through population genetic processes, again including selection.

• Stebbins, G. L. 1950. Variation and evolution in plants. New York: Columbia Univ. Press.

  Presents evidence that Mayr’s arguments apply equally well to plants.