Natural Selection, Fitness Determination, and Molecular Variation
The principal unit process in evolution is the substitution of one gene for another at the same locus.
Haldane, "The Cost of Natural Selection" ( 1957)
Although evolutionary biologists are known for their diversity of opinions, a majority would probably accept the two following generalizations. First, most natural populations contain abundant genetic variation. Morphological ( Endler 1986) and physiological ( Feder et al. 1987) variation is common, and variability for life-history variation also is apparent in natural populations ( Clutton-Brock 1988; Dingle and Hegmann 1982). Surveys of allozyme variation, mitochondrial DNA, and moderately repetitive DNA all reveal abundant genetic variation. Second, selection is common in natural populations. Antonovics ( 1971) summarized natural selection for tolerance to soil contaminants in plants, and Linhart and Grant ( 1996) summarized examples of natural selection generating and maintaining microgeographic variation in plants. Endler ( 1986) compiled studies of selection, which showed that selection was often intense, with selection coefficients exceeding .2 (figure 1.1) in about half the studies that reveal natural selection. We know little about how these two generalizations relate to each other. Furthermore, although selection is common and genetic variation is abundant, we do not know to what degree the abundance of genetic variation is influenced by selection.
In this book I examine the data relating molecular variation to selection in natural populations. After reviewing the methods used to detect genetic variation (chapter 2), I discuss the associations between levels of genetic variation and environmental heterogeneity (chapter 3). Next I present several case studies that demonstrate how genetic variation at a protein locus can influence biochemistry, whole animal physiology, and fitness (chapter 4).
The remaining chapters consider the implications of selection on many loci, the determination of fitness, and several of the evolutionary implications of selection for physiological variation. The following discussion in this chapter describes the focus of the later chapters of the book.