Natural Selection, Fitness Determination,. and Molecular Variation

Abstract 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.