Selection in Natural Populations

Both the beginning and the completion of this writing project were prompted by methodological innovations that profoundly influenced the questions asked by evolutionary biologists. I began the book to document the exciting developments in evolutionary biology fueled by studies of protein genetic variation. I was stimulated to finish the book, or at least to bring it to an end, by the realization that biologists were turning away from the use of protein polymorphisms, favoring the use of the rapidly expanding set of techniques to survey genetic variation of DNA. The common desire to use the most modern techniques has caused biologists to put aside some important issues, for the newest techniques are most appropriately applied to a different set of questions.

The use of electrophoresis and histochemical staining to survey genetic variation freed evolutionary biologists from the constraints that had limited classical geneticists to laboratory species. Electrophoretic techniques allowed evolutionary biologists to examine a wide variety of plants and animals, including such exotic species as coast redwoods, elephant seals, blue mussels, and American eels. The levels of genetic variation revealed by the electrophoretic surveys could not be accommodated by the existing models of fitness determination, stimulating some theoretical evolutionary geneticists to first propose and later assert that the protein genetic variation was adaptively neutral. At the same time, empirical geneticists found evidence of natural selection in their data. The neutralist-selectionist debate flared. But with more data and some thoughtful consideration, the neutralist and selectionist hypotheses were recognized as diametrically opposed and equally unrealistic alternatives. The natural world must lie between these extremes. Before the controversy was abandoned, laboratory and field studies inadvertently wedded the fields of physiological ecology and population biology, and revealed insights into the strength and variability of selection. The evolutionary insights from these interdisciplinary studies are the focus of this book.

Several strategies were employed to test whether protein polymorphisms were influenced by natural selection. Some biologists chose to sample a variety of samples and to test whether genetic variation changed over space in a pattern consistent with environmentally mediated selection. Others imposed selection in the lab or watched it in the field, while still others estimated selection coefficients associated with mating, survival, or fecundity. A few exemplary research programs include studies of enzyme kinetics, physiology, demography, and behavior, and these give us the fullest impression of how profoundly the genotypes segregating at a single locus can produce differences among individuals. Truly comprehensive studies, such as the studies of sickle-cell hemoglobin

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