Comments on Natural Selection
It would be more correct to say that natural selection is mainly concerned with preventing evolution, not causing it. Much selection is concerned with the elimination of low-fitness genotypes produced by mutation or recombination. It can be at a generally intense level, but vary so in direction and strength at different times or places that little cumulative change takes place.
G. Williams, Sex and Evolution ( 1975)
I never intended this summary of evolutionary genetics to be comprehensive. I have focused primarily on enzyme polymorphisms and have gained some critical insights by comparing and contrasting the variation of enzyme polymorphisms with the variation of DNA sequences. Like all books, this one reflects the writer's personal experiences and favored hypotheses. This book is mainly a summary of empirical data, and from this I want to emphasize several points.
Enzymes -- as well as functional proteins such as hemoglobin, transferrin, and haptoglobin -- are catalysts that allow metabolic reactions to proceed at temperatures much lower than would otherwise be possible; metabolic rates would be imperceptibly slow without the aid of enzymes. Enzymes must effectively bind to substrates, change conformation to catalyze a reaction, and then release the products of the reaction. All this must be accomplished in a range of temperatures and a set of concentrations of the reacting ingredients determined by the ecology and physiology of the species. But more than allowing reactions to occur at physiological temperatures, enzymes control and regulate metabolism to enable honeybees to fly, cheetahs to sprint, and ponderosa pine to capture carbon from the air. These are not trivial side effects of randomly assembled neutral mutations but adaptations orchestrated by natural selection. Some fish are biochemically tuned for bursts of speed, others for endurance swimming, and still others for neither of these. The metabolic characteristics in these fishes are no more inter-