Adaptation and Natural Selection

Adaptation and Natural Selection: A Critique of Some Current Evolutionary Thought is a 1966 book by the American evolutionary biologist George C. Williams. Williams, in what is now considered a classic by evolutionary biologists, outlines a gene-centered view of evolution, disputes notions of evolutionary progress, and criticizes contemporary models of group selection, including the theories of Alfred Emerson, A. H. Sturtevant, and to a smaller extent, the work of V. C. Wynne-Edwards. The book takes its title from a lecture by George Gaylord Simpson in January 1947 at the University of Princeton. Aspects of Williams' book were popularised by Richard Dawkins' in his 1976 book The Selfish Gene.

The aim of the book is to "clarify certain issues in the study of adaptation and the underlying evolutionary processes." Though more technical than a popular science book, its target audience is not specialists but biologists in general and the more advanced students of the topic. It was mostly written in the summer of 1963 when Williams utilized the University of California, Berkeley's library.

Adaption and Selection
Directed selection: What happens when certain values of a trait positively impact survival while values at the other extreme negatively impact survival? Assuming that the trait has a genetic component, over time the mean will move in the direction of the positive effect, and away from the negative effect. The shape of the distribution may change depending on a number of factors. With each generation, individuals that carry deleterious forms of the trait are likely to produce fewer off-spring than those that do not. Over a number of generations, the frequency of organisms with the disadvantageous trait will decrease. This process is known as directed selection. Why is it that the more genes that determine a trait, the more continuous is the distribution of that trait in a population? Does directed selection ever stop? That is, does the mean ever stop changing?

Conservative selection: Another possibility is that individuals with traits at either end of the population distribution are at a reproductive disadvantage compared with organisms that are closer to the mean. With each generation, the contribution of these "outliers" to the next generation will be reduced. The distribution mean will remain constant, but the standard deviation will decrease.The stronger the disadvantage the outliers face, the narrower the distribution will become. In the end, the size of the standard deviation will reflect the rate at which new variation enters the population. Disruptive selection: A third possibility is that the organisms with traits at the extremes of the population distribution actually have a reproductive advantage over those nearer the mean. This type of behavior would tend to expand the population distribution, while mating between organisms at the extremes would tend to bring the population distribution back to the original state. If organisms at the extremes preferred to mate with organisms with the same form of the trait, the population would split into two.

Summary(of the book)

 * 1) Introduction
 * 2) Natural Selection, Adaptation, and Progress
 * 3) Natural Selection, Ecology and Morphogenesis
 * 4) Group Selection
 * 5) Adaptations of the Genetic System
 * 6) Reproductive Physiology and Behavior
 * 7) Social Adaptations
 * 8) Other Supposedly Group-Related Adaptations
 * 9) The Scientific Study of Adaptation

In the first chapter Williams introduces some of his main arguments. Adaptation is "a special and onerous concept that should not be used unnecessarily". He writes that something should not be assigned a function unless it is uncontroversially the result of design rather than chance. For instance he considers mutations to be errors only, not a process that has persisted to provide variation and evolutionary potential. If something is considered (after critical appraisal) to be an adaptation, then we should assume the unit of selection in the process was as simple as possible, provided it is compatible with the evidence. For example selection between individuals should be preferred to group selection as an explanation if both seem plausible. Finally, Williams writes that the only way adaptations can come into existence or persist is by natural selection.

The next chapter deals with the idea of evolutionary progress. Firstly, it is argued that for natural selection to work, there have to be "certain quantitative relationships among sampling errors, selection coefficients, and rates of random change." It is put forward that Mendelian selection of alleles (alternative versions of a gene) is the only kind of selection imaginable that satisfies these requirements. Elaborating on the nature of selection, he writes that it only works on the basis of whether alleles are better or worse than others in the population, in terms of their immediate fitness effects. Survival of the population is beside the point, e.g. populations don't take any measures to avoid impending extinction. Finally he evaluates various ideas about progress in evolution, denying that selection will bring about the kind of progress that some have suggested. The author concludes that his view on the topic is similar to that of most of his colleagues, but worries that it is misrepresented to the public "when biologists become self-consciously philosophical".