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Positive and purifying (negative) selection: Classifying the forms of selection

Darwin thought of natural selection primarily in terms of fixation of beneficial changes. He realized that evolution weeded out deleterious changes, but he did not interpret this elimination on the same plane with natural selection. In the course of the evolution of Modern Synthesis, the notion of selection was expanded to include "purifying" (negative) selection; in some phases of evolution, this turns out to be more common (orders of magnitude more common, actually) than "Darwinian," positive selection. Essentially, purifying selection is the default process of elimination of the unfit. Nevertheless, defining this process as a special form of selection seems justified and important because it emphasizes the crucial role of elimination in shaping (constraining) biological diversity at all levels. Simply put, variation is permitted only if it does not confer a significant disadvantage on any surviving variant. To what extent these constraints actually limit the space available for evolution is an interesting and still open issue, and I touch on this later (see in particular Chapters 3, 8, and 9).

A subtle but substantial difference exists between purifying selection and stabilizing selection, which is a form of selection defined by its effect on frequency distributions of trait values. These forms include stabilizing selection that is based primarily on purifying selection, directional selection driven by positive (Darwinian) selection, and the somewhat more exotic regimes of disruptive and balancing selection that result from combinations of multiple constraints (see Figure 1-3).

Figure 1-3

Figure 1-3 Four distinct forms of selection in an evolving population: (A) Stabilizing selection (fitness landscape represented by solid line); (B) Directional selection (fitness landscape represented by solid line); (C) Disruptive selection (fitness landscape represented by solid line); (D) Balancing selection (fitness landscape changes periodically by switching between two dotted lines).

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