COMMENTS ON THE CAUSES OF NATURAL SELECTION.

Wade and Kalisz (1990) discuss approaches to identifying the causes of natural selection. They state that previous statistical analyses (e.g., Mitchell-Olds and Shaw, 1987) give "the impression that the quality of an analysis is equated with sophistication in multivariate statistics," and have "overshadowed the fact that the analysis of selection is essentially an ecological and not a statistical question." Wade and Kalisz recommend several procedures (experimental manipulation, explicit consideration of environmental effects, and understanding of ecological mechanisms), which are billed as an "empirical complement" to MitchellOlds and Shaw (1987). We have previously discussed interpretation and limitations of selection analyses (Mitchell-Olds and Shaw, 1987; all page references are from this paper). In addition to statistical issues, we advocated experimentation as an essential ingredient for biological interpretation. For example, we and many others have noted that correlation, even in a multivariate regression analysis, is not sufficient to imply causation ("Perhaps the most desirable approach to model validation is experimental manipulation to avoid the weaknesses inherent in a correlational approach," p. 1159). We also suggested "explicit consideration of environmental effects, . . . [which] permits detection of environment specific differences in the relationship of the characters to fitness" (p. I 1 5 5). Beyond experimental manipulation, we also have suggested that "functional analyses of the ecological importance of the characters may be used to support inferences from the regression approach" (p. 1 159). When results from correlational or regression analyses are in accord with known physiological or ecological mechanisms, then biological inferences are greatly strengthened. Biologists have long recognized limitations on biological interpretations based on correlational analyses, and the importance of experimentation for resolution of these questions. These points were emphasized in our 1987 paper, and have been discussed by experimentalists for many years (e.g., "causes and mechanisms must also be demonstrated with critical experiments," Endler, 1986, p. 201; additional references in Mitchell-Olds and Shaw, 1987). Wade and Kalisz present these ideas as their own original invention, with no reference to previous workers. Furthermore, their discussion mentions our 1987 paper, then appears to contrast their own views: "We [Wade and Kalisz] are concemed with the research program after this estimate of the fitness function is obtained. We suggest experimental manipulations.... We too are interested in the best unbiased estimate .... However, we treat even the best estimate of the selection gradient through multivariate analysis as an 'hypothesis'.... It can only suggest causation." We applaud Wade and Kalisz's suggestions for explicit consideration of environmental effects, experimental manipulation, and understanding of ecological (as well as physiological) mechanisms, having previously advocated this approach. Wade and Kalisz build on the previous work of many biologists, and suggest useful extensions to provide improved ecological understanding. Regression analysis is an important method for obtaining partial understanding of possible pattems of natural selection, and for suggesting hypotheses for future research. However, such correlational analyses are never sufficient for inference of causal pattems of natural selection. Attempts to understand causation are reinforced by experimental manipulations or examination of physiological or ecological mechanisms. "Our understanding of evolutionary processes will be greatly strengthened by intensive, long-term studies of wild populations employing a variety of techniques" (Mitchell-Olds and Shaw, 1987, p. 1160).