COMMENTARY Explanation and prediction and the maintenance of sexual reproduction

Evolutionary biology has often sat rather uneasily with fundamental principles of scienti®c explanation. Hempel (1965) has pointed out that, in science, there is an equivalence of explanation and prediction. In other words, a theory and/or a set of observations A can be said to explain a set of observations B if, and only if, B is predictable from knowledge of A. This criterion, widely accepted in epistemological philosophy, is often not satis®ed in adaptive explanations of the phenotype. Frequently, an evolutionary explanation for a pheno-typic trait is postulated, but one in which the trait is logically predicted by the explanatory idea only if quantitative values of unobserved parameters fall in a certain range. Measurement of the parameters can be dauntingly dif®cult. What, regrettably often, happen