Abstract
THE CHANGE IN MEAN FITNESS FOR TWO LOCI UNDER SELECTION CAN BE DESCRIBED BY FOUR TERMS: (i) the variance of fitness, (ii) a weighted between-gamete covariance, (iii) a function of recombination, linkage disequilibrium and the slope of the surface of mean fitness on disequilibrium, and (iv) a function of these two parameters and the curvature of the surface. Independent derivations of this equation by different methods, although thought at one time to be in disagreement, give algebraically identical results.