(1.) See C H Waddington, "Epigenetics and Evolution," Sympozia of the Society of Experimental Biology, 7:1956, 186-99: "Experiments in Canalizing Selection
," Genetics Research, (Camb.) 1:1960, 140-50: "Genetic Assimilation," Advances in Genetics, 10:1961, 257-93: "Towards a Theoretical Biology," Nature, 218:1968, 525-27.
A decelerating fitness function causes the trajectory to shift toward the less-steep region of the surface, corresponding to canalizing selection on development.
Equation (13) shows that whereas the strength of directional selection goes as [[Sigma].sup.2], the population variance, the strength of canalizing selection goes as [[Sigma].sup.4].