Having previously commented on R. A. Fisher's views on epistasis, I have noticed another relevant passage in The Genetical Theory of Natural Selection:

Each successful gene which spreads through the species, must in some measure alter the selective advantage or disadvantage of many other genes. It will thus affect the rates at which these other genes are increasing or decreasing, and so the rate of change of its own selective advantage. The general statistical consequence is that any gene which increases in numbers, whether this increase is due to a selective advantage , an increased mutation rate, or any other cause, such as a succession of favourable seasons, will so react upon the genetic constitution of the species, as to accelerate its increase of selective advantage if this is increasing, or to retard its decrease if it is decreasing. To put the matter in another way, each gene is constantly tending to create genetic situations favourable to its own survival, so that an increase in numbers due to any cause will in turn react favourably upon the selective advantage which it enjoys. The Genetical Theory of Natural Selection, Dover edn., pp.102-3

It would be hard to find a stronger statement of the pervasive role of epistatic fitness in evolution. But I dare say the myth that Fisher 'did not believe in epistasis' will persist.

Labels: Burbridge, Population genetics