Fisher's fundamental theorem of natural selection is an idea about genetic variance[1][2] in population genetics developed by the statistician and evolutionary biologist Ronald Fisher.
[3] It states: Or in more modern terminology: The theorem was first formulated in Fisher's 1930 book The Genetical Theory of Natural Selection.
[4] Fisher likened it to the law of entropy in physics, stating that "It is not a little instructive that so similar a law should hold the supreme position among the biological sciences".
[6][7] Largely as a result of Fisher's feud with the American geneticist Sewall Wright about adaptive landscapes, the theorem was widely misunderstood to mean that the average fitness of a population would always increase, even though models showed this not to be the case.
The sophistication that Price pointed out, and that had made understanding difficult, is that the theorem gives a formula for part of the change in gene frequency, and not for all of it.