Fisher's fundamental theorem of natural selection

Fisher's fundamental theorem of natural selection is an idea about genetic variance in population genetics developed by the statistician and evolutionary biologist Ronald Fisher. The proper way of applying the abstract mathematics of the theorem to actual biology has been a matter of some debate.

It states:

"The rate of increase in fitness of any organism at any time is equal to its genetic variance in fitness at that time."

Or in more modern terminology:

"The rate of increase in the mean fitness of any organism, at any time, that is ascribable to natural selection acting through changes in gene frequencies, is exactly equal to its genetic variance in fitness at that time".