The genetical theory of natural selection.

The first coherent general algebraic analysis of Mendelian population behavior. The work contains Fisher’s rigorous development of his “fundamental theorem of natural selection”–”the rate of increase in fitness of any organism at any time is equal to its genetic variance in fitness at that time.”

"Fisher's principle is an evolutionary model that explains why the sex ratio of most species that produce offspring through sexual reproduction is approximately 1:1 between males and females. It was famously outlined by Ronald Fisher in his 1930 book The Genetical Theory of Natural Selection^[1] (but incorrectly attributed to Fisher as original^[2]). Nevertheless, A. W. F. Edwards has remarked that it is "probably the most celebrated argument in evolutionary biology".^[2] Specifically, Fisher couched his argument in terms of parental expenditure, and predicted that parental expenditure on both sexes should be equal. Sex ratios that are 1:1 are hence known as "Fisherian", and those that are not 1:1 are "non-Fisherian" or "extraordinary" and occur because they break the assumptions made in Fisher's model.^[3] Many eusocial wasps, such as the Polistes fuscatus and the Polistes exclamans seem to exhibit such a ratio at times." (Wikipedia article on Fisher's principle, accessed 03-2017).

Along with Wright (No. 253.1) and Haldane (No. 254), Fisher established mathematical population genetics.