Evolutionary Theory Attains Maturity

Early in the debate between the biometricians and the Mendelian saltationists, Pearson's former student George Yule (1871–1951) attempted to reconcile the two sides. He failed because the statistical methods available were inadequate. Two decades later, while Morgan's team was mapping the Drosophila chromosomes, Ronald Fisher began to introduce new statistical techniques, many of which became standard. Fisher succeeded where Yule had failed: he assimilated Mendelian genetics into Darwin's natural selection model of evolution. These studies culminated in 1930.

In his 1918 paper, Fisher critically analysed Pearson’s claims. Contrary to Pearson, he showed that dominance is better explained by discrete Mendelian traits than by blending inheritance. By 1922 he had adopted Johanssen’s term ‘gene’. His critique of the biometric school made him appear anti-positivist; indeed, his philosophical position seemed close to Morgan’s. He was never explicit about mechanistic materialism and he is remembered as a theorist not an experimentalist, but he used a deliberate analogy with physics to unify genetics with natural selection: ‘… the whole investigation may be compared to the analytical treatment of the Theory of Gases’. Like a physicist, he worked with idealised initial conditions, appealed to theoretical entities, and sought mathematical laws to encapsulate the phenomena of heredity and evolution.