History and Philosophy of Science and Medicine - How Statistics Changed Natural Selection

André Ariew, University of Missouri, Columbia

Abstract: According to a standard story found in textbooks and historical treatments, the 20th century “modern synthesis” is a unification of Mendelian genetics with Darwin’s theory of evolution by natural selection. Yet, in his 1952 presidential address to the Royal Statistical Society R.A. Fisher—one of the pioneering modern synthesists—presents a sharp rebuke of Darwin’s version of natural selection. Fisher claims that Darwin was unaware that the argument for the theory of natural selection is “manifestly statistical” although much of the relevant statistical theory was available in Darwin’s day. Instead Darwin based his theory on the unrealistic rhetoric of Robert Malthus’s doctrine of over-reproduction which invokes non-existent violent struggles between organisms in order to provide the impression to his “thick-headed audience” that adaptive speciation is the product of cosmic forces. Is Fisher right that Darwin was unaware of the statistical methods of his day? What does it mean for natural selection to be “manifestly statistical”? What makes Darwin’s theory not statistical? What role does Malthus’s doctrine of overproduction play in Darwin’s theory? My attempt to answer these questions and evaluate Fisher’s critique has provided me with a long and fruitful research project in the history and philosophy of evolutionary science that has culminated in a book manuscript. I will be presenting the main discoveries and themes. Contrary to Fisher’s claim, Darwin was not only conceptually aware of the statistical methods of his day, he used them to further develop his theory of natural selection and principle of divergence. However, Darwin did not regard the phenomenon of adaptive speciation as manifestly statistical as Fisher claims, but manifestly ecological. I will explain the difference in terms of two kinds of natural large scale effects that emerge out of individual variation. I trace the distinction to a treatise written in 1713 by William Derham concerning harmonies of nature. The philosophical upshot of my corrective to Fisher is that the standard story about the modern synthesis is misleading. The modern synthesis version of natural selection is not only a different theory than that of Darwin’s version but a different kind of theory of the dynamics of evolution. No two ways of looking at population dynamics could be more different.