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The traditional history of the understanding of chance in evolution, as told by those like Depew and Weber (1995), goes roughly as follows: for Darwin, evolution is a non-statistical theory (because Darwin predates statistics) of a non-chancy process (natural selection, taken to be analogous to artificial selection). Francis Galton introduces statistics into the study of evolutionary theory in his work on regression and the Law of Ancestral Heredity. Sewall Wright, then, introduces a more robust notion of chancy evolutionary processes when he proposes that drift, in his shifting-balance model, is capable of actively, yet probabilistically, driving populations down the adaptive landscape, away from a selective optimum.
This history thus asks two questions: (1) When did evolutionary theory become statistical? (2) When did evolutionary processes come to be seen as chancy? While both these questions are certainly interesting and while the two standard answers to them may well be correct I argue that they miss a vital shift in thinking about chance in biology that was happening well before Sewall Wright. Two of Galton's students, W.F.R. Weldon and Karl Pearson, founded what would come to be known as the biometrical school, dedicated to the statistical study of evolutionary phenomena. Further, they both spent extensive time considering the philosophical grounding of their statistical study of biology. The concerns they had, however, do not map cleanly onto the two questions asked by the standard history: on each of these questions, Weldon and Pearson appear to differ very little from the views of their mentor Galton.
I argue, then, that it is time to deploy a new perspective when evaluating the views of these early evolutionists. Rather than searching for the ontic or reified sort of chance implied by the standard history's question (2), we can find more profitable results if we consider the work of those like Weldon and Pearson from a different angle: What is the relationship between (statistical) biological theories and the processes they describe? Further, when we examine the positions of Pearson and Weldon on this question, we find both that the two men, who are commonly thought to agree on nearly all points of interest, diverge in important and significant ways, and that this divergence parallels a heated debate in contemporary philosophy of biology: that between causal and statistical interpretations of natural selection, fitness, and genetic drift.