Darwin's phylogenetic reasoning

19 May 2017

Elliott Sober
Department of Philosophy
University of Wisconsin at Madison


Darwin is mostly remembered for his theory of evolution by natural selection, but common ancestry is a fundamental part of his theory, and he had interesting ideas about the epistemology of common ancestry. Darwin claimed that (1) similarity is evidence for common ancestry, and that (2) adaptive similarities provide weaker evidence for common ancestry than neutral or deleterious similarities provide. In this talk I'll use the law of likelihood to formulate and evaluate these two claims. The law of likelihood says that observation O favors hypothesis X over hypothesis Y precisely when Pr(O|X) > Pr(O|Y). It turns out that Darwin's claim (1) has a likelihood justification, given assumptions that are widely used in phylogenetics. However, violating those assumptions allows similarities to be evidence against common ancestry. As for Darwin's claim (2), it has a likelihood justification for some adaptive similarities, but not for others.

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