The (in)determinism of biological evolution: where does the stochastic character of evolutionary theory come from?

International audienceDoes biological evolution play dice? To what extent is it (in)deterministic? Biologists readily acknowledge that the theory of evolution is stochastic insofar as it only enables one to make probabilistic predictions as regards the way genic and genotypic frequencies change in populations over generations. The issue at stake in a recent and lively debate in philosophy of biology concerns the very origin and nature of such stochasticity. This debate seems to have been ignited, among others, by a section of Elliott Sober’s The Nature of Selection (1984): in this book, Sober examines the possibility that some macroscopic evolutionary phenomena might by influenced by some underlying microscopic indeterminism, and, in particular, through the “percolation” of quantum indeterminism. The debate truly began in 1996 when Robert Brandon and Scott Carson published an article in which they argue for an indeterministic point of view against Alex Rosenberg’s (1988, 1994) and Barbara Horan’s (1994) deterministic theses. In 1999, Rosenberg and Horan refined their arguments and elaborated a response to Brandon and Carson’s attacks, in collaboration with Leslie Graves. Other contributions to the indeterministic thesis came from David Stamos (2001) and Bruce Glymour (2001). Roberta Millstein (1996, 2000b, 2003b) assessed that the debate was going toward a dead end, and defended a rather agnostic attitude towards this issue of the origin and nature of the stochastic character of evolutionary theory. In this chapter, we first examine the main arguments exchanged in this debate, which happen to be somewhat polarised around two extreme theses. We then argue that the answer to the question at stake (i.e., the origin of the stochastic character of evolutionary theory) requires that one first answers the question of the relative contribution of the different factors of evolution. This leads us to defend a more nuanced vision of the origin of the stochastic character of evolutionary theory