For the most part, Todd discussed with us a work in progress on "Passive Diffusion Models, Explanatory Pluralism, and the Hierarchical Expansion of Evolutionary Theory." From that title the notion of "explanatory pluralism" proved particularly provocative.
Todd began with the question whether macro-evolutionary processes should be thought of as cumulative effects of micro-evolutionary processes. He asked us to entertain the following scenario. We can trace the development of numerous species within a clade back to a founding organism which was very small; indeed, its body-size was very near a limit of minimal body-size for an organism like it. Over the course of time the mean body-size of organisms in that clade shifted away from that boundary, i.e., shifted toward a larger body-size in a manner that can be modeled as random drift. Todd used Stephen Jay Gould's famous example to elucidate the notion of random drift away from a minimum boundary: the drunk who has a wall to his left and a ditch to his right is randomly staggering right or left but, over time, will predictably get away from the wall and into the ditch. Now, does this notion of random drift provide a sufficient causal explanation also in the case of the imagined macro-evolutionary process? Todd is not the only one who believes it does.
However, some view the "random drift" as the cumulative effect of numerous micro-evolutionary processes. Each of these is familiar enough: a variation in body-size is selected because of its adaptive value. Slightly more often than not larger body size has an adaptive advantage, and thus, the numerous branching lineages give the overall impression of drift away from the lower limit. Once all the branching lineages are filled in (and the attendant micro-evolutionary processes described), this could also provide a sufficient causal explanation of the macro-evolutionary process. And this is perhaps where the story ends and a lively discussion of Todd's explanatory pluralism begins.
Todd suggests that both causal accounts are adequate and that there is no question of deciding between them: these are just two ways of describing the same causal process -- we can describe it in terms or branching lineages, we can describe it merely as drift. Is this tenable or can there be only one true description?
Be that as it may. Todd points out that this focus on macro-evolutionary processes is an expansion of Darwinism in that it elaborates new models of explanation and that it helps us think about species-selection. Indeed, some of his publications probe related macro-evolutionary issues: "Hierarchical Theories of Macroevolution" appeared 1995 in the Annual Review of Ecology and Systematics (vol. 26, pp. 301-21); "Putting the Cart Back Behind the Horse: Group Selection Does Not Require that Groups Be Organisms" (Behavioral and Brain Sciences, vol. 17, 1994, pp. 622f.); and "Does Science Have a Global Goal? A Critique of Hull's View of Conceptual Progress" (Biology and Philosophy, vol. 9, 1994, pp. 85-97).
Alfred Nordmann
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