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“Variational tendencies: development as an ultimate cause”
The separation between ultimate and proximate causes is one of the most classical topics in the philosophy of biology. The ongoing debate over the Extension of the Evolutionary Synthesis (EES) includes a wide variety of philosophical topics, among which is a revision of such separation in the light of new evolutionary research agendas. In particular, it has been argued that some proximate causes, such as the development of organisms or their ecological interactions, play an evolutionary role in the sense that they are a necessary step in evolutionary transformations. While this has partially blurred the distinction between proximate and ultimate causes for local evolutionary changes, it has left relatively untouched the philosophical ideas about general tendencies in evolution, natural selection and chance remaining to be considered the main general driving evolutionary forces. Contrary to this view, in this paper I argue that the variational tendencies studied in evolutionary-developmental biology, or evo-devo, (evolvability, variability, modularity, robustness and plasticity) are ultimate causes in a sense that overcomes the specificities of local evolutionary changes. These properties, studied through genotype-phenotype maps, refer to structural features of how variation is produced in reproduction, and are increasingly being introduced in predictive models of evolution. I defend that these properties are better understood as propensities, analogous to how fitness and drift are usually seen in the philosophy of biology. While they are realized in particular developing systems, they refer to general features shared across taxa and affecting the production of variation in systematic ways, falling into the traditional category of ‘ultimate’ causes. I conclude with some ideas about how this consideration of variational tendencies can affect the notions of chance and contingency in evolution.