Gradualism as a Constraint on Theorising in Comparative and Evolutionary Psychology

In From Signal to Symbol (2021, p. x) Ron Planer and Kim Sterelny argue that any “adequate” theory of language evolution “must identify a plausible trajectory from great-apelike communicative abilities to those of modern humans where each step along the way is small, cumulative and adaptive (or at least not maladaptive: there might be some role for drift)”. They are not alone in invoking such a constraint. Gradualism is cited as an important assumption amongst those concerned with the evolution of cognition and the nature of animal minds going back to Darwin’s mental continuity thesis (Darwin 1871 [2013]). Gradualism is often invoked by scholars in pushing back against the anthropocentric allure of human uniqueness, the idea being that the postulation of the evolution of entirely novel cognitive capacities in our lineage alone is evolutionarily implausible. Indeed, Planer and Sterelny call the capacities such theories postulate “miracles” (p. 213). In this paper, I explore the evolutionary justification for such claims in comparative and evolutionary psychology in light of work on gradualism in evolutionary developmental biology (evo-devo). I ask, are evolutionary trajectories made up of “small, cumulative and adaptive” steps indeed more evolutionarily plausible than those that postulate entirely novel cognitive capacities within lineages? If so, why? One reason one might question the gradualist assumption (or at least suggest it needs to be applied with more care) comes from evidence that, although change at the genetic level is typically gradual, gradual genetic evolution is not always associated with gradual phenotypic evolution (Moczek 2008). As understanding of the relationship between genes and phenotypes in development has grown, so too has an appreciation of the important role played by neutral evolution and other processes in the evolution. At least from the perspective of evo-devo, these developmental processes undermine any bald gradualist assumption based in the gradualism of micro-evolution — even if genetic evolution is gradual, one cannot assume that phenotypic evolution will be. Another justification for gradualism lies in the randomness of variation. It is much more likely for large random phenotypic changes to be deleterious than small ones. Given this, we expect that most large phenotypic shifts will fail to persist and propagate in populations (Calcott 2011). Again, here, work in evo-devo on plasticity and other mechanisms of adaptation suggests that there are ways that developmental systems have evolved to make large adaptive shifts in phenotype possible (Moczek 2008) and undermines any bald gradualist assumption. This article explores these and other justifications for a gradualist assumption in comparative and evolutionary psychology. Ultimately, I offer a novel account of gradualism as a constraint on theorising in comparative and evolutionary psychology which better reflects contemporary evolutionary developmental biology. References Calcott, B. 2011. Wimsatt and the Robustness Family: Review of Wimsatt’s Re-engineering Philosophy for Limited Beings. Biology and Philosophy. 26:281-293. https://doi.org/10.1007/s10539-010-9202-x Darwin, C. 1871. The Descent of Man and Selection in Relation to Sex. Wordsworth Editions Limited (2013), Hertfordshire, UK. Moczek, A. P. 2008. On the Origins of Novelty in Development and Evolution. BioEssays. 30:432-447. https://doi.org/10.1002/bies.20754 Planer, R. J. and Sterelny, K. 2021. From Signal to Symbol: The Evolution of Language. The MIT Press.