Plasticity-Led (Not First) Evolution: A Matter of Causal Relevance

Abstract Phenotypic plasticity has long been a phenomenon studied in evolutionary biology, but in recent decades it has attracted renewed interest among biologists. This interest has culminated in the so-called plasticity-led evolution hypothesis, according to which phenotypic plasticity, under certain circumstances, is capable of acting as an evolutionary factor, influencing the direction of adaptive change and promoting the emergence of new phenotypic variation. This article aims to analyze this hypothesis and the controversies it has generated and to bring more clarity to a debate that is often characterized by ambiguity and conceptual confusion. We will show how this debate can best be understood as a debate about the explanatory range of the two rival (but not mutually exclusive) models, plasticity-led evolution and mutation-led evolution, and their ability to explain the origin of phenotypic variation. We will then make use of Woodward’s account of causality in biology to assess the “specific weight” that the causes theorized by plasticity-led evolution can have within evolutionary explanations. To this end, we will analyze one of the best-known cases of plasticity-led evolution in nature, the North American spadefoot toad. Our analysis leads us to conclude that models based on plasticity-led evolution can be regarded as autonomous explanans, irreducible to the action of natural selection on preexisting genetic variation. This is due to the causal role played by environmental stimuli, which is accorded greater significance than in classical approaches. Furthermore, the plasticity-led evolution hypothesis could pave the way for a reassessment of the role of development in evolutionary processes.