Evolving Systems. Phenotypic Plasticity In The Darwinian Research Program

Evolutionary biology has made remarkable progress in recent decades. Research in the field of epigenetic inheritance, the integration of development into the study of evolutionary processes (evo-devo) and its ecological dimension (eco-evo-devo) has led to a growing number of innovative empirical results. The theoretical implications of these discoveries for the structure of evolutionary theory, however, remain unclear and have become the subject of heated debate. According to some 'reformist' biologists, evolutionary biology cannot accommodate these new approaches without profound change; for them, an Extended Evolutionary Synthesis (EES) is the inevitable outcome. In contrast, 'conservative' biologists maintain that these findings do not contradict the classical theoretical structure and can be accommodated within it. The thesis aims to clarify this debate by focussing on a single phenomenon: phenotypic plasticity ( i.e. the ability of an organism to express different phenotypes in response to environmental variation). Phenotypic plasticity, long recognised in evolutionary biology, is frequently cited by proponents of the EES. Moreover, it operates at multiple levels (genes, organisms, environment) and spans several disciplines (quantitative genetics, population genetics, evo-devo and eco-evo-devo), making it particularly suitable for analysing possible changes in the structure of evolutionary theory. The thesis is divided into three main articles. The first, entitled 'Plasticity-Led Evolution: A Matter of Causal Relevance', analyses the debate surrounding the so-called 'Plasticity-Led Evolution Hypothesis' (PLE). It argues that the dispute revolves around the ‘explanatory range’ of models based on phenotypic plasticity relative to classical models, and on their capacity to account for the emergence of novel phenotypic variation. Drawing on the interventionist approach to causality in biology, the article contends that a distinguishing feature of PLE is the causal role it attributes to the environment and to developmental mechanisms. The second article, 'Changing While Trying to Stay the Same: The Case of Maladaptive Phenotypic Plasticity', examines the phenomenon of maladaptive plasticity. Long neglected, this phenomenon has recently regained attention and appears to play an important role in the generation of novel variation. The article argues that evolution via maladaptive plasticity can be understood as a case of PLE through a process of "creative stabilisation". Maladaptive phenotypic plasticity can indeed trigger a more rapid process of adaptive divergence, which, however, tends to restore the ancestral phenotype. In doing so, it can open new developmental pathways that later provide the basis for the emergence of further variation. This picture resonates with the early insights of Ivan Schmalhausen, one of the pioneers of research on phenotypic plasticity. The third article, 'The Double Side of Phenotypic Plasticity: Genetic Reaction Norm or Developmental Bias?', explores the inherent duality of phenotypic plasticity: it may be conceived either as a property of the genome (the genetic reaction norm) or as a property of the developmental system (developmental bias). The article compares these two approaches and highlights the specific issues that characterise a developmentally-oriented perspective. The analysis proceeds in two steps. First, it revisits the debate on the "proximate–ultimate distinction" (PUD) to clarify theoretical problems that arise when integrating developmental mechanisms into evolutionary inquiry. Second, it presents a comparative study of papers that adopt the two approaches while analysing the same model system, the horned dung beetle. The aim is to show the distinctive features and explanatory advantages of studying the developmental mechanisms that underlie plastic responses in these organisms.