Developmental noise : explaining the specific heterogeneity of individual organisms
Résumé
Recent research in molecular developmental biology has shown that the stochastic character of development (i.e., developmental noise) can produce phenotypic heterogeneity even in the absence of any other source of change (genetic and environmental). More precisely, developmental noise triggers phe- notypic heterogeneity amongst the members of a clonal population (synchronic heterogeneity) and even within an individual organism over time (diachronic heterogeneity), in a stable and homogeneous environment. This paper deals with such stochasticity in order to explore its epistemological relevance and role, both as explanans and as explanandum. First, I investigate whether developmental noise is part of the explanation of the physical characteristics of individual organisms (i.e., the phenotypic outcome of development). Then, I try to assess whether or not heterogeneity due to stochastic events in development can be explained by a selective-evolutionary history. My final aim is to argue for the two following theses. First, from the developmental point of view, I argue that developmental biologists need to take into account developmental noise in order to explain the uniqueness of each individual organism and its own heterogeneity over time, at the phenotypic level at least, that genetic and environmental changes cannot explain alone. Second, from the evolutionary point of view, I critically evaluate explanations of developmental stochasticity in term of adaptation, in particular the idea that noise is a trait that has been selected to increase the capacity of natural populations to evolve (“evolvability”). Then, I identify other ways in which biologists should try to explain developmental noise. I conclude by highlighting the limits of any univocal explanatory approach in biology.