Francesca Merlin

Developmental Noise: Explaining the Specific Heterogeneity of Individual Organisms

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 phenotypic 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.

Mapping Biological Transmission: An Empirical, Dynamical, and Evolutionary Approach

The current debate over extending inheritance and its evolutionary impact has focused on adding new categories of non-genetic factors to the classical transmission of DNA, and on trying to redefine inheritance. Transmitted factors have been mainly characterized by their directions of transmission (vertical, horizontal, or both) and the way they store variations. In this paper, we leave aside the issue of defining inheritance. We rather try to build an evolutionary conceptual framework that allows for tracing most, if not all forms of transmission and makes sense of their different tempos and modes. We discuss three key distinctions that should in particular be the targets of theoretical and empirical investigation, and try to assess the interplay among them and evolutionary dynamics. We distinguish two channels of transmission (channel 1 and channel 2), two measurements of the temporal dynamics of transmission, respectively across and within generations (durability and residency), and two types of transmitted factors according to their evolutionary relevance (selectively relevant and neutral stable factors). By implementing these three distinctions we can then map different forms of transmission over a continuous space describing the combination of their varying dynamical features. While our aim is not to provide yet another model of inheritance, putting together these distinctions and crossing them, we manage to offer an inclusive conceptual framework of transmission, grounded in empirical observation, and coherent with evolutionary theory. This interestingly opens possibilities for qualitative and quantitative analyses, and is a necessary step, we argue, in order to question the interplay between the dynamics of evolution and the dynamics of multiple forms of transmission.

The (In)Determinism of Biological Evolution: Where Does the Stochastic Character of Evolutionary Theory Come From?

Evolutionary theory is readily acknowledged to be stochastic in that it only enables one to make probabilistic predictions, for instance regarding changes in genotypic frequencies within given populations. However, the very origin of this stochastic character has been the focus of much philosophical debate. Is it due to an inherent indeterminism? Or rather to epistemic limitations? In this chapter, we review some of the major arguments that have been exchanged on the topic recently. We argue that settling the issue would require first to answer the question of the relative contribution of the different factors of evolution. This leads us to defend a more nuanced vision of the origin of the stochastic character of evolutionary theory.