Developmental bias

In evolutionary biology, developmental bias refers to the production against or towards certain ontogenetic trajectories which ultimately influence the direction and outcome of evolutionary change by affecting the rates, magnitudes, directions and limits of trait evolution. Historically, the term was synonymous with developmental constraint, however, the latter has been more recently interpreted as referring solely to the negative role of development in evolution. In modern evolutionary biology, the idea of developmental bias is embedded into a current of thought called Structuralism, which emphasizes the role of the organism as a causal force of evolutionary change. In the Structuralist view, phenotypic evolution is the result of the action of natural selection on previously ‘filtered’ variation during the course of ontogeny. It contrasts with the Functionalist (also “adaptationist”, “pan-selectionist” or “externalist”) view in which phenotypic evolution results only from the interaction between the deterministic action of natural selection and variation caused by mutation. The rationale behind the role of the organism, or more specifically the embryo, as a causal force in evolution and for the existence of bias is as follows: The traditional, neo-Darwinian, approach to explain the process behind evolutionary change is natural selection acting upon heritable variation caused by genetic mutations. However, natural selection acts on phenotypes and mutation does not in itself produce phenotypic variation, thus, there is a conceptual gap regarding the connection between a mutation and the potential change in phenotype. For a mutation to readily alter a phenotype, and hence be visible to natural selection, it has to modify the ontogenetic trajectory, a process referred to as developmental reprogramming. Some kinds of reprogramming are more likely to occur than others given the nature of the genotype–phenotype map, which determines the propensity of a system to vary in a particular direction, thus, creating a bias.