Developmental bias

Developmental bias refers to the greater accessibility of certain changes to development relative others. While there are several reasons this might be the case owing to various aspects of the gene regulatory network structure of development, my research into developmental bias has focused on plasticity and its potentially causative evolutionary role in so-called plasticity-led evolution.

Although plasticity has traditionally been regarded as an organismal trait under selection like many others, and hence a product and not cause of evolution, recent developments in evolutionary theory have called this perspective into question. Most notably the conceptualization of development as a constructive rather than programmed process has reshaped our thinking on this issue. Programmed development is the classical genes-as-blueprints understanding of how genotypes are translated into phenotypes. Constructive development, on the other hand, refers to an idealization of development in which a growing organism receives cues from both genetic and environmental factors. This allows for plasticity, as a form of developmental bias, to direct developmental and hence evolutionary trajectories. However, this perspective of plasticity as an evolutionary leader is not yet fully integrated into evolutionary biology as a whole. In my theoretical work on this topic I have approached this issue philosophically as well as palaeontologically.

Evolutionary explanations

Philosophically, I have worked towards mapping out the conceptual issues that cause this view of plasticity to be more readily embraced by some researchers than others. In short, this is due to differences in the simplifying idealizations adopted by evolutionary biologists, which vary due to field of study, methodological limitations, theoretical perspective and personal preference. For example, while a population geneticist might idealize evolution in a species or population as occurring in discrete generations, we know that in the natural world generations might not be so perfectly separated. Such idealizations are not made unnecessarily – they facilitate ease of computation, generality of models or strength of prediction – yet they nevertheless present a challenge to scientists as broadly shared or methodologically encoded assumptions have a tendency to become theoretical principles of a field. Such theoretically or methodologically integral principles are subsequently difficult to challenge or revise.

In the case of plasticity as a developmental bias, the key idealization or principle preventing a more wide-spread understanding of plasticity as a putative driver of evolution is the artificial distinction of selection, heredity and variation into three separate processes. In other words, novel variation is idealized as arising independently of selection (randomly) and selection between variants is assumed to not affect how they are inherited. Separating these process in this manner is a prerequisite for using the analytical tools of many sub-disciplines of evolutionary biology. But this separation also screens from view exactly the kind of causal complexity implied by plasticity’s suggested role as a driver of evolution.

For example, if the relationship between heredity and selection among variants may be altered, e.g. through emerging environmentally induced behavioural complexity, then the idealization of these processes as separate breaks down as they are no longer distinct. In our paper [pdf] on this topic, my co-authors and I argue that it is the challenge of this separated idealization of selection, heredity and variation that generates resistance to the notion of plasticity as an evolutionarily meaningful form of developmental bias.

Plasticity-led evolution in the fossil record

Plasticity-led evolution is conceptually challenging for the reasons given above. It is also difficult to study empirically in living populations because evolutionary change takes place over longer periods of time than are afforded to researchers. For this reason, paleobiological investigation of the fossil record is a tempting prospect when searching for evidence of plasticity-led evolution.

However, a challenge to detecting macroevolutionary evidence for plasticity-led evolution is the broad similarity between its predicted patterns in the fossil record and the predicted patterns of “regular” evolution. Both predict a shift in phenotypic variation following environmental change. However, “regular” evolution achieves this by gradual adaptation through randomly arising mutations yet plasticity-led evolution achieves this through environmentally mediated means.

In my palaeontological work on the issue of plasticity as a developmental bias directing evolution, I disentangle these predictions and provide directions for morphometric work seeking to identify the subtly distinct signature of plasticity-led evolution in the fossil record. This research is published here [pdf] and informs my ongoing work with Cambrian trilobite-like agnostids.