An investigation into the evolution of hierarchy and its consequences for evolvability

Abstract

This thesis is concerned with the evolution of hierarchical modules in a model of gene regulation, and the consequences thereof for evolvability. Developmental processes map genotypes to phenotypes, and translate random variation at the genetic level into biased, selectable variation at the phenotypic level. These developmental processes are themselves subject to evolution by natural selection and it might be the case that natural selection favours developmental architectures that facilitate phenotypic variation that is adaptive and enhances evolvability. One manner of developmental organisation that has inspired much interest is modular hierarchy. Such hierarchy - where one gene directs many others - has the potential to be very important to evolvability because it effectively rescales the variability of phenotypes, enabling natural selection to search combinations of modules rather than combinations of individual genes. However, the conditions where natural selection favours hierarchical organisation and the conditions where its consequences enable such rescaling are not well understood. Considering a developmental model based on a recurrent regulatory process, we describe conditions where natural selection favours the evolution of single-layer hierarchical modular structures, where independent ‘switch’ genes direct independent subsets of genes. We show that these structures increase evolvability by rescaling the genetic neighbourhood of phenotypes, from combinations of genes to combinations of modules, and that this makes high-fitness phenotypes more accessible to natural selection. This improved evolvability enables a micro-evolutionary process to better exploit a changing or static modular environment so long as sufficient long-term variation is maintained. We then investigate the underlying cause of the evolution of hierarchy. Interestingly, we find that the observable increase in evolvability (in particular, the ability to rescale the variability of phenotypes) is not required for natural selection to favour hierarchy in this model. Rather, hierarchy evolves due to a selective pressure for efficient phenotypic expression and because it is an efficient organisation for increasing the expression of many genes given limited regulatory connections. Thereby, we show that - in some cases - the causes and consequences of developmental hierarchy are not the same. That is, hierarchy evolves - and it increases evolvabilty - but increased evolvability need not be the reason it was favoured by selection.

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ORCID for Richard Watson: orcid.org/0000-0002-2521-8255

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