The Interplay Between Selection And Constraints On Adaptive Divergence And Phenotypic Evolution

Abstract: Under natural or sexual selection, individuals with advantageous traits or combinations of traits will be more successful than their peers at surviving and/or reproducing. Provided these traits are heritable, meaning that they have a genetic basis, the traits combinations which are selected for, will increase in frequency in the population. When selection is intense and persistent, adaptive traits may become ubiquitous in the population, and we may then say that this population has evolved and become adapted. However, this process might not always occur rapidly. This is because adaptive evolution occurs only when the individuals of a population are diverse in their trait combinations and when there is a significant amount of genetic variation for the trait(s) upon which selection is acting. However, if the adaptive optimum of a combination of traits is situated in a direction where there is little variation available, adaptation will be slowed down, and we will say it is constrained. This is precisely what I attempted to study in this thesis. The outcome of the interplay between selection and constraints might lead to evolution, to divergence between populations, and finally to the emergence of new species and biodiversity. By using different statistical techniques used in quantitative genetics or geometric morphometrics, combined with behavioral and breeding experiments, I tried to draw some conclusions on the role of constraints both in the early stages of adaptation and divergence (using isopod lake populations as a model system) and in the latter stages of divergence and speciation (using damselfly species as model organisms). My main conclusions are that in the context of strong divergent selection, constraints may be overcome and adaptation may proceed, provided that gene flow between populations is restrained.