A central goal of evolutionary biology is to understand the processes responsible for morphological, genetic and behavioural differentiation between sexes and among geographically distinct populations. Perhaps the most significant processes are genetic drift, natural selection, phenotypic plasticity and sexual selection. The main aim of this thesis was to investigate differentiation among individuals and populations of the sex-role reversed pipefish (Syngnathus typhle) and, consequently, determine which processes may be responsible for emerging patterns. This unique species is characterised by males predominately choosing amongst displaying females.
In this thesis I revealed, on a microgeographic scale, morphological differentiation without genetic divergence among populations. Interestingly, females differed in size whereas the males did not. For females in this sex-role reversed species, the costs of expressing a plastic phenotype may be outweighed by the potential gains from greater survivorship, higher fecundity or increased mating success. Thus, females gain the ability to make themselves as conspicuous and attractive to males as possible in the specific environment they are living. Moreover, behavioural experiments, which focussed on describing “personalities”, reproductive investment strategies, and mate-sampling tactics, also indicated that males as well as females had the behavioural plasticity required to adjust to the environment in which they live. To this end, using video playbacks as experimental stimuli may be especially rewarding in this species.
Overall, the studies in this thesis acknowledge the ability of species to fine-tune their phenotype to maximise fitness and, therefore, highlight the importance of considering patterns of differentiation in an environment-specific context.