The Evolution of Flight and Wing Shape in Heliconius Butterflies

Understanding how important ecological traits are driven by divergent selection and identifying the factors that shape their evolution can provide important insight on how species evolve. Flight is thought to have been one of the main drivers of insect diversification, allowing the colonisation of new ecological niches. Studies on flight have found that it is affected by anatomy, physiology and wing shape. However, wing shape has also evolved in response to a number of other selective pressures; the multiple selective pressures can constrain the effects of selection in certain directions, sometimes leading to trade-offs. The neotropical genus of Heliconius butterflies provides an excellent system to study different evolutionary processes owing to our extensive knowledge of their ecology and well-developed genomic resources. They are famous for their bright aposematic colour patterns, a classic example of Müllerian mimicry; but less is known about how mimicry affects other phenotypes, although evidence suggests that flight and wing shape are also involved in the mimetic signal. Furthermore, there is still a lot left to understand about how wing shape and flight mimicry are connected; and while the genetics of Heliconius colour patterns is well-researched, nothing is known about the genetic control of other ecological traits important in speciation such as wing shape. Using phylogenetically corrected ecomorphological analyses in over 600 specimens of the Heliconius and Eueides genera, I have demonstrated genus wide convergence of wing shape between mimics and habitat types with evidence of different selective regimes acting on the two wings, owing to aerodynamic constraints on the forewing. These patterns of convergence are strongly driven by one mimicry group, the silvaniform, which mimics the model species of Ithomiine and females converge more strongly with the models, suggesting constraints in males. In the second chapter, I use experimental manipulations of wing shape to determine whether wing shape differences between two sister species, H. elevatus and H. pardalinus, with divergent colour patterns can explain differences in flight behaviour (wing beat frequency and wing angle). Stronger divergence is found in the hindwing shape, consistent with previous results, and wing shape is correlated with flight measurements. However, wing differences measured do not appear to drive differences in flight, suggesting the two may be independently driven by mimicry. Finally, I carry out Quantitative Trait Loci (QTL) analyses to identify the underlying genomic structure of wing shape in F2 crosses of H. elevatus and H. pardalinus and identify two QTLs associated with wing shape. One QTL is found in a region of elevated divergence, consistent with evidence of divergence with gene flow in these species. Therefore, in this thesis, I present an integrative approach to understanding the evolution of flight and wing shape in Heliconius. I demonstrate the importance of understanding the interactions between different ecological factors driving wing shape evolution and underline the need to understand how wing shape and flight are connected. Finally, I identify the first QTLs involved in wing shape in Lepidoptera and provide further insight on the evolutionary history of two sister species.