Understanding how selection interacts with genetic variation to produce biodiversity is a central theme in evolutionary biology. Many studies have taken advantage of the rich diversity of colouration in animals to tackle this, as colour is both ecologically relevant and a clearly visible phenotype. This has carried over into the ‘omics’ era, with plenty of studies addressing evolutionary questions by examining the genomics of colouration in natural populations. These studies tend to focus on discrete colour variation and pigmentation. However, most phenotypic variation is continuous, and little is known about the genetics of structural colour. Heliconius butterflies display warning colouration that boasts both striking diversity, alongside near-perfect convergence between mimetic species. Repeated evolution of pigment colour patterns is driven by the repeated use of a small set of genes. On the western slopes of the Andes, convergent iridescence has also evolved between the co-mimics Heliconius erato and Heliconius melpomene, which appears to vary continuously. In this thesis I (1) describe clinal variation in iridescence across hybrid zones between iridescent and non-iridescent subspecies of H. erato and H. melpomene and highlight a common selective agent (mimetic warning colouration), yet different migration-selection balance between the species. 2) I demonstrate a striking difference in levels of population structure between the co-mimics across their hybrid zones. However, in both species variation in iridescence is independent of population structure and is maintained by selection despite gene flow. (3) I describe the genetic architecture of iridescence in Heliconius, for the first time, using association mapping. Lack of power to estimate genetic architecture for H. melpomene prevented a thorough between-species comparison. However, I find potential evidence of overlapping genomic regions responsible for variation in iridescence. This thesis lays the groundwork for future research narrowing down the genetic underpinnings of iridescence in this system
