Abstract
Understanding how adaptive traits arise and are maintained is crucial to evolutionary biology, especially in landscapes increasingly altered by human activity. In insects, colour polymorphisms are often an evolutionary response to predation or environmental heterogeneity. Widespread deforestation has dramatically reshaped freshwater ecosystems in New Zealand, in turn altering predator communities and the distribution of chemically defended species. This thesis investigates the genetic and environmental drivers of melanism in the stonefly Zelandoperla fenestrata, a colour-polymorphic species that is a part of a Batesian mimicry system with the chemically defended stonefly Austroperla cyrene.
I genotyped a 7 bp indel at the ebony locus that had previously been linked melanism in 48 Z. fenestrata population samples across the lower South Island. This allowed me to map the frequency of the melanic allele across the geographic range of Z. fenestrata and to analyse how environmental factors influence the distribution of melanism. My results indicate that melanism is widespread across clade 1 of Z. fenestrata and is present in three of four geographic sub-clades. However, it was unevenly distributed, ranging from complete absence to complete fixation among different population samples. Notably, northern population samples were largely fixed for melanism, while north-western population samples lacked the melanic allele entirely. Eastern and Western population samples were predominantly polymorphic.
Statistical analyses revealed a strong association between melanism and forested habitats, with significantly higher melanic allele frequencies in forested streams compared to deforested areas. Conversely, elevation showed a negative relationship with melanism frequency, while distance to the coast had no significant effect. These patterns closely mirror the distribution of A. cyrene, supporting the hypothesis that melanism in Z. fenestrata is maintained primarily through frequency-dependent selection associated with Batesian mimicry. While melanism appears to be maintained as a polymorphism in the Eastern sub-clade, this is not the case in the other sub-clades. The Western sub-clade showed relatively low, patchy frequencies of melanism, which may have been driven by post-glacial recolonisation and are consistent with strong mimicry-based selection in forested habitats. In contrast, the Northwestern sub-clade lacked melanism entirely, likely reflecting the long-term absence of the selective conditions or limited gene flow introducing the melanic allele. In contrast, the Northern sub-clade appeared fixed for melanism despite regional deforestation, suggesting possible historical founder events followed by reduced adaptive flexibility under recent habitat change.
By sequencing a small fragment ebony across 151 clade 1 samples, I found that there is likely a single evolutionary origin of melanism within clade 1. In contrast, North Island Z. fenestrata exhibited melanic and non-melanic individuals despite lacking the characteristic indel associated with melanism in clade 1, suggesting that a different genetic mechanism controls mimicry in these populations. The occurrence of similar melanic phenotypes in both regions represents convergent evolution between North Island populations and clade 1. These results demonstrate how a conserved pigmentation gene, such as ebony, can underpin complex, geographically structured adaptive variation in human-modified landscapes and highlight the interplay between population history and ecological selection in maintaining mimicry polymorphisms.