One of the challenges for evolutionary biology is to explain complex adaptations in terms of the genes underlying them. Changes in an organism’s genes (genotype) can alter an organism’s appearance (phenotype) – whether you have brown or blue eyes, for example – but understanding how these changes lead to precise matching of organism to its environment is more difficult. This question is at the heart of the field of ecological genetics. One of the things ecological geneticists are interested in is how natural selection leads to changes in the organisation and function of an organism’s genes and how selection acting on phenotypes can lead to long-term changes in the genome (the entire DNA sequence of an organism).
Mimetic butterflies offer us one way of approaching this problem. Unlike in many natural systems, it is very obvious what adaptive value changes in wing pattern have because we know that mimicry protects against predation. Unlike many larger animals, butterflies can be bred quite easily under experimental conditions, allowing us to study their genetics by looking at how different wing patterns are passed on between parents and offspring – how colour patterns are inherited. As a result, many different species of butterfly are used as models to understand how evolution takes place in natural populations.
One model for Batesian mimicry is the African butterfly Papilio dardanus. When Papilio dardanus was bred in captivity it was clear that the polymorphism was determined by a single locus, termed H, with different female wing patterns determined by alternative alleles (gene variants) at H. The complexity of the wing patterns in terms of black pattern and background colour led to the hypothesis that H was determined by a ‘supergene’ or a tightly-linked complex of multiple genes which were inherited as a single unit.
We are now working with Papilio dardanus to uncover how supergenes evolve, how wing colour pattern is determined in development, and how the huge diversity of wing patterns within this species in maintained.