An eye for change: Evolutionary consequences of visual plasticity

For a long time, biologists have been trying to understand the patterns of biodiversity. Darwin showed that the environment is an important factor in speciation, as it exerts the force of natural selection on properties of organisms. If this selection is aimed at heritable properties, then, gradually, a genetic difference will evolve and the property under selection will change. However, some properties are plastic and can change in response to the environment, without genetic change. This phenomenon, known as phenotypic plasticity, might affect speciation, but it is unclear if plasticity promotes or impedes speciation, if it plays a role at all. In this thesis, I studied the association between speciation and phenotypic plasticity of the visual system in African cichlid fish. Cichlids constitute one of the most species-rich vertebrate families and are found in a wide range of aquatic habitats, differing in light conditions. I explored the association between plasticity and speciation at multiple levels, combining gene expression data, data on the behaviour of fish and data on the habitats in which the cichlids occur. My results show that visual plasticity varies considerably between species, at all studied levels, but does not play a consistent role in the diversification of African cichlids, neither positively nor negatively. The results in my thesis also underline the importance of studying evolutionary questions at multiple levels, combining for example gene expression data with data on the behaviour. Only then will we be able to understand the evolutionary consequences of phenotypic plasticity.