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My thesis aimed to investigate the influence of selection on an animal invading a novel habitat and how subsequent differences in natural selection, sexual selection and genetic drift might influence phenotypic variation among populations. The Pacific leaping blenny on Guam (Alticus arnoldorum) provided a novel opportunity to address these aims as it has made one of the most extreme ecological transitions possible, it is a fish that lives its entire adult life on land. Furthermore, the presence of multiple populations of this fish around the island allowed the investigation of how differences in selection pressures and gene flow might generate or limit phenotypic variation between populations that otherwise occupy the same broad habitat type (intertidal rocks on land). First, I examined the influence of natural selection in the form of predation on colonising a novel environment (here, a shift onto land). The colouration of male and female blennies from five populations was examined along with the colour of their respective backgrounds. I found the body colour of all populations closely resembled the habitat on land. A subsequent predation experiment confirmed that this background matching functioned to reduce predation and was therefore adaptive. These results suggested that closely resembling the colour of habitat backgrounds has probably aided the Pacific leaping blenny in successfully colonising land. Second, I examined the relative influence of natural selection (predation), sexual selection and gene flow on phenotypic variation among populations on land. Results suggested that variation in natural selection and sexual selection had a predictable influence on phenotypic expression: sexual selection has promoted the exaggeration of ornaments, while natural selection has reduced the conspicuousness of these features. Most notably, this population divergence in ornamentation has occurred despite high gene flow. Overall, my thesis demonstrates how adopting an integrative approach is essential for isolating the mechanisms leading to phenotypic divergence during the invasion of new environments, and how populations can subsequently diverge in response to variation in social factors and predation once colonisation has occurred.