 Ecological genetics is the broad field of studies that investigates the relationship between genetic change and features of the biotic and abiotic environment. In population genetic theory, the role of the environment in adaptive evolution tends to be hidden in the recursion equations for gene frequency dynamics, where the common assumption of a constant selection coefficient implies a constant relationship between genotype, phenotype and fitness irrespective of environment. For ecological genetics, in contrast, the central focus is the causal relationship between genetic variation and environmental variation, where conspicuous (especially discrete) phenotypic variation is assumed to have an underlying genetic component. The advent of molecular tools and the development of molecular ecology within ecological genetics has greatly broadened the conceptual scope of the field within the last fifteen years.