 The focus of traditional ecological genetic research on the current action of natural selection has been broadened in several ways over the last two decades. First, whereas the early studies tended to focus on evolution in single populations, there is now a significant emphasis in ecological genetics on the population genetic structure of metapopulations and the roles of migration, extinction, and colonization on evolutionary and adaptive processes (e.g., Goodacre 2002). Secondly, whereas the earliest studies emphasized conspicuous polymorphisms or chromosomal rearrangements and their influence on fitness, the advent of biochemical genetics in the late 1960s significantly broadened the phenotype, beginning with the application of electrophoretic methods to population studies. These studies revealed abundant “hidden polymorphism” in the new, biochemical phenotype of enzyme mobility. These methods extended the domain of ecological genetics from the classic “conspicuous phenotypic polymorphisms” in color, shape and behavior to the physiological domain of enzyme function. The new emphasis on functional biochemical phenotypes, however, did not change the explanatory or causal framework of the field. Determining the role of natural selection in maintaining enzyme polymorphisms, such as the fast/slow polymorphisms of alcohol dehydrogenase (an enzyme involved in the detoxification of environmental alcohols), superoxide dismutase (which catalyzes the removal of free oxygen radicals), or the esterases (which are involved in the detoxification of pesticides in many insects), became a primary focus of investigation with the goal of finding a selective basis for the enzyme variants in terms of differences in their physical and kinetic properties. Indeed, the roots of controversy between the selectionist and neutralist schools over the maintenance of “balanced” polymorphisms (see Lewontin 1974) lie in the controversy over random genetic drift versus natural selection in early ecological genetic research (see below).