The Variable Niche Hypothesis Revisited an Analysis of Intra- and Inter-Specific Differences in Bill Variation in Parus

Abstract

The “Variable-Niche” or “Niche-Variation” hypothesis (van Valen 1965) was propounded as one of several explanations for the maintenance of genetic variation in wild populations. It suggests that different individuals in a population may be differently suited to different parts of the species’ nichespace. The model was formulated to consider the maintenance of continuous variation in populations and had three requirements. First that “individuals of some set a survive or reproduce better than those of some set b in some environment A, while the reverse is true in some environment B”. Secondly that “the above difference between a and b is in part genetic”, and thirdly that “some appropriate mechanism of distributional or mating preference exists”. Van Valen suggested that this last requirement could be met either by appropriate differences between groups in habitat preferences, or by nonrandom mating with respect to these groups. Despite an insistence that the model could explain continuous variation, it is easiest to see how it might work if the groups represent discrete classes within the population, especially sex (Willson 1969).