Population Variation in Continuously Varying Traits as an Ecological Genetics Problem

The niche variation hypothesis is an adaptive explanation for variation within populations and for, the differences in variation between populations in morphological, physiological or behavioral traits. It has received only partial support from empirical tests and has been criticized on theoretical grounds. Recent quantitative genetic models have made an advance by exploring the effects of mutation, migration, mating pattern and selection on phenotypic variance. These models are reviewed and their most important features are integrated in a new model. In this model population variation is in a state of balance between the opposing forces of mutation and immigration, which tend to elevate variation, and selection and possibly genetic drift tending to decrease it. Populations exhibiting different levels of variation are interpereted as having different equilibrium points, and it is the task of empirical studies to determine the relative magnitudes of the opposing factors. An example is given from studies of Darwin's finches. Geospiza fortis varies more than G. scandens on Isla Daphne Major, Galápagos, in several morphological traits including beak and body size. This is explained, assuming equal mutation rates in the two species, as the result of more frequent genetic input to the G. fortis population, through occasional hybridization with immigrant G. fuliginosa, and relaxed stabilizing selection. Stabilizing selection is less intense on G.fortis than on G. scandens because the G. fortis population has a broader niche; there is both a within-phenotype and betweenphenotype component to the broad niche of G. fortis. The success of theory in explaining population variation is discussed, and it is concluded that empirical studies lag far behind theory.