Population genetics largely rests on a ‘standard model’ in which random genetic drift is the dominant force, selective sweeps occur infrequently, and deleterious mutations are purged from the population by purifying selection.
Studies of phenotypic evolution in nature reveal a very different picture, with strong selection and rapid heritable trait changes being common.
The time-rate scaling of phenotypic evolution suggests that selection on phenotypes is often fluctuating in direction, allowing phenotypes to respond rapidly to environmental fluctuations while remaining within relatively constant bounds over longer periods.