Does selection intensity increase when populations decrease? Absolute fitness, relative fitness, and the opportunity for selection

The variance in relative fitness, commonly called the opportunity for selection, is a measure of the maximum amount of selection that can occur in a population. I review the relation between fitness variance and population growth, showing that fitness variance is higher during periods of population decline. This is true both for survival and for commonly used models for variable descendant number (Poisson, negative binomial, generalized Poisson). Empirical evidence suggests that not just the opportunity for selection but also the actual selection occurring is commonly greater during such periods of population reduction.