What is individual quality? An evolutionary perspective

In studies of population ecology, demography and life history evolution, among-individual differences in traits associated with survival and reproduction are often attributed to variation in 'individual quality'. However, often intuitive quality is rarely defined explicitly, and we argue that this can result in ambiguity about what quality actually is. Here we consider the various ways in which the concept of quality is currently applied, and show that subtle differences in intended meaning have very important consequences when the goal is to draw evolutionary inferences. We also propose a novel approach that is consistent with all current ecological uses, but also allows the concept of quality to be integrated with existing evolutionary theory.