   It has also been held that trait fitness and fitness difference are both something more than and different from the average of individual fitnesses and fitness differences. On this view, trait fitness is a property of populations and, in part, their population structure—that is, the ways in which traits are distributed in the population. An example (to be explored further below) is reflected in cases where the fitness of a trait depends not just on its incidence in a population, but also on its variance within. A trait’s variance in a population cannot be reflected in individual fitness any more than the entropy of a volume of gas can be reflected in the properties of any of the individual molecules that compose it and contribute to its entropy. This suggests that trait fitness is not reducible to, definable in terms of, or even entirely supervenient on the relationship between individuals (e.g., organisms) and their environments.