ABSTRACT
When selection acts on viabilities, the fitnesses are probabilities of surviving. Whether selection acts on viabilities or on fertilities, fitness describes how the frequencies of traits can be expected to change. Fitness is analogous to solubility. The only problem with the propensity interpretation is that it fails to provide a noncircular interpretation. As the propensity interpretation of fitness states, there are two ways to find out about the fitnesses of traits in a population. In addition, the variation in fitness that occurs within a population will have its source in the complex nexus of relationships that connects organisms to their environments and to each other. Thedosius Dobzhansky discovered fitness differences among various chromosome inversions in Drosophila. If fitness explains the change in trait frequency that occurs in the Drosophila population, then the fly's thick thorax does not.
