   The disagreement about whether fitness is a property of biological individuals (genes, genotypes, organisms, families, populations and other groups or even species) as opposed to a property of traits of these individuals, has made significant another debate about the explanatory role of fitness in the theory of natural selection. Among those who endorse trait fitness (differences), and not individual fitness (differences), as a fundamental explanatory factor in natural selection, there is a dispute about whether its explanatory role reflects its causal force, or whether trait fitness is a purely statistical concept employed in the theory of natural selection to make demographic predictions about future trait distributions, without entering into any causal nexus among the objects so to speak. Thus, Matthen and Ariew (2002, 56) write, “Unlike vernacular [individual] fitness, predictive [trait] fitness is not a cause of selection, or of evolution for that matter…. [P]redictive fitness is a measure of evolutionary change not a cause.” This view confers an obligation on those who embrace it to explain how the theory of natural selection can provide contingent explanations without trading in causal relations among its explanatory variables.