   Some philosophers (notably Sober, 2013) have argued that evolutionary fitness is a property of populations and not of individual organisms, or alternatively that fitness is a property of traits and not of the individuals that possess them. Individual fitness values are, they argue, empirically inaccessible and predictively useless. If trait fitness is defined in terms of the average fitness value of individuals bearing the trait, then it will turn out that there are individual fitnesses and individual fitness differences, even if it is the case that evolutionary biologists’ interests are confined to trait fitnesses. This claim has been challenged (cf. Pence and Ramsey 2014): sometimes biologists estimate trait fitnesses by studying individual fitnesses (Endler 1986).