  The question arises however of what sort of a probabilistic propensity the definition invokes. One candidate that has not found much favour among philosophers is that the probability invoked by the definition is long run relative frequency. To begin with, owing to periods of environmental and genetic instability, actual frequencies may vary rapidly while still approaching their long run relative frequencies (conditional on the counterfactual assumption that environment and genes had been stable). This makes it difficult to confirm measures of fitness so defined. Moreover, treating single case probabilities as long run relative frequencies is subject to a long-standing objection. In the biological case it is particularly serious owing to the role of small numbers (e.g., founder effects) in evolution. Finally, if long run relative frequencies reflect dispositions of organisms or populations to leave descendants, the question arises whether these dispositions are grounded on some occurrent properties or not. If not then long run relative frequencies turn out to be or immediately give way to objective chances. (See Earman 1985, 149.)