  The theory trades pervasively in probabilities. To take a simple case: if there are three possible combinations of alleles at a given locus in a population, we can characterize the outcome of a reproductive cycle as ‘chance’ if each of the three possible combinations occurs at a frequency determined strictly by the laws of probability. In any given case of reproduction, we would say, which genotype emerged is a matter of chance.  Given the fact that evolutionary biologists, especially in so far as they take their cues from population genetics, deal with large populations conceived as ‘gene pools’, and think of evolution as long run changes in the frequencies of different combinations of genes from generation to generation, it is clear that, in this sense, chance permeates contemporary Darwinism.  The models of population biology provide a means of assigning probabilities to various outcomes, given information about population size, rates of mutation and migration (themselves given as averages and estimates). That is, as Darwin notes, being relatively better adapted increases an organism’s ‘chances’, i.e. increases its probability, of leaving viable offspring.  It does not guarantee it.  Since natural selection is a stochastic process, Darwinians from Darwin to the present rightly characterize it in terms of influencing the ‘chances’ of a given outcome, given variables such as selection pressure, population size or mutation rates.