Tesina elaborada para obtener el MPhil en la Universidad de Cambridge, Inglaterra, 1987Most biologists would nowadays agree that a good shorthand formula that describes evolution is "the change in frequencies of alleles in a population's gene pool". Another formulation widely accepted is that of the conditions for evolution to be produced by natural selection: that there be "differential reproduction of heritable fitness variation". As it happens, the causal appeal that distinguishes driven "deterministic" selection processes from purely statistical error sampling (drift) relies on an appeal to such a troublesome concept as fitness differences. Some authors have tried to avoid the problem by making use of statistical concepts (as "nonrandom" or "consistent") to qualify natural selection processes, and recently Hodge(1987) emphatically defended the use of "nonfortuitous" as the proper qualification for differential reproduction of heritable variation due to natural selection, because it stresses the causal character of the difference. The analysis of what the causal appeal in this case amounts to is central for the understanding of how explanations of evolution through natural selection are ultimately justified. Parallel to the so called "logic" of inferences based on natural selection, there is the task of giving a satisfactory general account of its causal workings.