In general, as pointed out above, those features and behaviors that are considered adaptations are explained teleologically. This is simply because adaptations are features that come about by natural selection.

Among alternative genetic variants that may arise by mutation or recombination, the ones that become established in a population are those that contribute more to the reproductive success of their carriers. "Fitness" is the measure used by evolutionists to quantify reproductive success. But reproductive success is usually mediated by some function or property. Wings and hands acquired their present configuration through long-term accumulation of genetic variants adaptive to their carriers. How natural selection yields adaptive features may be explained with examples where the adaptation arises as a consequence of a single gene mutation.

One example is the presence of normal hemoglobin rather than hemoglobin S in humans. One amino acid substitution in the beta chain in humans results in hemoglobin molecules less efficient for oxygen transport. The general occurrence in human populations of normal rather than S hemoglobin is explained teleologically by the contribution of hemoglobin to effective oxygen transport and thus to reproductive success.

A second example, the difference between peppered-gray moths and melanic moths is also due to one or only a few genes. The replacement of gray moths by melanics in polluted regions is explained teleologically by the fact that in such regions melanism decreases the probability that a moth be eaten by a bird. The predominance of peppered forms in nonpolluted regions is similarly explained.

Not all features of organisms need to be explained teleologically, since not all come about as a direct result of natural selection. Some features may become established by random genetic drift, by chance association with adaptive traits, or in general by processes other than natural selection. Proponents of the neutrality theory of protein evolution argue that many alternative protein variants are adaptively equivalent. Most evolutionists would admit that at least in certain cases the selective differences between alternative amino acids at a certain site in a protein must be virtually nil, particularly when population size is very small. The presence in a population of one amino acid sequence rather than another, adaptively equivalent to the first, would not then be explained teleologically. Needless to say, in such cases there would be amino acid sequences that would not be adaptive. The presence of an adaptive protein rather than a nonadaptive one would be explained teleologically; but the presence of one protein rather than another among those adaptively equivalent would not require a teleological explanation.