"Some evolutionary biologists are of the opinion that it is not necessarily the fittest that survive through the evolutionary process, but those that are best adapted to the requirements of evolution. Others have emphasized that survival of the organism is not as important as its fecundity. In both cases the problem of predictability remains. In a symposium volume celebrating 100 years of Darwinism the prominent geneticist Waddington (1960, p. 385) evaluates the matter of fecundity. He states:
Natural selection, which was at first considered as though it were a hypothesis that was in need of experimental or observational confirmation, turns out on closer inspection to be a tautology, a statement of inevitable although previously unrecognized relation. It states that the fittest individuals in a population (defined as those which leave most offspring) will leave most offspring.
Another problem associated with the untestability of evolutionary theory is that the theory explains too much. Grene (1959) points out that "whatever might at first sight appear as evidence against the theory is assimilated by redefinition into the theory." Evolutionary theory is broad enough to accommodate almost any data that may be applied. Two ecologists Birch and Ehrlich (1967) emphasize this. They state:
Our theory of evolution has become, as Popper described, one which cannot be refuted by any possible observations. Every conceivable observation can be fitted into it. It is thus 'outside of empirical science' but not necessarily false. No one can think of ways in which to test it.
No matter what is observed, there usually is an appropriate evolutionary explanation for it. If an organ or organism develops, it has positive survival value; if it degenerates, it has negative survival value. If a complex biological system appears suddenly, it is due to preadaptation. "Living fossils" (contemporary representatives of organisms expected to be extinct) survive because the environment did not change. If the environment changes and an evolutionary lineage survives, it is due to adaptation. If the lineage dies, it is because the environment changed too much, etc. Hence the concept cannot be falsified. Platnick (1977) states that this type of situation "makes of evolutionary biologists spinners of tales, bedtime storytellers, instead of empirical investigators."
(Ariel A. Roth, 'DOES EVOLUTION QUALIFY AS A SCIENTIFIC PRINCIPLE?'
from: http://www.grisda.org/origins/04004.htm )