Over at The New Atlantis Stephen L. Talbott has a great discussion of the vacuity of the idea of “fitness” as used in Darwinian theory. As we all know, Darwinian theory “predicts” that the “fittest” organisms will survive and leave more offspring. And what makes an organism “fit” under the theory? Why, the fact that it survived and left offspring. There is an obvious circularity here:
This is the long-running and much-debated claim that natural selection, as an explanation of the evolutionary origin of species, is tautological — it cannot be falsified because it attempts no real explanation. It tells us: the kinds of organisms that survive and reproduce are the kinds of organisms that survive and reproduce.
Darwinists counter the tautology charge by attempting to demonstrate that there are independent criteria (so-called “engineering criteria”) that explain reproductive success. For example, if a wolf runs faster, it will be more fit, and therefore the trait that gives the wolf the extra speed (longer legs perhaps) explains its fitness, not merely the fact that it did survive and reproduce.
However, the appeal to engineering criteria in the abstract does not by itself get us very far. As philosopher Ronald Brady reminded us when discussing this dispute in an essay entitled “Dogma and Doubt,” what matters for judging a proposed scientific explanation is not only the specification of non-tautological criteria for testing it, but also our ability to apply the test meaningfully. If we have no practical way to sum up and assess the fitness or adaptive value of the traits of an organism apart from measurements of survival rates (evolutionary success), then on what basis can we use the idea of survival of the fittest (natural selection) to explain evolutionary success — as opposed to using it merely as a blank check for freely inventing explanations of the sort commonly derided as “just-so stories.”
Here is the key sentence:
If we have no practical way to sum up and assess the fitness or adaptive value of the traits of an organism apart from measurements of survival rates . . .
What are you talking about Barry. Isn’t it obvious that a trait like the longer legs that help our wolf run faster will necessarily be beneficial in terms of fitness? Actually, no, it is not obvious. Ask any engineer and he will tell you there are always tradeoffs associated with engineering decisions. You want a faster car? Make it lighter. Is it a “better” car? Well, if by “better” you mean “faster,” of course it is. But if by “better” you mean “safer” maybe not, because a lighter car might not be as structurally sound as a heavier car. The same is true for engineering traits in animals. Talbott quotes two of the most famous Darwinists in history:
George Gaylord Simpson opined that ‘the fallibility of personal judgment as to the adaptive value of particular characters, most especially when these occur in animals quite unlike any now living, is notorious.” And in 1975, the geneticist Theodosius Dobzhansky wrote that no biologist ‘can judge reliably which ‘characters’ are useful, neutral, or harmful in a given species.’
One evident reason for this pessimism is that we cannot isolate traits — or the mutations producing them — as if they were independent causal elements. Organism-environment relations present us with so much complexity, so many possible parameters to track, that, apart from obviously disabling cases, there is no way to pronounce on the significance of a mutation for an organism, let alone for a population or for the future of the species.
None other than the famous Richard Lewontin (he of the “divine foot in the door” quotation) has illustrated the point:
A zebra having longer leg bones that enable it to run faster than other zebras will leave more offspring only if escape from predators is really the problem to be solved, if a slightly greater speed will really decrease the chance of being taken and if longer leg bones do not interfere with some other limiting physiological process. Lions may prey chiefly on old or injured zebras likely in any case to die soon, and it is not even clear that it is speed that limits the ability of lions to catch zebras. Greater speed may cost the zebra something in feeding efficiency, and if food rather than predation is limiting, a net selective disadvantage might result from solving the wrong problem. Finally, a longer bone might break more easily, or require greater developmental resources and metabolic energy to produce and maintain, or change the efficiency of the contraction of the attached muscles.
In summary, because all engineering decisions involve tradeoffs, there is no way to tell whether a particular engineering trait, in isolation, caused an organism to be more fit. And this drives us back to where we started. The only way to measure “fitness” is by reproductive success, which is obviously tautological if “fitness” is defined as “reproductively successful.”