Is natural selection tautological?
Summary: The claim that evolutionary theory is a tautology rests on a misunderstanding of the theory. Fitness is more than just survival.
The simple version of the so-called 'tautology argument' is this:
Natural selection is the survival of the fittest. The fittest are those that survive. Therefore, evolution by natural selection is a tautology (a circular definition).
The real significance of this argument is not the argument itself, but that it was taken seriously by any professional philosophers at all. 'Fitness' to Darwin meant not those that survive, but those that could be expected to survive because of their adaptations and functional efficiency, when compared to others in the population. This is not a tautology, or, if it is, then so is the Newtonian equation F=ma, which is the basis for a lot of ordinary physical explanation.
Writing in National Review Online in December of last year, conservative commentator Tom Bethell expressed the main point more clearly:
“Darwin's claim to fame was his discovery of a mechanism of evolution; he accepted “survival of the fittest” as a good summary of his natural-selection theory. But which ones are the fittest? The ones that survive. There is no criterion of fitness that is independent of survival. Whatever happens, it is the “fittest” that survive — by definition."
Let us begin our reply to this argument in the most direct way possible. It is asserted that within evolutionary theory, the fittest organisms are defined as those who survive. This is the crux of the argument, and it is completely incorrect. In reality, the fittest organisms are the ones who, based on their physical characteristics and the environment in which they find themselves, would be expected to leave the most offspring.
Let us imagine that we have perfect information about the environment in which a population of organisms finds itself. Let us further suppose that we are aware of the full range of extant heritable variation within the population. In those circumstances we could make some definite statements about the future evolution of that population. A group of scientists could examine that information and come to a consensus about which members of the population were the fittest. Plainly there are criteria for fitness independent of mere survival.
This is not the whole story, however. Predicting the future is only a very small part of what evolutionary biology is all about. Most of the interesting events in evolution took place in the distant past. Unraveling and explaining that past presents scientists with a problem almost perfectly opposite to the one considered in the previous paragraph. Instead of trying to predict the future evolution of a species given information about its present environment, now we are trying to understand ancestral environments given information about what sorts of creatures survived.
…In this context scientists will, indeed, hypothesize that traits that persisted and developed over long periods of time did so because of the fitness advantages they conferred on their possessors. But here’s the catch: that’s the beginning, not the end, of the investigation.
The assumption that the trait under investigation emerged from the prolonged result of natural selection is used to generate testable hypotheses about the creatures in question. In his book Plan and Purpose in Nature, biologist George C. Williams provides the following example:
Productive use of the idea of functional design, in modern biological research, often takes this form: an organism is observed to have a certain feature, and the observer wonders what good it might be. For instance, dissection and examination of a pony fish shows it to have what looks like a light-producing organ, or photopore, and even a reflector behind it to make it shine in a specific direction. So we accept the conclusion that the organ is good at producing light, but the obvious question then becomes, What good is light? The pony fish photopore is deep inside the body. Can it really be adaptive for a fish to illuminate its own innards?
The organ is situated above the air bladder, and the light shines downward through the viscera. The pony fish is small and its tissues are rather transparent. Some of the light gets through and produces a faint glow along the ventral surface. But what is the use of a dimly lit belly? Perhaps it makes the pony fish more difficult to see in the special circumstances in which it lives. It inhabits the open ocean, where it may move toward the surface as darkness approaches, but spends the daylight hours far below at depths where the light is exceedingly dim by our standards, detectable only as a murky glow from above.
Williams goes on to describe how this hypothesis led to experiments that confirmed that the pony fish’s glow has the intensity it ought to have if its primary function was to provide camouflage. This is a nice illustration of how selection-based reasoning is used in scientific practice.
The reasoning used by scientists in this way is comparable to what historians do in trying to understand why certain events happened the way they did. An historian studying nineteenth century America might begin his investigation with the fact that the North won the Civil War. From this starting point he will naturally ask himself what advantages the North had that allowed them to emerge victorious over the South. But the assumption that the North had such advantages will not be the sum total of his investigation. And no one would consider it reasonable to object to his work on the grounds that it is based on circular reasoning.
Well, populations of organisms that survive through long stretches of evolutionary history are likewise the victors in a war, this time for survival. It is perfectly reasonable to assume that those that survived had certain advantages over those that did not. Determining the precise nature of those advantages might pose a difficult practical problem, but the assertion that those advantages existed is surely unproblematic.
We have thus provided two answers to the tautology objection. The first is that its central premise, that there are no criteria of fitness independent of survival, is false. The second is that natural selection is not applied in practice in the simplistic way the phrase “Survival of the fittest,” suggests. Instead, scientists use selection based reasoning to develop specific, testable hypotheses about the organisms under investigation.
"Survival of the fittest" is sometimes claimed to be a tautology. The reasoning is that if we take the term "fit" to mean "endowed with phenotypic characteristics which improve chances of survival and reproduction" (which is roughly how Spencer understood it), then "survival of the fittest" can simply be re-written as "survival of those who are better at surviving".
However this criticism fails to consider that the expression "survival of the fittest", when taken out of context, is actually a very incomplete account of Darwinian evolution. The reason is that this expression does not mention a key requirement for Darwinian evolution, namely the requirement of heritability. Darwin's mechanism of evolution through natural selection implies that heritable variations lead to differential reproductive success, and therefore (precisely because they are heritable) become over-represented in the next generation. If the characters which lead to differential reproductive success are not heritable, then no meaningful evolution will occur, "survival of the fittest" or not. In other words, Darwinian evolution by natural selection does not simply state that "survivors survive" or "reproducers reproduce"; rather, it states that "survivors survive, reproduce and therefore propagate any heritable characters which have affected their survival and reproductive success".
When the full picture is considered, no tautology exists: the complete mechanism leading from heritable fitness-impacting differences, through differential reproductive success ("survival of the fittest"), to actual adaptive evolution (change in the makeup of lineages toward better adaptation) is a valid, informative reasoning, hinging on the testable hypothesis that such fitness-impacting heritable variations actually exist.
It would be romantic to think that a huge, overwhelmingly accepted branch of science, such as the theory of evolution by natural selection, could be brought crashing down by somebody spotting an elementary flaw in its logic that could be comfortably explained to bright schoolboy. There's one in the eye for the so-called experts!
Sadly, it would also be somewhat far-fetched.