If one phrase could be said to have percolated into popular consciousness as summing up Darwin’s theory of natural selection, that phrase would be “survival of the fittest”. Which is rather a pity since that phrase is not how scientists state the principles of Darwinism. That’s because the phrase is tautological.
In Darwinian terms “fitness” is the ability to survive and reproduce. As stated by H. Allen Orr: “In the crudest terms, fitness involves the ability of organism … to survive and reproduce in the environment in which they find themselves”.1
Given that “fitness” is the tendency to survive and reproduce then, obviously, it is the fittest who tend to survive and reproduce. The maxim “survival of the fittest” is thus a tautology.
And boy has that caused problems! Not, I might add, problems for biologists, but problems for some who comment on biology, from creationists to philosophers of science. I had been under the impression that all this had been clarified decades ago, but on reading the article on biological fitness in the Stanford Encyclopedia of Philosophy, written in 2015 by Alex Rosenberg and Frederic Bouchard, I was surprised to find that this is still a live issue among philosophers.
The worry is that, if the phrase “survival of the fittest” is tautological, then perhaps the whole concept of Darwinian “fitness” is tautological and thus empty of any actual content, and then maybe the whole of Darwinism becomes “metaphysics” rather than science. This accusation has been made by creationists, and even by luminaries such as Sir Karl Popper.
The reply is actually quite simple: Yes, the phrase “survival of the fittest” is tautological; but no, the concept of “fitness” is not tautological, and nor is Darwinian natural selection. That’s it; that’s the resolution of the “problem”.
Darwinian “fitness” is simply an accounting of how likely an animal is to leave descendants. If a particular trait — say bigger antlers, greater swiftness, or a better sense of smell — tends to result in an animal leaving more descendants, then the trait has greater “fitness”. There is nothing tautological about that concept.
Now let’s suppose that animals differ in their traits (which is a known fact; deer of a given species do differ in antler size, et cetera), and further, let’s suppose that traits tend to be passed to an animal’s descendants. Thus a trait bestowing greater fitness can be passed on to offspring who thereby also have greater fitness (this inheritance of genetic traits is also a known fact).
Under those conditions “natural selection” occurs. The “natural” here means “automatically”, or “logically/tautologically entailed” by the above conditions. If those conditions hold then Darwinian evolution occurs.
That is the essence of Darwinism, as stated in all the textbooks.2 But, in the above, I did not make any statement about the “survival of the fittest”. That’s because that phrase is a mere tautology, already entailed in the definition of “fitness”.
But it is only that phrase that is tautological; there is nothing tautological about the concept of “fitness”, nor about natural selection. The phrase “survival of the fittest” is not part of a formal statement of Darwinism for that reason: it is a redundant phrase that adds nothing.
And, since it is not part of a formal statement of Darwinism, the fact that it is tautological carries no implication that Darwinian evolution itself is tautological. If one takes three information-containing sentences and adds a tautology, then the combination of the four is not tautological, since the combination has the information content of the first three sentences!
Indeed, the phrase “survival of the fittest” was not part of Darwin’s Origin of Species until the fifth edition. It was added on the advice of Herbert Spencer and Alfred Wallace, who thought that it would help people to understand natural selection. Well, maybe it does. Pointing out a tautology can sometimes help people understand (for example, maths teachers spend their lives pointing out tautologies to their pupils). But one should not mistake that phrase for the actual Darwinian theory itself.
Sadly, exactly that mistake has caused problems ever since Popper initiated it. So many people now associate the phrase “survival of the fittest” with Darwinism that many consider it essential to rescue the phrase from being a tautology.
The Stanford article by Rosenberg and Bouchard says:
Darwin’s theory of natural selection provides an explanation of all three of these features of the biological realm … by explaining the process of “the survival of the fittest.”
This mistakenly places the phrase “survival of the fittest” at the centre of Darwinian logic. The article continues by saying:
[Darwinian theory’s] … reliance on the concept of `fitness’ makes it imperative that conceptual problems threatening the explanatory legitimacy of this notion be solved.
After failing to resolve the “conceptual problems” they end the article with:
As these debates suggest, far from being merely a 19th century slogan, understanding the meaning of the “survival of the fittest” is of philosophical and biological urgency.
Philosophical and biological urgency? Wow, sounds important! And yet the answer is easy and obvious: “Survival of the fittest” is a tautology. Sorted! Rosenberg and Bouchard are confusing that one phrase for the concept “fitness” itself. They are mistaking Spencer’s commentary on Darwin’s theory for Darwin’s theory.
Suppose that we define “speed” as the distance travelled divided by the time taken. That is not tautological. But suppose I then add a comment: “objects with a higher speed travel further in a given time”. Given the definition of “speed” that comment is tautological. But my adding a tautological comment does not make the concept “speed” itself tautological!
I feel like I’m belabouring a very obvious point, but such a blunder is exactly what many philosophers from Popper to Rosenberg and Bouchard have committed. They seem to regard the phrase “survival of the fittest” as sufficiently primary that they need it to be non-tautological. And so they try to achieve that by trying to find another account of “fitness” under which “survival of the fittest” is true but not tautological.
For example, Rosenberg and Bouchard explicitly ask for a redefinition of “fitness”, in order to rescue “survival of the fittest”, when they say:
It appears therefore that evolutionary theory requires a definition of fitness that will protect it from the charges of tautology, triviality, unfalsifiability, and consequent explanatory infirmity. If no such definition is in fact forthcoming, then what is required by the theory’s adherents is an alternative account of its structure and content or its role in the research program of biology.
This is akin to trying to find another definition of “speed” such that the phrase “objects with a higher speed travel further in a given time” is no longer tautological but still true. This is an utterly misguided and hopeless quest based on mistaking the status of the phrase “survival of the fittest” in the first place.
Of course biologists doing their day jobs have never let this issue worry them: they intuitively know that there is nothing wrong with the concept of “fitness”, and if one particular phrase is tautological then so what?
I confess to being baffled that this argument is still “live” in philosophical circles. The resolution I’ve given above is not new, indeed all of it was said by Dawkins decades ago in his book The Extended Phenotype (the relevant chapter, “An agony in five fits”, is online here and is well worth reading).
And yet, since then, many have tried to come up with convoluted analyses of the concept “fitness”, all to try to rescue the phrase “survival of the fittest”, all to avoid acknowledging that that one phrase is indeed a mere tautology (whether usefully illustrative or not).3 Yet it does not matter, since it is merely Spencer’s commentary on Darwin’s theory of natural selection, not a statement of the theory itself.
 The technical definitions of “fitness” get more intricate since likelihood of surviving and reproducing is always a probability, and thus to evaluate the “fitness” of a trait one needs to average over populations; the Orr article deals with such complications.
 An influential statement of the principles of Darwinism was by Richard Lewontin in 1970 (from “The Units of Selection”, Annual Review of Ecology and Systematics, quoted from here). Note the absence of any “survival of the fittest” statement, since it would be superfluous.
“As seen by present-day evolutionists, Darwin’s scheme embodies three principles …
1. Different individuals in a population have different morphologies, physiologies, and behaviors (phenotypic variation).
2. Different phenotypes have different rates of survival and reproduction in different environments (differential fitness).
3. There is a correlation between parents and offspring in the contribution of each to future generations (fitness is heritable).
These three principles embody the principle of evolution by natural selection. While they hold, a population will undergo evolutionary change.”
 Attempts to analyse “fitness” in such a way as to prevent “survival of the fittest” being tautological include this well-known one by Gould, called Darwin’s Untimely Burial. Many others have taken a similar line, including Massimo Pigliucci, Eric Scully, Jason Rosenhouse, and Tam Hunt. As I see it, all of these are misguided.