The argument that natural selection is a tautology and therefore lacks explanatory power is one of the silliest tropes that creationists have used to impugn evolution. Here’s a decent explanation of why:

Natural selection is in one sense a tautology (i.e., Who are the fittest? Those who survive/leave the most offspring. Who survive/leave the most offspring? The fittest.). But a lot of this is semantic word-play, and depends on how the matter is defined, and for what purpose the definition is raised. There are many areas of life in which circularity and truth go hand in hand (e.g. What is electric charge? That quality of matter on which an electric field acts. What is an electric field? A region in space that exerts a force on electric charge. But no one would deny that the theory of electricity is valid and can’t explain how motors work.)—it is only that circularity cannot be used as independent proof of something. To harp on the issue of tautology can become misleading, if the impression is given that something tautological therefore doesn’t happen. Of course the environment can ‘select’, just as human breeders select.

That explanation, it may surprise you to know, is from Creation Ministries International, part of their list of “Arguments we think creationists should NOT use.” Lest I be accused of taking this out of context, their essay does go on to express doubt that natural selection is a sufficient explanation for life’s diversity:

Of course demonstrating this doesn’t mean that fish could turn into philosophers by this means—the real issue is the nature of the variation, the information problem. Arguments about tautology distract attention from the real weakness of neo-Darwinism—the source of the new information required. Given an appropriate source of variation (for example, an abundance of created genetic information with the capacity for Mendelian recombination), replicating populations of organisms would be expected to be capable of some adaptation to a given environment, and this has been demonstrated amply in practice.

So Creation Ministries International, young-Earth creationists who believe that the Genesis account of creation in six days is literally true, accept that the claim that natural selection can drive adaptation “has been demonstrated amply in practice.”

Advocates for intelligent design frequently object to being lumped together with creationists. Intelligent design is a purely scientific theory, you see, not a thinly veiled attempt to lend scientific credibility to religious ideas. Obviously, these very serious scientists would never use an argument that is so silly that young-Earth creationists reject it. Obviously.

All Darwinian “theory” is banality and tautology, without a shred of explanatory power. It’s all the same logic. Stuff happens and survivors survive. —Michael Egnor

Darwinism (natural selection acting on random mutation) is either a tautology (the survivors survive)— or a claim that the tautology is a mechanism for creating vast amounts of new information. —Eric Anderson

I haven’t read Ferguson’s book, but it sounds like he’s offering the traditional Darwinian tautology: The fittest firms survive. And what, in economic history, defines fitness? Well, survival. Because, you see, those that survive are, uh, those that survive. —David Klinghoffer

…we are astonished at [Massimo] Pigliucci’s grand claims for the tautology of “natural selection” to produce adaptive complexity which is about as well demonstrated in the real world as magic. —Denyse O’Leary

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. —Barry Arrington

“The survival of the fittest” has the same problem. It is circular. Fitness is defined in terms of survival, and there is no independent criterion of fitness. —Tom Bethell

Critics have long complained that “natural selection” is an oxymoron, and “survival of the fittest” is a tautology (see Darwin’s House of Cards by Tom Bethell, Chapter 5). Yet today still, 160 years after Darwin’s blunder, many evolutionary biologists still don’t get the distinction. —Unsigned post at Evolution News & Science Today