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.