Artificial selection and natural selection are different forms of the same process. Treating the relationship as a mere analogy assumes that differences are greater than they actually are.
Natural selection simply requires certain conditions. When they occur, natural selection will occur:
The only difference between natural selection and artificial selection is whether the difference in reproductive success is driven by naturally occurring processes, or whether the selection is imposed by humans. Explore Evolution obscures this in two ways. First, by asserting that the relationship is an analogy, rather than a generalization from the human activity. Second, by referring not to a human activity, but to the action of "intelligence."
This shift is subtle, but is a powerful rhetorical opening move. After introducing an example of shepherds selectively breeding woollier sheep, Explore Evolution asks:
Is it possible that something like this process occurs in nature—only without any intelligence to guide it?Explore Evolution, p. 87
The same question could as easily be posed whether "something like this process occurs in nature—only without any [human] to guide it," but would seem much less profound. And as Explore Evolution acknowledges, it is easy to see how forces other than humans could exert selective pressure on populations of living things.