Here’s an excerpt from John Herman Randall’s Aristotle (Chapter 9: "Aristotle’s Functionalism Illustrated in Biological Theory"; Section: "Aristotle's Natural Teleology versus “Design”"), Page 228-229:
Since the various religious traditions not unnaturally identified “nature,” the system of ends toward which natural processes are discovered to be directed, with the “will of God,” as Plato’s creation myth had already done, “final causes” were taken as the conscious purposes of the Deity, and as such were held to be ipso facto efficient causes, themselves acting to bring about their own realization. In sharp contrast, for Aristotle “final causes” and “natural ends” are in no sense whatever to be taken as “purposes”: they involve no conscious intent, except in the one case where conscious intent is obviously involved, human action and art. And final causes or ends are for Aristotle never to be identified with efficient causes: never for him does what a process brings about itself bring about the process. For Aristotle a final cause is always a necessary condition of understanding, a principle of intelligibility; it is never a “whence of motion,” an arche of action.
In the second place, “final causes,” as they were developed during the predominance of the religious traditions, tended to become a way of showing how under the ministrations of God’s providence everything in the universe conduces to the self-centered purposes of man. In sharp contrast, Aristotle’s natural teleology is, in the technical sense, wholly “immanent.” No kind of thing, no species, is subordinated to the purposes and interests of any other kind. In biological theory, the end served by the structure of any specific kind of living thing is the good—ultimately, the “survival”—of that kind of thing. Hence Aristotle’s concern is always to examine how the structure, the way of acting, the “nature,” of any species conduces toward the preservation of that species, and enables it to survive, to exist and to continue to function in its own distinctive way. This Aristotelian emphasis on the way in which kinds of living things are adapted to their environment brings Aristotle’s thought very close to the functional explanations advanced by evolutionary thinkers: in both cases the emphasis is placed on the survival value of the arrangement in question.