Evolutionary theory makes it impossible for biological species to be natural kinds (or at the very least, if naturalness comes in degrees, they're very marginal natural kinds). Biological species were, for Aristotle, a central example of forms. Even if there is a good way to make sense of forms of some kind, evolutionary theory again, for the same reasons, makes it clear that there can't be forms corresponding to biological species.
This seems to have profound consequences for many of the hopes of rationalism; many earlier attempts to provide rationalist accounts of human nature seem to have been heavily dependent on there being a form of humanity (and even self-proclaimed critics of rationalism like Kant often make claims which seem to require something very much like such a form). This is especially clear in the case of ethics. On the Aristotelian, form-centered view of human nature, variations in humanity are deviations from the basic form. This made it at least conceivable that undesirable features of humanity could be explained away as merely deviations (if in some cases depressingly frequent ones), leaving open the possibility that the form could serve a normative purpose, that one could derive what humans ought to do from the form of humanity.
Undercutting this is a major change. On the other hand, Newtonian physics already seems to carve up the world along lines very different from those suggested by Aristotle's forms, and as a result skepticism about viewing any aspect of the world in terms of forms was already well established long before Darwin. Certainly there were many critics of Aristotelian natural-law morality before Darwin, including, for example, the whole empiricist tradition, which also rejected forms in general.
So, how much difference did the additional reasons Darwin provided for being skeptical of biological forms make? And, conversely, did the rejection of forms in the British Empiricist tradition help in any way in making something like an evolutionary theory seem conceivable to Darwin and his contemporaries?
 While evolutionary theory does provide some ability to distinguish the normal from the abnormal (as discussed by Millikan), the lines are much murkier, and not revealed by reason, and prospects for plausibly lining up the evolutionarily normal with the morally good are far worse.