Arguments like these convince me that a lot of philosophy is a kind of cargo cult mathematics. Practitioners don't do actual reasoning; they construct assemblages of words that mimic mathematical arguments, but fall far short of what a mathematician would consider acceptable.
Let's look at some of the techniques Groothuis employs:
1. Reliance on vague terms that one cannot possibly measure, test, or verify, such as "essential nature", "intrinsically valuable", and "human dignity". (If you have no argument at all, then you can always decry some practice you don't like by claiming it offends "human dignity"; it's a favorite ploy of Robert George.)
2. Quotation fabrication: Darwin never spoke of "less favored races", as Groothuis claims, and the term "favored races" that appears as a subtitle in On the Origin of Species actually refers to what biologists now call "varieties". If you google the phrase "less favored races", you find that it appears largely in creationist websites and a Republican congressional candidate.
3. An incoherent argument that concludes "But (4) is false, because of (5)" and "Therefore (6) is false because of (5)" But the terms (5) and (6) refer to nothing at all!
4. Rewriting history to claim that "our moral intuitions and the history of Western law" provides support for believing that "every human being, irrespective of race" possesses "intrinsic human dignity". Really? Whatever happened to slavery in the history of the US? How about all the Christian Southerners who claimed that slavery was ordained by God? How were black people treated in the US Constitution? In what year were women allowed to vote? If the history of Western law shows us anything, it shows us that our "moral intuitions" are not precisely fixed and are subject to change.
5. Pretending rigor by explaining grade-7 concepts like "modus tollens" and "reduction ad absurdum". Bad arguments don't get better when you use Latin.
But the silliest thing of all is the attempt to defeat a scientific theory, the theory of evolution, using moral reasoning. This makes no sense at all; it's like trying to justify a claim about chemistry by appealing to political theory.
I feel sorry for Groothuis's students.