This week we have been considering a view called ethical egoism and I gave you the example of the Plank of Carneades. When I give this dilemma in class, sometimes my students will argue for the egoistic choice by noting that in the moment of choice your survival instinct would kick in. You would naturally want to fight off the other person and giving up the plank would be unnatural. The implication is that things that are natural are good.
This style of argument is similar to what is given by a different but somewhat related system of morality. Evolutionary ethics is the view that we have a moral obligation to aid the evolution of our species. Natural selection hones the human race to be ever more adept at survival. To fight against this could only serve to weaken our race, and this is surely a bad thing. The human race naturally progresses towards a greater goal, so it would be wrong to interfere. However, living in a society instead of in nature creates an artificial environment in which evolution is stymied. Therefore, we should try to correct this as much as possible.
If this sounds at all familiar it should: this is the logic employed by the Nazis. Typically when someone compares something to 
the Nazis, they do so fallaciously (a phenomenon documented here). In logic, a ‘false analogy’ is reasoning that just because two things are similar in one way, they must be similar in an unrelated way. This is exactly how much politicking goes on. I’ve heard people say (and I’m not endorsing anyone), “President Trump distrusts the press. You know who else had a problem with the press? Hitler.” A couple years back it was, “President Obama blames the rich for the problems in America. You know who else blamed a group of people? Hitler.” The problem is that, just because someone is similar to Hitler in one way doesn’t mean they are similar to him in other, objectionable ways . Otherwise, we could say, “President Lincoln drank water. You know who else drank water?” However, in the case of evolutionary ethics, we are no longer appealing to irrelevant similarities. The Nazis reasoned that the weak would die off naturally but due to artificial circumstances (society) they survive. Worse, they are allowed to breed and thereby pollute the gene pool. The obvious conclusion is that they should not be allowed to survive. So, the Nazis euthanized them (for a gnarly example of what this would be like, watch season 2 of The Man in the High Castle, but not before reading the excellent book by my favorite author, Philip K. Dick.
Hopefully, you haven’t found the Nazi case too convincing because it is logically fallacious. Appeal to Nature is a fallacy on which we assume something is good just because it is natural (for our purposes, assume “natural” means “occurs without human engineering”). So, for example, in the movie “Friday,” Smokey says weed is from the Earth, implying it is good because it is natural. You know what else comes from the Earth? Lead. Does that mean you should shoot yourself (this isn’t a proof that marijuana is bad, but this argument certainly is)? When the evolutionary ethicist appeals to the “nature” part of natural 
selection, she is making the same assumption. Even if evolution happens in nature, so does murder. That doesn’t make murder good.
The same should be said about the way my aforementioned students argued. They said that since we naturally have a survival instinct, we should employ that instinct. Again, just because something happens in nature, doesn’t mean it’s good. You may want to kill in order to survive, but that doesn’t mean you should. In fact, we have all kinds of natural desires we should avoid. We naturally desire to hit people, take things that aren’t ours, and eat the M&M’s out of the trail mix so dad is left with peanuts and salt-caked raisins. But we shouldn’t.