There’s a fantastic theory in science, called gravity. Gravity describes things being attracted to large masses. Despite this rather concrete description of reality, humans have not confined themselves to a prescription of gravity. Instead, we have built planes and rockets and space shuttles and we’ve had the cheek to launch things right out of our solar system. There’s another scientific theory called germ theory. A very accurate description of reality, indeed. But very few people are arguing we should create a political system which permits the free passage and spreading of germs. In fact, much like aviation did with gravity, antibiotics are a way of saying we can control the implications of this scientific description. A scientific description is not a prescription at all.
That is, unless you don’t like the description. If you don’t like the description you can try to convince people its false, not with evidence, but by relating the description of reality to something which, tentatively, could be the prescription of that description.
Readers, you’re right! Without an example, that idea is unclear and clunky. So I shall illuminate it with the example provided by the question. People who do not like Darwin’s descriptive theory of reality have decided to try to tar describing biology with eugenics and mass murder. The premise they are working on is this: there is no conceivable difference between how nature works without management and how nature should work with management (unless the manager is God). Therefore, if the somewhat trite summary of evolution “survival of the fittest” is apt for the scary world beyond cities and politics, then it should be apt for the political world too.
I hope the problem is clear.
Without adding a great many extra premises, one cannot get from ‘evolution is true’ to ‘mass murder to right’. Don’t be a moron!