Thought experiment: a bunch of people are stranded for decades on a deserted island. The children of one of the families all die from an unfortunate genetic trait. That family passes on no breeding heirs and their gene pool on that island ends. That unfortunate genetic trait ends with them, yes? While it is still possible the trait exists in some of the other people, the fact is, the frailty of that trait causes those genes to be weeded out of the gene pool.

We see this in the world. Sickle cell anemia is correlated with an increased resistance to malaria. People without the sickle cell trait were more likely to die from malaria. The survivors were more likely to develop sickle cell anemia.

Survival of the fittest is obvious. Natural selection is obvious. I’m no expert on these matters, but I just don’t see what is so hard to understand about this.