A much better example of counter-intuitive evolution, one that's literally textbook, is sickle-cell anaemia. It's a genetic disease (bad) that offers protection

A much better example of counter-intuitive evolution, one that's literally textbook, is sickle-cell anaemia. It's a genetic disease (bad) that offers protection against malaria (good), so it's more common in areas where malaria is a fatal problem for significant numbers of people. This illustrates the principle that genes do more than one thing, and can be simultaneously adaptive and maladaptive at the same time. As a hypothetical example, maybe the same genes that make a man fractionally more likely to be gay could protect his sister from ovarian cancer or stop him going bald.