An important thing to realize is that evolution doesn’t actually care or think. For a human, if a small change makes a significant difference in say, car safety, it’s relatively easy for other humans to adopt this same change in all cars, and it quickly spreads. Evolution, on the other hand, can only effect gene pools through breeding rates and survival. An acne resistance gene might be all upside, but if people with zits still breed at the same or nearly the same rate as people without then it won’t spread at anything like a noticeable rate.
An important thing to realize is that evolution doesn’t actually care or think. For a human, if a small change makes a significant difference in say, car safety, it’s relatively easy for other humans to adopt this same change in all cars, and it quickly spreads. Evolution, on the other hand, can only effect gene pools through breeding rates and survival. An acne resistance gene might be all upside, but if people with zits still breed at the same or nearly the same rate as people without then it won’t spread at anything like a noticeable rate.