Sorry this reply is rather late, but evolution and selection of the fittest doesn't always produce the best or perfect results. It produces organisms that are good enough to survive and reproduce. Anything that would effect life expectancy or quality of life enough to stop an organism to reproduce would not be selected for, but problems can creep up where people continue to reproduce just fine. The need to wear glasses is an example of this. Why would evolution select for worse eyesight when survival requires being able to see well enough to perhaps hunt, or run from predators, or be able to harvest, or sew clothes, or whatever. Yet there's plenty of people who have had eyesight before glasses were a thing and continued to procreate. Glasses are only a thing because the problem existed in humans. Evolution is random and throws spaghetti at the wall to see what sticks. This means that genes which cause worse quality of life, while not necessarily affecting one's ability to reproduce, can still survive and continue to be passed on through the generations through genetics.