If most mutations are deleterious, how have organisms evolved to become more complex?
Most mutations are NOT deleterious. (Where did you read that?) Most mutations are neutral. They are neither beneficial, nor deleterious. The ones you *hear* about are the harmful ones, because they have the most obvious effects. But most mutations are quiet … they either don’t do anything at all, or they have very small quiet effects (a improvement to a digestive enzyme, a small change to the cell walls of bone cells, a small change to a blood clotting regulator, etc. etc.) that would hardly be noticeable from generation to generation … but over millions of years lead to significant change. And second, it doesn’t matter anyway. Even if most mutations were deleterious, it’s the beneficial ones that matter. The deleterious ones are discarded quickly by natural selection because individuals that get those mutations don’t reproduce as many offspring. It’s the beneficial ones that get propagated into the population.
