Not all complex mutations are positive either. Frequently its a negative mutation that may interfere with something MORE negative, resulting in a net positive effect, which may then (based upon possible increased survival rates for future generations) mutate further for a more complex result.
2 examples; african slaves used in the US died from dehydration. African americans have a high rate of heart disease, a hypothesis is that salt retention was developed in the population, resulting in hypertension for future generations. SO here we have some people that adapted to something horrible, with something thats horrible aswell, but a little less horrible than the first thing in the short game.
Another example is sickle cell anemia. Rampant in some areas of africa. Nobody wants sickle cell.. not even malaria. If you have sickle cell, you have a resistance to malaria, a net positive. Sure you have to deal with the complications of anemia, but its not going to take you out.
There's no art to it. Some things add on slowly with positive effects, some negative, some have no real effect at all. The only reason we're so proud of what we have and are so amazed by it all, is because we havent seen anything better. Next time you think your eyes are amazing, look at a mantis shrimp. It can see colors you cant even comprehend. Your eyes are dollar store garbage in comparison.