[Runeite51]

Though that might just be the case for most things, I don't think it would be fair to say that for all things. Two traits could develop in conjunction with another, and trait 1 gives an advantage, while trait 2 is a disadvantage. The most in-your-face example I think is the stereotype of nerds and glasses. In short, a good enough trait could get you laid and fed even though you have a crappy trait that came with it.

edit: alternatively, a bad enough trait could completely derail the development of a good train. Maybe dolphins born with 200iq brains develop some other kind of bad trait as a result of the same mutation that doesn't allow for the 200iq trait to permeate.

[freeone3000]

The characteristic one for this in humans is sickle cell anemia. It really sucks for the people who have it but it sucks less than malaria, so it’s persisted in the gene pool… in regions with endemic malaria.

[Consultant32452]

The poor eyesight you're referring to comes with age, generally after the primary ages of procreation. That's why, on an evolutionary timescale, there wasn't much benefit to weeding out the need for glasses. Maybe into the future we will see that. Conditions which cause poor eyesight from birth get weeded out.