If there was a fish, that through some genetic freakyness, was born with a defect that meant it couldn't breath in water - and instead had to go to the surface for air - you'd think "what a dumb fish". In terms of fish this one has a serious defect, this fish surley can't be any good, a fish that can't breath in water is a stupid, useless fish: it deserves to drown and die, particularly before it has the chance to procreate and give birth to more stupid fish that drown in water. Maybe though that fish isn't so dumb, maybe that fish lives in shallow waters and can afford to come up for air every now and again, maybe that fish, because it can breath above water, can bury itself in the sand and sleep and lay its eggs out of the danger of predators. Maybe that dumb fish that drowns gives birth to an even stupider fish; one that can barely swim because it's got two thin inefficient sticks instead of fins. All the other fish are really laughing at this fish (even the fish that can't breath in water): not only will it drown in water the dumb fish can't even swim! Surely that stupid drowning, sinking fish needs to be given a good dose of natural selection and killed off? Maybe though that fish isn't so dumb, maybe that fish can use its thin sticks to drag itself allong the sand and go rummaging for food or bury itself further away from the water and lay its eggs in a nice dry warm place where its offspring have a greater chance of survival. Maybe one of those little drowning fish that can't swim gives birth to another fish that isn't even a fish at all anymore but something quite different that lives on land and couldn't survive in water at all.
The point to my little story isn't to demonstrate the workings of evolution (they are such an oversimplification and acceleration that is quite probably inaccurate) but they demonstrate a valuable point: something may appear as a defect when held up against the norm but in the longer term that defect may place its host at a distinct advantage. The question should not be whether the mutation is at an apparent disadvantage compared to its peers but whether it survives and is successful. Likewise taking an existing mutation and placing it a past environment is not an accurate measure of its success: to try and state that the last fish in the sequence is defective because it wouldn't survive in its previous environment of water is a fallacy: although not descended directly from a gorilla stick me side by side with it in an environment similar to one of our far, far ancestors (i.e. one similar to its own) and the thing would probably kill me within minutes if I'm lucky to survive that long. Though stick a gorilla in my environment: get it to put on a suit everyday, commute to work, earn its keep and do the shopping the poor thing would survive maybe a little longer than I did but it would soon starve to death or end up in a zoo (possibly a smart move: free lodgings and feed). From the gorillas point of view within his environment I am defective but within my own I am (as a species) very succesful.
For some reason we have a strong cultural prejudice against mutations in our race - ADHD, autism, dyslexia, bi-polar disorder to name a few - and we view these things as something that needs repairing, more specifically we view them as drowning fish. We need to adjust this view and start looking at the advantages these mutations give us in our environment. After the hysteria of the MMR jab I listened to the head of the society on Autism explain the important and irreplaceable role people with autism have played throughout our recent history from breaking the German's codes, designing and building aircraft and ultimately helping us win the second world war to designing and building our modern communications systems and computer software. Likewise the cultural and economic contribution made by people who "suffer" from the so called "unstable" and "mentally ill" condition of mania such as Stephan Fry, Thom Yorke, etc. These drowning fish have enriched our society and pushed our cultural, intellectual and technological evolution on faster and beyond anything our race could have achieved without these 'defective' genes in our pool.
The measure of whether a gene is successful is if it can propagate and ultimately survive and if it can then by definition it isn't defective: in fact it may serve a purpose (in other species it has been suspected that genes that cause suicidal thoughts could be an evolutionary method of population control). The fact that in our current environment, like our drowning fish, they appear to suffer is also no indication of a defect as maybe in our modern fast changing, heavy paced life these peoples adaptations may place them at an advantage to be better able to cope with that future environment. Or perhaps these so called 'defects' are part of our species make-up to deliberately produce small pockets of mutations to ensure our adaptation and ultimate success.