Why did evolution permit "useless" and "destructive" traits such as mental illness?

"However, to me it seems like things such as depression shouldn't exist, if evolution is merely trying to improve an organism."

Evolution is not "trying" to do anything. It doesn't care about improving organisms or anything else. Evolution is merely what happens when some things live and some don't.

That said, it is true that evolution tends, in the long run, to eliminate "bad" traits. But think for just one second - how can this happen unless the "bad" traits show up in the first place? It takes time - sometimes a very, very long time - to eliminate deleterious genes from a population. During that time, there will be individuals carrying these negative traits. Evolution doesn't ever produce perfection. It simply takes what works well enough.

And that's ignoring the fact that you're assuming that mental illness is hereditary, which it almost certainly isn't. Evolution can only work on traits that are determined by genetic makeup. Mental illness depends on a lot more factors than just genes, and thus, natural selection is unlikely ever to make much of an impact on it.

Gardenga is right. We have lots of genes which are not doing us much good, just random genes that are always changing. No organism is set in a perfect formula - various features that are not too good at the moment, might be needed for an organism to evolve when circumstances change. No organism or machine as complex as a human will ever be perfect anyway, we are a good "design" able to adapt to many things, but not perfect.

Short answer, genetics

most conditions you mention hit either after reproductive years or don't kill the person with it until after they have passed on their genes.

the other way is that it is a recessive trait, meaning that it only shows if both genes are recessive. otherwise we have non-disjunction conditions such as down's.

there is also the environment, while not a psychologist, it seems that depression tends to only affect people who sit around or after massive traumatic experiences

Genetic traits exist in a range of variations that recombine in may ways, some being detrimental under certain environments. There is no goal or aim at improvement. It is sufficient that the combinations of alleles tend, on average, to produce more favorable combinations than harmful combinations for alleles to persist in the gene pool.

As Radman said "Errors and infidelity, even wastefulness, can cause individual failure, but also provide innovation and robustness, ensuring the perpetuation of life."

http://www.stat.ucla.edu/~dinov/courses_students.d...

:o