[ugexe]

Its a stretch to say natural selection put decay in the brain for a good reason. Not every trait was the result of positive or negative selection. Traits can occur as side effects of another trait (of another trait etc etc) that was positive or negatively selected.

If a new, sub optimal trait does not negatively affect the specimens ability to reproduce then there is no reason to try and select against it. Such a circumstance would not imply the trait was positively selected for.