This makes me wonder what effect absolutely zero radiation would have on an individual. And on a population (e.g. would evolution be harmed by having no radiation at all?)
DNA copying not being perfect would be the main cause in my opinion.
There is one error on average for every billion pairs copied. The human DNA has 3 billion pairs. So every time a single(!) cell is copied, three mistakes are made.
You don't need any radiation for evolution.