Let's talk briefly about Muller's rachet. What he's saying there is that a sexual population of a finite size is going to exhibit a rachet mechanism. When all members of the population are loaded with mutations, they can't restore the unloaded variant. Here I try showing this schematically. Here's a healthy wild type of cell. And they're all fine. But as time goes on, chances are you're going to see each one is going to get some kind of mutation. And there's no way to restore the undamaged original. Now maybe that's fine but maybe it isn't. But, say you have a sexual population. Even if you have mutated every single member of the population, their mutations will not necessarily be in the same place. So, through sexual recombination you're going to be able to regenerate some of the wild types.
So sex has this advantage of being able to restore the wild type population. In other words, the undamaged cell. This is a good theoretical, intellectual reason to tie in the whole idea of sex in DNA repair. So the main emphasis I'd like to hypothesize: UV radiation is a driving force in evolution.