How did Evolution produce Symmetry?

Ok..I'm new here so take it easy on me for a while...But here is my opinion on the whole symmetry thing.. Let's just say (hypathetically speaking) that someone was born with a short arm. Was that an advantage? Probably not so therefore the traits wouldn't be passed on to the offspring. But if a fish was born with one fin and one of its offspring had 2 and swam faster and better, then it would be seen as an advantage so the traits of the two finned fish will be passed on to the offspring and then the one finners will eventually be weeded out...That's my opinion and theory... Oh and there is a really good program on sexual selection and sexual characteristics by the Discovery Channel.. Can't remember the name but it's quite informing and interesting..I'll get back to you all on the name of it ASAP.