I didn't mutate as in they all of a sudden develop a fifth leg or anything - i meant a change in the alleles within the population. It's not a 50/50 chance, but it could go either way.
Here ya go. The text is ambiguous at time, but some passages are interesting and plausible.
Organisms are in a constant state of flux - look at any populations from humans to fruit flies: they have adapted, perhaps slightly, in the past dozen generations. Have you ever been in a house that was built 200 years ago? You have to stoop to walk in some of them - humans have grown to be much taller. Eventually, following enough adaptations, the species will evolve into something different (assuming they don't die out for whatever reason). That happens over many, many thousands-millions of years. That seems to be what you are saying?