J Thomas-- What you say fits well with the neo-Darwin model of evolution. One example you might be interested in that clearly does not is the tuberculosis bacteria. Google 'tuberculosis strain w' for more info. It turns out this sort of thing happens more than was previously thought (of course it wasn't thought to happen at all until fairly recently) This is a case of motivated continuation on my part- the old model predicted a cure that turned out to be a recipe for making an incurable disease-- uh I want to understand better.