Sorry, I haven't had much time lately.
I see you draw your conclusions too soon, Tuppence. If yeast reproduces very slow, does this mean it stopped evolving? Nope. Evolution is based on reproduction, so if something reproduces, it is bound to be exposed to 'natural selection' (=evolution).
When less change is observed in a species, this is due to the environment, not to 'evolution that has stopped.' If it is better for a species NOT to change, then we do not observe CHANGE (duh). And evolution does not equal 'change', evolution means 'selecting those that are suited to the environment.' And if yeast in its present form is suited to its environment (whatever that may be; the outer skin of a grape or a parasitic variation that lives on humans or something else; you didn't specify) then there is no reason so assume that its needs to change.
The fact that yeast hasn't died out seems evidence enough to me that it is doing just fine, slow rate of reproduction or not. So why change?
As long as a life form reproduces, it is ALWAYS evolving. As far as I know, it is IMPOSSIBLE to stop evolving, unless you are talking about cloning.