Thanks for all the replies, folks. I'm guessing that Cooper's is an Aussie thing - not available in the States (though I'll keep my eye open).
I'm about to make a nice porter using WLP023 Burton Ale Yeast and my thought is to harvest enough to put aside and freeze small packets I can use for future starters. I want to avoid genetic drift, too, but I'm thinking the best way is to get 10 or so second gen cultures in the freezer then I can make the next 10 batches from that. I would be nervous about going multi-generations because I'd have no way of checking to see if it had evolved... even with precautions that would be impossible to control.
So, my real question is whether it would be preferable to get my crop from the top 'pancake' created by the top fermenting yeast in the primary, wait til after racking to secondary to harvest from the bottom of the primary (not using much hops), or wait until bottling to collect from the bottom of the secondary. I suppose logically the top crop would be the least 'evolved', so maybe that would be my target?
I could try all three and compare....