[mrob]

Brewers reused yeast skimmed from the top of fermenting beer ("barm"). Wikipedia's "History of beer" article quotes a 1557 source mentioning this, and it probably goes back much earlier. The motivation here is probably speed and reliability of fermentation, which are obvious benefits to people not aware that yeast is a microorganism, but it also incidentally breeds for alcohol tolerance (especially considering the popularity of strong beers). Reusing wooden brewing equipment without sanitizing between batches has similar effects.