Yeah, it doesn't seem like 2 generations would create any problems, I have some whitbread at 3rd generation now with no weird off-flavors.
(I apologize in advance for the population genetics lecture!!!) I haven't read much about mutations in yeast strains on this forum, but natural selection is HIGHLY unlikely the be the culprit. NS actually has little or nothing to do with evolution, really.
Neutral Theory (which is now generally accepted) will give some insight to what could possible be happening, but not likely as it's only been 2 generations. As Neutral Theory states, almost all genetic variability is caused by random mutation and genetic drift, meaning it is actually kind of likely that a random mutated allele could have spread throughout the population (by random genetic drift and chance, not NS) and created slight off-flavors.
