Let's look at the polar bear and grizzly bear. One group lives on polar ice caps and their survival strategy involves hunting seals. One group lives in forests and they are omniverous. The other group will select for teeth suited for eating plants, etc. etc. This speeds the process of evolution.
Reproductive isolation is more a case for degeneration than for evolution.
The ice bear has lost information, i.e. colour pigment, which he will never regain. The proto-bear, as you call it, is the mutual ancestor with 'inactive' information which could be used in different ways. The icebear has used different genetic information, but has lost the rest which was not necessary for him in his environment. He has thus indeed, adapted, but his possibilities to muatte have diminished. There are three cheetahs.