Gamanit
The animals don’t “choose” anything, different pressures make those with certain characteristic’s offspring and descendants become dominant over time. It would actually make sense that genetically similar animals (Like horses and donkeys) would evolve the same sort of responses to similar pressures.
benigmum,
I agree with and appreciate your distinction between common descent and the discussions as to what mechanisms led to that descent came about. Allot of people lump it all together.
As far as the Shi Tzu thing: the point is that the reason for the difference in appearance between Shi Tzus and wolves is because Shi Tzus are GENETICALLY different then wolves, just not enough to theoretically prevent a hybrid from being fertile. There is really no reason to assume that genetic changes would stop at the point that it would affect the fertility of the offspring of a hybrid between a shi tzu and the original wolf population.
The amounts of time talked about when discussing these things are so vast that it is very very hard to even begin to imagine them and the amount of time things could have had to change….