Sparrows and archaeopteryx are different species. One cannot become another. This is not what is being discussed.
You are dealing with specific traits that have been emphasized among select populations of the same species, and which will dissapear (return to the original) if not kept selectively enhanced. It's simple to prove this in a tank, try it. Some of each generation will exibit traits of the originating generation and continue interbreeding with the ones that have the emphasized traits till they are all back in the original state (outside influences removed).
This is the reason (specific traits being exhibited among some members of each generation that are not common to the entire generation) that allowed breeding the specific 'big tail' traits to begin with. It's the same with people, look at 'family' traits and how they are carried on or not carried on by subsequent generations according to the mates chosen by each generation. But the're all still human and each has various dominant and subordinate traits (height, color of eyes, skin, hair, etc. that could be selectively bred to make different 'racial' groups over time.
So long as we are talking about animals of the same species, unusual traits might well receed into dormancy. However, if we continue our experiment to the point where our two groups are no longer able to interbreed, then this is no longer the case.
This is what is called the fallacy of begging the question.