Phoenix,
I'm just throwing this out there for you to consider. I have no proof, just an hypothesis I'm toying with.
Recent studies seem to indicate that
P.t.amoyensis was the first tiger, evolving two million years ago in eastern China and then populations migrated north, south and west through various routes, where they persisted through many pleistocene glaciation events.
It is also beleived that geneflow between populations has existed until fairly recent times.
My hypothesis - actually, supposition is probably a better word - is that the original tigers were white (advantageous in a snowy habitat like China, and especially during glaciation events). At some point a mutant gene occured producing orange tigers, which were better adapted to the warmer regions. Between glaciations, the orange gene flowed between different tiger populations and during these warmer times the orange tigers predominated as they were better suited to the environments at that time. During glaciations the white tigers were prevalent. Essentially, tigers alternated between orange and white in the same way the Peppered Moth in the UK went from pale to dark, it's just the tigers do it over a much larger time scale.
If my supposition is correct, I would expect to find white tigers in wild populations of the other subspecies. Logically there would be a higher frequency of them in the Amur population. The reason none have been seen may be because the population of that subspecies is so low that the gene is unable to express itself (or is already extinct in the
altaica genome).
At this point, I don't believe my idea can be proved.
Hix