Hi Frank, everyone,
Overnight I've thought of a few more things I need to say...
It's natural to assume that species radiate out from the larger, main landmasses to the smaller islands. After-all there is usually more diversification on the larger land masses. And this certainly appears to be the case with
Musa - on the large land masses of Indochina, Philippines and Indonesia. And
Musa itinerans var. formosana certainly crossed a land-bridge from China to Taiwan during the glacial maximum which occurred about 20,000 – 18,000 years ago, and it was at this time that the sea level was at its lowest, roughly 120-150 m lower than it is today.
The entire Philippine Arc of volcanic islands, which includes the Batan Islands of the Philippines, Lanyu Island and the Ryukyu Archipelago were formed during the Tertiary 65 to 1.8 million years ago by submarine volcanic activity. The island of Taiwan is even younger and was formed less than 6.5 million years ago by the Luzon Volcanic Arc crashing into the Chinese continental margin and in doing so forced the land mass of Taiwan up from the sea bed.
It is evident that certain plants that were native to this arc of islands (Namely Cycads and Palms from my point of view) made their way to the newly formed island of Taiwan. Quite probably some of the Taiwan landmass includes some of those original islands.
Bananas were NOT on any of these original arc islands, and
Musa itinerans arrived in Taiwan from China about 18,000 years ago. In that time
Musa itinerans has covered almost all the high ground on the Taiwan mainland, forming huge clumps. It couldn't have crossed over to Lanyu, as the oceanic trench separating the two is over 1500 m deep.
This is my distrubution map (in red) for
Musa itinerans var. formosana, follow that link to see the full interactive map.
I've made a few notes on the map - (1) is Lanyu Island, (2) is green island, which did have a land-bridge with Taiwan, and which might be the origin of
Musa insularimontana. (3) This area of Taiwan interests me, because it is quite inaccessible, there are no roads near the coast, part of the area is closed off as a military base, and I have no idea if bananas can be found at low elevation. If there are, this would be an obvious origin of
Musa insularimontana.
Have I convinced you yet?
Best Phil.