the question you posed as an example, I would be unable to give a exact answer to, but in general as animals spread out from Ararat in the years following the flood, they would encounter different pressures from competition, predators, food sources, and probably other considerations. eventually they would begin to settle to the locations now common to them.

Ah, but that doesn't do it, you know. The explanation has to account for why the *marsupials* all ended up in the same place (more or less). What was it about being a marsupial that accounts for them ending up in the same location?