So how come white Australians are still white Australians?
Well, I guess that's a mystery ;^D
You either build a ship for millions of animals or you take only a few (let's say several thousand, which is still a lot for one ship) and let them evolve into the millions of species we have today in only 5000 years.
Heck, if you ask me, neither scenario is really plausible but who am I to judge ;^P