I want to start this off saying I'm not a conventional believer in God.. I do not claim Christianity as my religion basically because of the corruption that has slowly and utterly fallen onto the many churches of that world (I'm not saying all Christians are corrupt... just that too many are for me to want to associate myself with those types..)
As far as the Biblical Noah story.. I can only give a few reasons why it makes sense at all. FIRST to the Jewish people the 'world' was in fact only a small portion of the actual world, in fact not even the whole of the Middle East.
I believe 'God' (or whatever you want to call it) saw that his people, the Jews, had become corrupt and twisted (this is a theme with the Jewish people throughout the Bible.. and not to knock on Jewish people but... couldn't he have chosen someone else who didn't defy him every three minutes? heh). According to the non-canonical Book of Enoch the world had become corrupted in many ways and so God sent Enoch as a messenger to warn the Jewish World that he was going to destroy them if they didn't play nice. Didnt work out.. so God said, "I liked that Enoch guy, nice fellow", "I will save his grandson, Noah. Yep." So Noah wasn't chosen just because he was 'right with God', but because who he was related too. Makes you wonder about what kind of God that 'god' is...
Anyway... before this gets any longer: the flood was most likely contained only to that region... there are a few other flood stories around the world, but the majority of these tales originate from the Middle East. Since at the time the ME was the center of the civilized world, its the most logical explanation. That being said Noah would only need to gather animals SPECIFICALLY native to that region. If they existed in other regions it would be pointless to bring them aboard.
I know some would say I'm taking the Noah story out of context.. but if you think about it alot of ancient Hebrew writings were written in a very specific way (for example they tended to ignore things outside of the Hebrew World) so it makes since that a 'world-wide flood' might really only mean their world, the "Chosen People's World". And that's not taking the flood as a metaphor, just taking it as what is most logical.
Once again I'm sorry for a very long post.