The correct answer is - reproductive isolation.
If certain species are isolated in their reproduction over a longer period of time, they tend to diversify, and evolve into numerous new species that have unique traits and start to look much different that the species with which their ancestors where inhabiting the same area prior to that. We can easily see this in the present, with the flora and fauna of Australia, which was a continent that was isolated for millions of years from the rest of the world, so the speciation is high, unique, and unlike other places in the world that were connected, and this was due to the reproductive isolation of the species.
