You are absolutely right and as you mentioned with bees and birds... some species might bully out some other species out of their niche and then the native population has to find a new one or die out. Species that live in extreme conditions nowadays are the ones that were pushed out, bullied out by some other species, but they have had enough genetic potential to overcome that and adapt on other conditions! For a species to survive bulling they need to be wast in numbers and the interesting thing is in the new environment those on the periphery of the bell curve are the most likely to have the most potential to adapt on new conditions because they weren't perfectly adapted in their original niche. Populations need these underachievers because they have some gens that the majority doesn't have and those genes are making them less viable but in the new environment they just might blossom, saving the population from extinction. Populations need the weak!
And nicely explained dawh!
And another thing:
For species to diverge (for a fraction of the population to diverge from the other living in the same area and inhabiting the same niche) they need to be isolated from the main population (during Earths history a lot of geological changes took place as well) whether by uprising mountains, gaps created by earthquakes, small groups of animals arriving on islands... Isolation (which means that there's no reproduction, no mixing genes) is necessary for a new species to develop. Also populations can diverge if some members prefer reproducing with members of the same population that have a certain characteristic and if this continues over a vast period of time those two subpopulations might diverge to the point where reproduction between the members of the two subpopulations is no longer possible.
Some species simply change so much in time without diverging that the recent population has little in common with the population they have developed from.
And I think that this is the most that I have ever written in a post!