Originally Posted by CyndiLouSan
Is it because where they were living(niche) has been altered so much that they can no longer exist as the species they are so they have to change into a new species to exist?
There's no "have to" about it. If a niche is altered then different attributes may improve the chances of bearing progeny. Over time the differences may accumulate to the point where interbreeding is difficult. At that point we'd say - from the outside - that a new species appeared. That doesn't mean that the old species had to die out. It's all about probability and proportions. It may be true that if the alteration is severe enough, only those with the different attributes survive in sufficient numbers or that the newer types outcompete the older ones for that niche. But it's always a matter of individuals living and dying and competing against one another.