Climate change, disease, migration one way with no way back, competition from other species… so many variables. Honestly, the best thing here is to do some proper reading or sign up for a course.
The comment about evolution between species is just nonsense. Humans and fish have a common ancestor… humans and plants have a common ancestor. It was a very long time ago but thats facts. Populations undergo genetic drift when they are separated from one another over significant enough timescales. The example of Darwin’s finches is well known and was enough to convince the great man of his theory. Speciation occurs when those changes are large enough such that the offspring of a mating is either not viable or sterile.