Simplified to the extreme for the retard:

• Species A are small organisms that can move quickly to catch prey. This is advantageous as food is scarce and predation from larger organisms is a concern. They are very successful and come to inhabit a large area.

• One of these areas actually has an abundance of prey and a lack of any other predators. Members of Species A in this area no longer need to run to catch their food, thus moving quickly is no longer a particularly beneficial trait. Nor is being small. Over time, this population of Species A become larger and slower, as offspring with these traits are no longer victims of predation or starvation.

• Eventually, members of this new population are on average over five times the size of other members of Species A and are simply physically incapable of mating with them. They are now Species B.