Evolution.
Evolution is a series of changes that occur to species, which make them better suited to their environment. The changes that occur are normally related to the harsh situations species may encounter in their new habitat.
All species have evolved and changed, from single-celled microorganisms to elephants. Cells for example, where simple organisms and didn't have all the organelles and types they now have. Elephants, also varied a lot in size and appearance.
Hope it helped,
BioTeacher101
