But the fossil records of most species show rapid changes in their forms. That evolution should happen over a very short period of time should not be surprising to anybody who really thinks about it. Evidence that ecosystems can change virtually in the blink of an eye abounds in the world today. For example, entire forests of a single genus of trees on the eastern coast of the United States disappeared when a deadly fungus appeared early in the century. If evolution truly occurred slowly, with only slight changes over millions of years, animals would never be able to adapt to such rapidly changing environments.
All animals have variations within their species.
Often, some variation in the population will be better suited to a specific environment. If the environment shifts in that direction, it will give animals of that variation an advantage. Soon, most of the surviving animals carry the favored variation. If no animal carries a favorable variation, the stress placed on the animals by the new environmental conditions can interfere with the reproductive process, creating mutations. If these mutations do not provide the animal an advantage in the new environment, or don't happen at all, the species will go extinct.