Today’s modern cheetahs can run 60 mph to catch prey. Their ancestors could originally only run 20 mph. There was selective pressure in the environment of cheetahs that demanded for faster cheetahs to catch more prey because there was a battle for survival.
There was variation in the ancestral population. Some cheetahs could run faster than others. These cheetahs were able to catch more prey, which resulted in more survival and reproduction. This was a heritable trait, so it was passed on from one generation to the next.
Eventually, faster cheetahs dominated the population because they were the fittest. Then the cheetahs’ prey got faster because they evolved. This called for faster cheetahs. There was a variation within the population and some cheetahs were faster than others. The faster ones were again able to catch more prey and therefore survived and reproduced more.
As time went on, cheetahs kept on growing faster because their prey grew faster too and so the fastest and fittest cheetahs became the majority of the cheetah population.