The idea of evolution presents that all species evolved and adapted to their environments over time. Charles Darwin is the person who created this thought in the first place. Overpopulation and limited resources presents the idea that species evolved themselves to be able to mate successfully and gather food successfully.
This can lead to evolution by getting certain traits that help the species survive, such as the cheetah's "Cleopatra eyeliner". The "eyeliner" is actually a fur pattern that helps keep the sun out of their eyes. Bears go into hibernation so that they can sleep throughout winter and still be alive.