Overproduction of offspring/ Struggle for Survival
Many animals have evolved and have changed over time. Many animals have used the process of natural selection the changes that occur from mutations and environments.
Differential Survival and Reproduction
Organisms that are in a population are all different from one another. These differences are caused by mutations that may lead to changing the phenotype of the organism. For example, the deer in the picture they each have a different color.
Change in Gene Pool
Having a lot of an organism but, not all are capable of surviving. Finding a home, space, find a mate and the ability to scare predators away. The organisms with the beneficial traits have a better chance for survival. In this case, the small male deer struggle b/c they don't have the beneficial trait that helps them survive from their predators.
An organisms capacity to survive in an environment. Individuals with beneficial traits tend to live longer than the individuals that don't. In this scenario, the deer with longer antlers has a better chance of defending itself from predators.
To change all genes of a population. As seen in the image above, all the deer are different colors or are bigger than others.
Natural selection has been going on during centuries and it's what has kept many organisms alive, including us humans.