Natural Selection & Evolution
The idea of natural selection was first proposed by Charles Darwin. Natural selection is the idea that in a population of a given species, there will be some organisms with traits, or adaptations, that will help it survive better. The organisms that survive best will then pass these traits on to the next generation, and these traits will then become more common. As certain traits become more common, the appearance of an entire species can change.
For a more specific breakdown of natural selection, along with some additional facts, watch the YouTube video above.