Natural Selection is an explanation put forth by Charles Darwin to explain how species change over time. Populations over-reproduce, there is a struggle for existence, because no two individuals are alike, whichever variations an organism possesses that gives it an advantage, might make it survive and reproduce. This is survival of the fittest. Over time, if enough variations are passed down, a new species may evolve. Nature itself ultimately "selects" the best adapted individuals for survival. An example is the peppered moth in Industrial Revolution England. As trees became covered in soot, the white moths which previously dominated the area where consumed by predatory birds--natural selection. The black moths blended in better-an adaptation that gave it a survival advantage. Over time, the black moths outnumbered the light colored moths. Sexual selection goes hand in hand with Darwin's Theory of Natural Selection. In nature, males compete against other males for females to mate with. They do this by displays and by aggressive behaviors. The most successful males will pass their traits to the next generation and the least successful will leave few or no offspring. Artificial selection is when humans select the traits they deem beneficial. People have been breeding livestock and plant varieties for decades. Dog breeds and race horses are other examples of where artificial selection comes into play. Via artificial selection, people decide which organisms will be mated to hopefully get the outcome they desire.