Natural selection is the theory that only the strong survive. For example, the animals that can outrun their predators live to pass on their speedy genes; the slow are eaten.
Natural selection is part of Charles Darwin's Theory of Evolution. England's peppered moth is a great example of natural selection. Before 1800, most peppered moths were light colored. Then came the Industrial Revolution. The soot from the factories darkened the once light-colored tree trunks, making the light moths immediately visible — and edible — to birds. Dark moths blended into the trees, surviving and having dark-colored young.