Darwin's Natural Selection Theory

The core of Darwin's theory of evolution is the pillar of natural selection theory. For example, the related species is characterized by the diversity of natural species, and the important content is supported by over-breeding, survival competition, genetic variation and survival of the fittest. For Darwin, he believes that both animals and plants, both of which have a strong fecundity, can reproduce a strong descendants. But when the environmental load force tolerated in an environment is too large, there will be a struggle between the same species, resulting in a small number of individuals who can grow safely and survive, but will also retain the better traits.
About the natural selection, There is a well-known example. There are a group