Natural populations of any species usually contain variation. The environment around an individual organism is everything physical and biological that is not the organism. An organism may have characteristics that make it very well suited for a particular environment. That organism will probably leave more of its offspring than other varieties less well suited. Slowly the frequencies of the best suited variety will increase as the other varieties fail to compete for limited resources. Natural selection thus acts on varieties that exist and weeds out the less fit by limited resource competition. Natural selection does not create new varieties. If the environment becomes too severe for all of a species varieties the species might become extinct. Some fossil species do not look like anything known to be alive today so extinction is inferred. Meteor hits like Yucatan or Arizona's meteor crater caused extinction of local populations. Dodos were killed unnaturally by people.