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The motto "Evolution is a fact" requires a detailed explanation. Otherwise, it can be taken different ways by advocates of different points of view, leading to confusion; see [[Define your terms]].
The motto "Evolution is a fact" requires a detailed explanation. Otherwise, it can be taken different ways by advocates of different points of view, leading to confusion; see [[Define your terms]].
==Changes in populations of species==
Sometimes, "Evolution is a fact" refers to the common observation that the number of individuals of a certain species varies with time in a certain [[range]]. For example, the wolf or rat population tends to increase whenever and wherever human beings establish ranches or cities. More sheep, more prey for the wolves. More garbage, more food for the rats.
Natural changes, such as [[desertification]] generally reduce populations. In general, [[ecology]] studies how animals and plants increase or decrease their prevalence in the various regions on earth and underwater.
So, the observation that the size and extent of a population can change over time, varying with climate and other conditions, is rather straightforward and uncontroversial. People who hate wolves and rats know that if they don't feed them, they'll go away. But if they leave sheep unattended in unfenced pastures, wolves will come. Populations of species of animals grow and shrink in response to the environment, as do trees and other plants.