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Several basic observations establish the theory of evolution, which explains the variety and relationship of all living things. There are genetic variations within a population of individuals. Some individuals, by chance, have features that allow them to survive and thrive better than their kind. The individuals that survive will be more likely to have offspring of their own. The offspring might inherit the useful feature.
Evolution is not a random process for creating new life forms. While mutations are random, natural selection is not. Evolution is an inevitable result of imperfectly copying, self-replicating organisms reproducing over billions of years under the selective pressure of the environment. The result is not perfectly designed organisms. Rather, the result is individuals that can survive better than their nearest neighbor can in a particular environment. Fossils, the genetic code, and the peculiar distribution of life on earth, which demonstrates the common ancestry of all organisms, both living and long dead, provide a record of evolution. Evolution can be directly observed in artificial selection, the selective breeding for certain traits of domestic animals and plants. The diverse breeds of cats, dogs, horses, and agricultural plants serve as examples of evolution.
Although some groups raise [[Objections to evolution|objections to the theory of evolution]], the evidence of observation and experiments over a hundred years by thousands of scientists supports evolution.<ref name=DeVries/> The result of four billion years of evolution is the diversity of life around us, with an estimated 1.75 million different species in existence today.<ref name=Cavalier-Smith/><ref name=>{{cite web |url=http://www.enviroliteracy.org/article.php/58.html | title=How many species are there? |accessdate=2008-01-05 |last=Sedjo|first=Roger|date=2007 |publisher=Environmental Literacy Council}}</ref>
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