Individual organisms differ, and some of the variations are heritable. Organisms produce more offspring than can survive, and many that do survive do not reproduce. Because more organisms are produced than can survive, they compete for limited resources. Each unique organism has different advantages and disadvantages in the struggle for existence. Individuals best suited to their environment survive and reproduce most successfully. These organisms pass their heritable traits to their offspring. Other individuals die or leave fewer offspring. This process of natural selection causes species to change over time. Species alive today are descended with modification from ancestral species that lived in the distant past. This process, by which diverse species evolved from common ancestors, unites all organisms on Earth into a single tree of life.
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