Although different species look very different, the unity among organisms becomes apparent from an analysis of internal structures, the similarity of their chemical processes, and the evidence of common ancestry. Biological evolution accounts for a diversity of species developed through gradual processes over many generations. Species acquire many of their unique characteristics through biological adaptation, which involves the selection of naturally occurring variations in populations. Biological adaptations include changes in structures, behaviors, or physiology that enhances reproductive success in a particular environment. Extinction of a species occurs when the environment changes and the adaptive characteristics of a species do not enable it to survive in competition with its neighbors.
Fossils indicate that many organisms that lived long ago are now extinct.
Extinction of species is common. Most of the species that have lived on the Earth no longer exist. The solid Earth is layered with a thin brittle crust, hot convecting mantle, and dense metallic core. Crustal plates on the scale of continents and oceans constantly move at rates of centimeters per year in response to movements in the mantle. Major geological events, such as earthquakes, volcanoes, and mountain building, result from these plate motions.