The term "extinction" is a familiar concept to most people. It is defined as the complete disappearance of a species when the last of its individuals dies off. Usually, complete extinction of a species takes very long amounts of time and does not happen all at once. However, on a few notable occasions throughout Geologic Time, there have been mass extinctions that totally wiped out the majority of species living during that time period. Every major Era on the Geologic Time Scale ends with a mass extinction.
Mass extinctions lead to an increase in the rate of evolution. The few species that manage to survive after a mass extinction event have less competition for food, shelter, and sometimes even mates if they are one of the last individuals of their species still alive. Access to this surplus of resources to meet basic needs can increase breeding and more offspring will survive to pass their genes down to the next generation. Natural selection then can go to work deciding which of those adaptations are favorable and which are outdated.
Probably the most recognized mass extinction in the history of the Earth is called the K-T Extinction. This mass extinction event happened between the Cretaceous Period of the http://evolution.about.com/od/LifeOrigins/a/Mesozoic-Era.htm">Mesozoic Era and the Tertiary Period of the Cenozoic Era. This was the mass extinction that took out the dinosaurs. No one is completely sure how the mass extinction happened, but it is thought to be either meteor strikes or an increase in volcanic activity that blocked out the sun's rays from reaching the Earth, thus killing the food sources of the dinosaurs and many other species of that time. Small mammals managed to survive by burrowing deep underground and storing food. As a result, mammals became the dominant species in the Cenozoic Era.
The largest mass extinction happened at the end of the Paleozoic Era. The Permian-Triassic mass extinction event saw about 96% of marine life go extinct, along with 70% of terrestrial life. Even insects weren't immune to this mass extinction event like many of the others in history. Scientists believe this mass extinction event actually happened in three waves and were caused by a combination of natural disasters including volcanism, an increase of methane gas in the atmosphere, and climate change.
Over 98% of all living things recorded from the history of the Earth have gone extinct. The majority of those species were lost during one of the many mass extinction events throughout the history of life on Earth.