All species are capable of producing offspring faster than the food supply increases.
All living things show variation; no two individuals of a species are exactly alike.
Because there are more individuals than can possibly survive, there is a fierce struggle for existence and those with a favourable variation in characteristics are necessary for survival will possess an advantage over others.
These favourable variations are inherited and passed on to the next variation.
Over long periods of geologic time, these successful variations produce great differences that result in new species.
The background of the process is natural selection.
At the end of Paleozoic first land vertebrates appeared reptile Dimetrodon
Mass extinction at the end of Permian 90% of species extinct
http://www.pbs.org/wgbh/evolution/change/deeptime/index.html Evolution of life
http://www.pbs.org/wgbh/evolution/extinction/dinosaurs/index.html What happened before 65 millions of year s ? K/T border (= cretaceous / tertiary) – extinction of dinosaurs and a lot of other species (approximately 75 % of species)