suecica.[229] This happened about 20,000 years ago,[230] and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process.[231] Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms.[232]

Speciation events are important in the theory of punctuated equilibrium, which accounts for the pattern in the fossil record of short "bursts" of evolution interspersed with relatively long periods of stasis, where species remain relatively unchanged.[233] In this theory, speciation and rapid evolution are linked, with natural selection and genetic drift acting most strongly on organisms undergoing speciation in novel habitats or small populations. As a result, the periods of stasis in the fossil record correspond to the parental population and the organisms undergoing speciation and rapid evolution are found in small populations or geographically restricted habitats and therefore rarely being preserved as fossils.[234]

Extinction
Further information: Extinction


Tyrannosaurus rex. Non-avian dinosaurs died out in the Cretaceous–Paleogene extinction event at the end of the Cretaceous period.
Extinction is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation and disappear through extinction.[235] Nearly all animal and plant species that have lived on Earth are now extinct,[236] and extinction appears to be the ultimate fate of all species.[237] These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass extinction events.[238] The Cretaceous–Paleogene extinction event, during which the non-avian dinosaurs went extinct, is the most well-known, but the earlier Permian–Triassic extinction event was even more severe, with approximately 96% of species driven to extinction.[238] The Holocene extinction event is an ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Present-day extinction rates are 100–1000 times greater than the background rate and up to 30% of current species may be extinct by the mid 21st century.[239] Human activities are now the primary cause of the ongoing extinction event;[240] global warming may further accelerate it in the future.[241]

The role of extinction in evolution is not very well understood and may depend on which type of extinctio