Radiation and extinction
The term radiation (or adaptive radiation) refers to the proliferation of species over a relatively short period of geological time. One of the most spectacular adaptive radiations is the one popularly known as the Cambrian Explosion that occurred about 542 million years ago.
Extinctions refer to the dying out of species without any descendants. The fossil record provides direct evidence of how organisms radiated over time, as well as evidence of extinctions that befell certain lineages. Although extinctions are fairly common in the fossil record, there are five catastrophic mass extinction events that resulted in the dying out of more than 50 - 75% of existing organisms over a relatively short time period.
After each extinction event, there appears to have been a distinct adaptive radiation of organisms to occupy the vacated habitats. A classic example of this is the prolific radiation of mammals after the extinction of the dinosaurs at the end of the Cretaceous. During the coming weeks, I will discuss the various extinction events and the recovery thereafter, which often involved new adaptive radiations/diversifications.
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