Biodiversity Change Through Geological Time

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The graph plots percent extinction occurrences versus time in millions of years before present time, starting 550 million years ago. Extinction occurrences increase and decrease in a cyclical manner. At the lowest points on the cycle, extinction occurrences were between 2% and 5% percent. Spikes in the number of extinctions occurred at the end of geological periods: end-Ordovician, 450 million years ago; end-Devonian, 374 million years ago; end-Permian, 252 million years ago; end-Triassic, 200 million years ago; and end-Cretaceous, 65 million years ago. During these spikes, extinction occurrences approximately ranged from 22% to 50%.
Percent extinction occurrences as reflected in the fossil record have fluctuated throughout Earth’s history. Sudden and dramatic losses of biodiversity, called mass extinctions, have occurred five times. Source: OpenStax Biology 2e

OpenStax Biology 2e

The number of species on the planet, or in any geographical area, is the result of an equilibrium of two evolutionary processes that are continuously ongoing: speciation and extinction. Both are natural “birth” and “death” processes of macroevolution. When speciation rates begin to outstrip extinction rates, the number of species will increase; likewise, the number of species will decrease when extinction rates begin to overtake speciation rates. Throughout Earth’s history, these two processes have fluctuated—sometimes leading to dramatic changes in the number of species on Earth as reflected in the fossil record.

Paleontologists have identified five strata in the fossil record that appear to show sudden and dramatic (greater than half of all extant species disappearing from the fossil record) losses in biodiversity. These are called mass extinctions. There are many lesser, yet still dramatic, extinction events, but the five mass extinctions have attracted the most research. An argument can be made that the five mass extinctions are only the five most extreme events in a continuous series of large extinction events throughout the Phanerozoic (since 542 million years ago). In most cases, the hypothesized causes are still controversial; however, the most recent mass extinction event seems clear.

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e


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