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Extinction

Extinction occurs when the last individual of a species or taxa dies (though some scientists argue that it's when the last chance of reproduction vanishes). In most cases, extinction is completely natural. It is commonly believed that about 99.9% of all species that have ever existed are extinct. New species rise up through evolution and adaptation to replace extinct ones, sometimes displacing them and other times adapting to niches that the old species could not manage. It is common for a species to have a life span of a several million years from the date of first appearance, though some survive largely unchanged for hundreds of millions of years (e.g. certain shark, fish, crocodile, turtle) or even billions of years (e.g. cyanobacteria) for a variety of reasons.

Extinction does not mean there are no descendants. Daughter species often evolve to carry on most of the genetic information from the extinct species, and occasionally a throwback offspring can exhibit most or all the traits of the extinct species; this is sometimes called pseudoextinction. In some cases, though, the extinct species did not produce a viable daughter species, and the entire line is now extinct.

Today, a common environmental concern is the extinction of species. In the last few centuries, thousands of species have gone extinct due to radical environmental change imposed by human societies. Some species have been hunted into extinction and others are squeezed out of their niches by land development. In some cases, newly introduced predators and competitors for food squeeze out other native species, endangering them.

The most forceful cause of extinctions have been mass extinction events which happen in geologically short periods of time. Five mass extinctions throughout biological history have been intense enough to leave conclusive fossil evidence.

Web Resources On Extinction

Paleontology and Extinction
PBS Feature on Extinction and Evolution

Book Resources On Extinction

Extinction : How Life on Earth Nearly Ended 250 Million Years Ago by Douglas H. Erwin
Extinction: Bad Genes or Bad Luck? by David M. Raup

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