Extinction (biology) - biology.

« III ROLE OF MASS EXTINCTION IN EVOLUTION Historically biologists—most famous among them British naturalist Charles Darwin—assumed that extinction is the natural outcome of competition between newlyevolved, adaptively superior species and their older, more primitive ancestors.

These scientists believed that newer, more highly evolved species simply drove less well-adapted species to extinction.

That is, historically, extinction was thought to result from evolution.

It was also thought that this process happens in a slow and regularmanner and occurs at different times in different groups of organisms. In the case of background extinction, this holds true.

An average of three species becomes extinct every million years, usually as a result of the forces of naturalselection.

When this happens, new species—differing only slightly from the organisms that disappeared—rise to take their places, creating evolutionary lineages ofrelated species.

The modern horse, for example, comes from a long evolutionary lineage of related, but now extinct, species.

The earliest known horse had four toes onits front feet, three toes on its rear feet, and weighed just 36 kg (80 lb).

About 45 million years ago, this horse became extinct.

It was succeeded by other types ofhorses with slightly different characteristics, such as teeth better shaped for eating different plants, which made them better suited to their environments.

This patternof extinction and the ensuing rise of related species continued over the course of 55 million years, ultimately resulting in the modern horse and its relatives the zebrasand asses. In mass extinctions, entire groups of species—such as families, orders, and classes—die out, creating opportunities for the survivors to exploit new habitats.

In theirnew niches, the survivors evolve new characteristics and habits and, consequently, develop into entirely new species.

What this course of events means is that massextinctions are not the result of the evolution of new species, but actually a cause of evolution.

Fossils from periods of mass extinction indicate that most new species evolve after waves of extinction.

Mass extinctions cause periodic spurts of evolutionary change that shake up the dynamics of life on Earth. This is perhaps best demonstrated in the development of our own ancestors, the early mammals.

Before the fall of the dinosaurs, which had dominated Earth for morethan 150 million years, mammals were small, nocturnal, and secretive.

They devoted much of their time and energy to evading meat-eating dinosaurs.

With theextinction of dinosaurs, the remaining mammals moved into habitats and ecological niches previously dominated by the dinosaurs.

Over the next 65 million years, thoseearly mammals evolved into a wide variety of species, assuming many ecological roles and rising to dominate Earth as the dinosaurs had before them. IV THE CURRENT EXTINCTION CRISIS Most scientists agree that life on Earth is now faced with the most severe extinction episode since the event that drove the dinosaurs extinct.

No one knows exactly howmany species are being lost because no one knows exactly how many species exist on Earth.

Estimates vary, but the most widely accepted figure lies between 10 and13 million species.

Of these, biologists estimate that as many as 27,000 species are becoming extinct each year.

This translates into an astounding 3 species everyhour. Humans are the cause of this latest mass extinction.

With the invention of agriculture some 10,000 years ago, humans began drastically modifying or replacing theworld’s terrestrial ecosystems to produce farmland.

Today industrial pollution destroys ecosystems even in remote deserts and in the world’s deepest oceans.

Burningfossil fuels for heat and energy releases greenhouse gases such as carbon dioxide into the atmosphere, contributing to the warming of Earth ( see Global Warming) and causing polar ice to melt and sea levels to rise.

In addition, we have cleared forests for lumber, pulp, and firewood.

We have harvested the fish and shellfish of theworld’s largest lakes and oceans in volumes that make it impossible for populations to recover fast enough to meet our harvesting needs.

And everywhere we go,whether on purpose or by accident, we have brought along species that disrupt local ecosystems and, in many cases, drive native species extinct.

For instance, Nileperch were intentionally introduced to Lake Victoria for commercial fishing in 1959.

This fish proved to be an efficient predator, driving 200 rare species of cichlid fishesto extinction. This sixth extinction, as it has come to be known, poses a great threat to our continued existence on the planet.

As the sum of all species living in the world'secosystems, known as biodiversity, dwindles, so too go many of the resources on which we depend.

Humans use at least 40,000 different plant, animal, fungi, bacteria,and virus species for food, clothing, shelter, and medicines.

In addition, the fresh air we breathe, the water we drink, cook, and wash with, and the many chemicalcycles—including the nitrogen cycle and the carbon cycle, so vital to sustain life—depend on the continued health of ecosystems and the species within them. The list of victims of the sixth extinction grows by the year.

Forever lost are the penguinlike great auk; the passenger pigeon; the zebralike quagga; the thylacine; theBalinese tiger; the ostrichlike moa; and the tarpan, a small species of wild horse, to name but a few.

More than 16,000 plants and animals are threatened by extinction.Each of these organisms has unique attributes—some of which may hold the secrets to increasing world food supplies, eradicating water pollution, or curing disease. A subspecies of the endangered chimpanzee, for example, has recently been identified as the probable origin of the human immunodeficiency virus, the virus thatcauses acquired immunodeficiency syndrome (AIDS).

But these animals are widely hunted in their west African habitat, and just as researchers learn of theirsignificance to the AIDS epidemic, the animals face extinction.

If they become extinct, they will take with them many of the secrets surrounding this devastatingdisease. Frogs and other amphibians produce a variety of compounds in their skins that are potentially medically useful, including antibiotics and painkillers.

However, a third ofall amphibian species worldwide are considered threatened.

Many species are susceptible to the chytrid fungus, which can be deadly.

The fungus has struck amphibianpopulations in different parts of the world, adding to threats from climate change and pollution.

A number of species of frogs in tropical regions have disappeared andare thought to be extinct. V SPECIES CONSERVATION In the United States, legislation to protect endangered species from impending extinction includes the Endangered Species Act of 1973.

The Convention on InternationalTrade in Endangered Species of Wild Fauna and Flora (CITES), established in 1975, enforces the prohibition of trading of threatened plants and animals betweencountries.

The Convention on Biological Diversity, an international treaty developed in 1992 at the United Nations Conference on the Environment and Development,obligates more than 160 countries to take action to protect plant and animal species. Scientists meanwhile are intensifying their efforts to describe the species of the world.

So far biologists have identified and named more than 1.75 million species—amere fraction of the species believed to exist today.

Of those identified, special attention is given to species at or near the brink of extinction.

The World ConservationUnion (IUCN) maintains an active list of endangered plants and animals called the Red List.

In addition, captive breeding programs at zoos and private laboratories arededicated to the preservation of endangered species.

Participants in these programs breed members of different populations of endangered species in an effort toincrease their genetic diversity, thus enabling the species to better cope with further threats to their numbers. All these programs together have had some notable successes.

The peregrine falcon, nearly extinct in the United States due to the widespread use of the pesticide DDT,rebounded strongly after DDT was banned in 1973.

The brown pelican and the bald eagle offer similar success stories.

The California condor, a victim of habitatdestruction, was bred in captivity, and small numbers of them are now being released back into the wild.. »