Written by Jyotsna Iyer, a second-year undergraduate student.

Extinction broadly refers to the end of the existence of a species on earth.

Species is one of the basic classifications of organisms, and refers to a group of similar organisms capable of interbreeding. A species is declared extinct when the last organism of that species dies without leaving being survived by an offspring. However, in numerous cases, extinction is definitively predicted long before the last of the species dies.

This happens in cases where it becomes impossible for the organisms of the species to interbreed. For instance, in cases where all the living organisms from a particular species are of the same biological sex, reproduction becomes impossible.

As it is hard to determine the exact range of geographical distribution of these species, sometimes a species that is declared extinct is later found to have ‘reappeared’. This is known as the Lazarus taxa phenomenon. Extinction of species is a natural process, and it has been historically recorded that most species go extinct in about 10 million years since the beginning of their existence. However, there are species that defy this generalization and are known as ‘living fossils.’

These species survive for hundreds of millions of years with little changes in their structure and anatomy. One such example is the horseshoe crab, which has survived for more than 450 million years, making them probably older than the dinosaurs. Extinction is also classified into specific types. Pseudo extinction refers to when a species that goes extinct leaves behind an evolved but similar species. On the other hand, terminal extinction refers to the extinction of a species without leaving a successor. Terminal extinction can either occur gradually over millions of years, or a species could undergo sudden mass extinction due to certain circumstances.

What causes species to go extinct?

1. Invasive/ more dominant species: Charles Darwin proposed the ‘survival of the fittest theory, proposing that the species that are best adapted are the ones that survive in a competitive environment. This is one of the major natural causes of the extinction of species.
   
   When a certain species has to compete for resources and survival with a more evolved or better-adapted species, it is likely to go extinct. Invasive species refer to non-indigenous species that are newly introduced in an environment, that adversely affect the natural habitat and the indigenous species. Black rats are one such invasive species that led to the extinction of many bird, mammal, and reptile species on numerous islands.
   
2. Climate change: Climate change can prove to be fatal to species that are biologically suited to survive only within a particular climate range, and do not have the ability to adapt to the changing conditions. Polar bears are one of the mammal species that are vulnerable to extinction at present due to global warming.
   
3. Loss of habitat: The natural habitat of a species involves the all-encompassing environment that a species thrives in. The destruction of this natural habitat could expose the species to circumstances that it isn’t evolved to face and survive. Destruction of natural habitat was the cause of the extinction of species such as the Tarpan, clouded leopard, and golden toad.
   
4. Catastrophic events/ natural disasters: Rare catastrophic environmental events and natural disasters are capable of causing mass extinction of a species, like in the case of dinosaurs going extinct. The meteor strike that wiped out dinosaurs is one well-known example. Scientists have stated that there could have been at least 5 such mass extinctions in the past 540 million years, responsible for wiping out between 70% and 90% of all extinct species.
   
5. Poaching and hunting: Human activities such as poaching and hunting cause the population of a species to decrease at an unnaturally high rate. The quagga and passenger pigeons are two of the species that were driven to extinction by hunting by humans. At present, the rhinos are an endangered species due to hunting and are still at threat due to poaching and loss of habitat.
   
6. Pollution: Pollution leads to the contamination and destruction of air, land, and most importantly water. Around 700 marine species are currently facing the threat of extinction due to plastic pollution alone.
7. Lack of genetic diversity: If the population of a species is low, that is reflected in a lack of genetic diversity. This leads to the interbreeding of harmful traits, which could lead to a decrease in the species’ survival ability. For instance, susceptibility to a particular disease is a harmful trait that could be amplified due to interbreeding in a lack of genetic diversity. This could make the species vulnerable to extinction by being affected by a disease.

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