Allopatric Speciation

Mayr (1970) defined speciation as 'the creation of species'. Species can be defined as one of the basic units of biological classification, a group of organisms capable of interbreeding and producing fertile offspring. The process of speciation is possible by a few mechanisms, one of them being - Allopatric Speciation.

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In this scenario, a population splits into two geographically isolated populations by some - geographic barrier / extrinsic barrier / unfavourable habitat. Once a barrier is placed, speciation is only a matter of time (e.g. mountain range for terrestrial animals, land mass for aquatic animals). The geographical barrier leads to reproductive isolation and eventually reproductive barrier. The organism undergoes evolutionary change over many generations because:

• they become subjected to dissimilar selective pressures,
• different mutations arise in them,
• they independently undergo genetic drift.

When they come back into contact, they are no longer capable of exchanging genes. 

Example - Northern Spotted Owl and Mexican Spotted Owl

Photo by Andy Chilton on Unsplash

Extinction

Extinction refers to the disappearance of an animal group (e.g. a species), from the evolutionary period. It is not an unusual event, but rather a natural phenomenon. It is estimated that 99.9% of all species that ever lived on earth are now extinct. 

Extinction appears to be the ultimate fate of all species. The rate of extinction spikes in occasional mass extinction events. The 2 ways in which species may become extinct are:

• negative role of environmental selection in evolution (species develops a way of life such that a change in environment would prevent its persistence). 
• a species may become extinct as it is transformed into another. 

Major Extinction events:

1. Permian-Triassic extinction - Though it is less known, it was the most severe and approximately 96% of species were driven to extinction. 
2. Cretaceous-Tertiary extinction - The non-avian dinosaurs went extinct.
3. Holocene extinction - Ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Present-day extinction rates: 10-1000 times greater than the background rate.