種の形成

Speciation occurs when a species splits into two or more new species, usually due to geographic isolation and genetic divergence. Over time, these factors result in populations that can no longer successfully breed with each other. This happens through two primary mechanisms: allopatric and sympatric speciation.

In allopatric speciation, the formation of a geographic barrier, like a mountain range, can cut off gene flow between two populations. Once they are separated, different climates, food sources, or predators can act as selective pressures. Over time, evolutionary processes such as natural selection and genetic drift can lead these populations to evolve into separate species.

This mechanism may also happen when a small group splits off, like birds that fly to a new island. This is known as a founder event, a common form of allopatric speciation. Over time, they undergo natural selection in the new environment, which stops gene flow between the two populations.

But reproductive isolation that leads to speciation does not require a geographic barrier.

In sympatric speciation, new species form within the same geographic area when groups within a population stop breeding with each other.

For example, in plants like corn, a mistake during cell division can create an individual with more than two complete sets of chromosomes. This condition is called polyploidy. Because the chromosome numbers do not match, polyploids usually cannot successfully breed with the original plants.