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Isolation and speciation

A new species often arises when a population is split, for example by a mountain, a sea or a river. Each part gets its own mutations, and natural selection can pull them in different directions if the environments differ. After a long time the differences are so large that the individuals no longer have fertile offspring if they meet again. Then there are two species. The isolation can also be that they reproduce at different times, or that they do not recognise each other's signals. Time and a lack of gene flow are what it takes.

isolation+time→two species\text{isolation} + \text{time} \rightarrow \text{two species}a common path to speciation

Symbols

geneflowgene flowexchange of alleles between populations

Example

As long as the individuals still have fertile offspring together, they are the same species.

When that exchange stops and the differences grow, there can be two species.

Speciation happens in the population over generations, not by one individual transforming.
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Part of Biology 1: Evolution.