How is a species distinguished by the ability to interbreed?

In biology, the ability to interbreed is a fundamental criterion for defining a species. Under this concept, a species is defined as a group of natural populations that can interbreed with one another but are reproductively isolated from other such groups.1

This ability is significant because it establishes species as independent evolutionary units. Individuals within a species share a common gene pool, meaning genetic changes can spread throughout that population but cannot pass to members of other species.1

When populations are unable to interbreed, they are separated by reproductive isolating mechanisms (RIMs), which are categorized into two types:
* Prezygotic RIMs: These prevent fertilization from occurring, such as through behavioral, mechanical, or temporal differences.2
* Postzygotic RIMs: These take effect after fertilization, reducing the viability or fertility of hybrid offspring.2

It is important to note that this definition primarily applies to sexually reproducing organisms. It does not apply to organisms that reproduce asexually, such as bacteria, which are classified using other criteria like morphology and genetic constitution.13