What defines a population that is substantially reproductively isolated from other populations?

The concept of an evolutionary significant unit (ESU) is crucial for understanding how populations are categorized based on their reproductive isolation. An ESU is defined as a population that is not only genetically distinct but also has a significant impact on the evolutionary trajectory of the species to which it belongs. This definition encompasses populations that are only capable of interbreeding within their group, which signifies substantial reproductive isolation from other populations.

When populations are reproductively isolated, they do not exchange genetic material, which allows them to evolve independently over time. This isolation can be due to geographic barriers, behavioral differences, or other forms of isolation. The importance of defining ESUs lies in conservation biology, as identifying these units aids in the preservation of biodiversity by ensuring that different genetically distinct populations are recognized and protected.

In contrast, other options like biospecies focus on the biological species concept, emphasizing potential interbreeding and genetic exchange, while genospecies refers to groups that are genetically similar but may not have clear reproductive barriers. A hybrid population typically consists of individuals resulting from the mating of different species or subspecies, which does not align with the notion of reproductive isolation. Therefore, the evolutionary significant unit is the most accurate term that captures the essence of substantial reproductive isolation among populations.