Final answer:
A population is a group of individuals of the same species that interbreed within a given region to produce fertile offspring. The biological species concept uses the ability to interbreed as the principal criterion for species definition, despite some exceptions with hybrid offspring.
Step-by-step explanation:
With regard to sexually reproducing species, a population is a group of individuals of the same species that occupy the same region and can interbreed with one another. A species is effectively a group of individual organisms that interbreed and produce viable, fertile offspring. This biological species concept is foundational as it uses the ability to interbreed as its criterion.
According to this concept, species are reproductively isolated from other groups, ensuring that gene flow only occurs within the species. This reproductive isolation is what differentiates one species from another by preventing inter-species mating that would result in fertile offspring. For example, Homo sapiens is a species because all humans can interbreed with one another and produce fertile offspring.
However, it is important to note that there are exceptions to this rule as some species can produce hybrid offspring. These instances often suggest an ongoing speciation process and may indicate that the involved species have not become completely reproductively isolated.