Under the biological species concept, sexually reproducing organisms belong to the same species when they can freely interbreed to produce fertile offspring; species boundaries are therefore maintained by reproductive isolation, the set of barriers that prevent gene flow between populations. These barriers are partitioned by their timing relative to zygote formation: prezygotic barriers (temporal, habitat, behavioral, mechanical, and gametic isolation) act before fertilization, while postzygotic barriers (zygote mortality, hybrid inviability, and hybrid sterility) act after a zygote forms by preventing it from developing into a viable, fertile adult. The framework belongs to evolutionary biology and taxonomy, and its central limitation — the species problem — is that it cannot be applied to asexually reproducing organisms, which do not interbreed at all.
Under the biological species concept, sexually reproducing organisms belong to the same species when they can freely interbreed to produce fertile offspring; species boundaries are therefore maintain…