The biological species concept defines species by reproductive compatibility and fertile offspring, while morphology, genetics, and exceptions complicate boundaries in practice.
Coverage 2015–2020 · Updated 15 Jul 2026
The biological species concept defines a species as a group of organisms that can breed with one another and produce fertile offspring.
Successful reproduction allows gene flow within the species, whereas reproductive isolation limits gene flow between species. During speciation, separated populations usually diverge gradually rather than becoming different species in one step.
To apply the concept: identify whether breeding is possible, determine whether offspring are viable and fertile, and distinguish actual reproductive isolation from populations merely being unable to meet. Because divergence is continuous, the point at which two populations receive different species names can be partly arbitrary.
Two geographically isolated populations may never meet in nature but could still produce fertile offspring if brought together. Their separation alone therefore does not prove that they are different biological species.
The concept is difficult to test for fossils, asexual organisms and geographically isolated populations, and competing species definitions may give a different boundary.
This objective is assessed through structured response, commonly using Discuss / Outline / State.
Discuss / Outline / State / Define
Build the answer around this relationship: Members of the same biological species can produce fertile offspring.
Using interbreeding alone without stating that offspring must be fertile.
Representative question
Outline the criteria that should be used to assess whether a group of organisms is a species.
For SL, A3.1 is really one evidence map. Variation explains why individuals differ. Morphology and binomial nomenclature help group and name organisms. The biological species concept uses interbreeding and fertile offspring, but boundaries can be difficult during gradual speciation. Chromosomes and karyograms add cellular evidence. Genomes, SNPs, genome size, and whole genome sequencing add molecular evidence. The skill is choosing the right evidence for the question.
