The biological species concept is a practical way to define species using reproduction. It emphasizes gene flow within species and reproductive barriers between species, helping biologists classify biodiversity and interpret evolutionary relationships.

Core idea: species defined by interbreeding

The biological species concept (BSC)

The BSC focuses on reproductive compatibility, not just appearance. Under this concept, members of the same species share a common gene pool because alleles can be exchanged through mating and successful reproduction.

Key criteria in the specification

• Interbreed: individuals mate and produce offspring under natural conditions (not only in captivity or artificial lab settings).

• Viable offspring: offspring survive long enough to develop normally.

• Fertile offspring: offspring can reproduce and pass genes to the next generation; fertility maintains gene flow across generations.

Why “viable and fertile” matters

Hybrid offspring sometimes form between different populations, but the BSC treats populations as separate species when hybrids do not effectively transmit genes.

• If offspring are not viable, gene flow stops immediately.

• If offspring are sterile, gene flow stops after one generation.

• If hybrids are viable and fertile and reproduce successfully in nature, the populations may be considered the same species under the BSC.

Reproductive isolation and boundaries between species

Reproductive isolation as the boundary

In the BSC, species boundaries are maintained when reproductive isolation prevents sustained gene flow.

This schematic summarizes sympatric speciation: reproductive isolation evolves within a single geographic area as the population subdivides into groups that mate preferentially within themselves. The diagram emphasizes that the key transition is reduced gene flow, which allows the groups to diverge genetically. It helps connect “reproductive isolation” to the practical outcome of independent evolutionary lineages. Source

This makes the concept especially useful for understanding how populations remain genetically distinct even when they live near one another.

Interbreeding “in nature”

A critical detail is whether interbreeding occurs under natural conditions, because:

• forced matings in captivity may bypass normal mate choice or timing

• artificial methods can create offspring that would rarely (or never) occur in the wild Therefore, the BSC prioritises the real ecological and behavioural context in which mating occurs.

Strengths of the biological species concept (what it does well)

Matches evolution through gene flow

Because evolution acts on populations across generations, the BSC connects classification to the movement (or restriction) of alleles.

• Within a species: gene flow tends to homogenise allele frequencies across populations.

This graph shows allele frequency ($p$) over time in multiple subpopulations that exchange migrants, illustrating how gene flow causes allele frequencies to converge. As migration continues, population differences shrink and the system approaches a shared equilibrium frequency. This visual reinforces why sustained gene flow helps maintain a single, shared gene pool within a species. Source

• Between species: lack of gene flow allows populations to remain independent lineages.

Operational usefulness for many animals

For many sexually reproducing organisms (especially animals), evidence such as mating behaviour, fertility of offspring, and patterns of hybridisation can provide strong support for species classification under the BSC.

Limitations (where the BSC is difficult to apply)

Asexual organisms

Organisms reproducing primarily asexually do not form species boundaries through interbreeding, so the BSC is not directly applicable. Classification must instead rely on other lines of evidence.

Fossils and extinct species

Extinct organisms cannot be tested for interbreeding or offspring fertility, limiting the BSC’s usefulness in paleontology where only morphology or molecular remnants may be available.

Allopatric populations

Geographically separated populations may be potentially able to interbreed, but cannot be observed doing so. In such cases, the BSC can become inference-based and may depend on indirect evidence.

Hybridisation in nature

Some species pairs produce occasional hybrids. The BSC requires judgement about whether hybridisation is:

• rare and evolutionarily insignificant (little gene flow), supporting separation, or

• frequent enough to merge gene pools, supporting a single species designation.