Explain how the concept of reproductive isolation is central to the biological species concept.

IB DP Biology Standard Level (2023 syllabus) — A4.1 Evolution and speciation · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring, but are reproductively isolated from other such groups.

Model answer (4 marks)

Reproductive isolation stops gene flow between two groups, so they cannot interbreed.

Because no genes are exchanged, each group accumulates its own mutations, experiences genetic drift and is shaped by natural selection, creating genetic differences.

Members that can still interbreed share a common gene pool and produce fertile offspring, which is the defining unity of a species.

When reproductive isolation becomes complete, the two populations can no longer interbreed even if they meet, so they are recognised as separate species.

Examiner tips

Use the word ‘gene flow’ and ‘interbreed’ – key terms in the mark scheme.
Show the causal chain: isolation → no gene flow → genetic divergence → separate species.

Common mistakes

Confusing reproductive isolation with ecological isolation; they are different. Forgetting to mention that fertile offspring is the criterion for a species.

Mark scheme (4 marks)

Reproductive isolation prevents gene flow between populations / prevents interbreeding between different species
Without gene flow, isolated populations accumulate different alleles / genetic differences over time through independent mutation, genetic drift, or natural selection
Members of the same species share a common gene pool / can interbreed to produce fertile offspring, which defines their unity as a species
Once reproductive isolation is complete / absolute, the two populations are considered separate species because interbreeding is no longer possible even if they come into contact

Key terms in this question

reproductive isolation · biological species concept