Explain how the concept of a species is used to define the boundary between two closely related organisms, and outline one limitation of applying this concept in practice.

IB DP Biology Standard Level (2023 syllabus) — A3.1 Diversity of organisms · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A species is a group of organisms that can interbreed and produce fertile offspring. Two closely related organisms are placed in different species when they are reproductively isolated and cannot produce fertile offspring together. One limitation of this concept is that it cannot be applied to asexually reproducing organisms, fossils, bacteria or to situations such as ring species or hybridising species where the boundary is unclear.

Examiner tips

  • Use the phrase "reproductive isolation" to show understanding of the species concept.
  • Mention that fertile offspring is the key criterion.
  • State a specific limitation (e.g. asexual organisms).
  • Keep the answer concise and to the point.

Common mistakes

  • Confusing viable with fertile offspring.
  • Failing to mention reproductive isolation.
  • Giving a general statement about species without linking it to interbreeding.

Mark scheme (4 marks)

  1. A species is defined as a group of organisms that can interbreed / mate with one another
  2. to produce fertile / viable offspring
  3. Two organisms belong to different species if they cannot interbreed to produce fertile offspring / there is reproductive isolation between them
  4. One limitation: the biological species concept cannot be applied to asexually reproducing organisms / fossils / bacteria, OR ring species / hybridising species complicate the boundary

Key terms in this question

species

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