The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring. Members of different species are reproductively isolated because they do not normally exchange alleles through successful reproduction.
This core definition is required at both SL and HL in A3.1 Diversity of organisms.
How the Concept Works
The concept focuses on reproduction rather than appearance. Two organisms may look similar but belong to different species if they cannot produce fertile offspring; conversely, individuals within one species may vary greatly in appearance while remaining capable of successful interbreeding.
Successful reproduction involves three conditions:
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The organisms can mate under natural conditions.
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Fertilization produces viable offspring that survive.
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The offspring are fertile and can reproduce themselves.
For example, a horse and a donkey can produce a mule, but most mules are sterile. Horses and donkeys are therefore classified as separate species because there is a barrier to gene flow between their populations.
Case | Application of the concept |
|---|---|
Organisms produce fertile offspring | Usually classified as the same species |
Organisms cannot reproduce together | Classified as different species |
Organisms produce sterile hybrids | Classified as different species |
Asexual organisms or bacteria | The concept is difficult to apply because reproduction does not involve normal interbreeding; this limitation is an HL extension |
A common misconception is that organisms belong to the same species simply because they look alike. Appearance is central to the morphological species concept, but the biological species concept depends on reproductive compatibility and fertility.
Exam Technique
For a state question, write: “A species is a group of organisms that can interbreed and produce fertile offspring.” For an explain question, add that reproductive isolation prevents gene flow between species. When discussing limitations, identify why the concept cannot be applied effectivelybutnot merely that classification is “difficult.”