Lesson 5 of 5 / Variation, selection and species
Decide what makes a biological species
Is looking different enough to establish two species?
I can explain the biological species concept and apply its reproductive criterion with appropriate limits.
I can explain the biological species concept.Syllabus
Syllabus 8876, Core 4(e). Explain the biological species concept.
About 5 min
Make a guess. You are not marked.
Two birds look almost identical. Are they definitely the same species?
Show the answer
What matters is fertile interbreeding.
Under the biological species concept, species are defined by reproductive isolation, not appearance.
The key idea
A biological species is a group of natural populations whose members can actually or potentially interbreed and produce fertile offspring, while being reproductively isolated from other groups.
Explore the idea
Test the reproductive evidence
This supports the same biological species despite visible variation.
Explanation
The biological species concept focuses on reproduction, not appearance alone. Members of the same species can actually or potentially interbreed under natural conditions and produce fertile offspring. Reproductive isolation limits gene exchange with other species.
Individuals can look different and still belong to one species, for example through sex, age or variation within a population. Conversely, very similar-looking populations can be reproductively isolated. A photograph alone is therefore often insufficient evidence for this species concept.
Producing a hybrid is not by itself enough. Fertility matters because fertile descendants can continue gene exchange. The commonly used horse-donkey example illustrates a usually sterile hybrid, but the biological concept concerns natural populations and reproductive isolation rather than a rule based on one exceptional cross.
Geographically separated populations may never meet, so potential interbreeding can be difficult to test. The concept is also difficult to apply to fossils and organisms that reproduce asexually. These limits explain why biologists may need additional evidence; they do not make appearance a reliable substitute for every case.
Step by step
- 1
Ask about interbreeding
Can members actually or potentially reproduce with one another under natural conditions?
- 2
Ask about fertility
Can the offspring continue reproducing and enable gene exchange?
- 3
Check the limits of the evidence
Isolation by distance or lack of living specimens may prevent a direct test.
Worked example
Two colour forms
Two differently coloured populations overlap naturally and repeatedly produce fertile offspring together. Does colour alone justify classifying them as different biological species?
One way to explain it
No. The interbreeding and fertile offspring support membership of the same biological species despite colour variation. Appearance alone does not establish reproductive isolation.
Why this answer works
- Apply the reproductive criterion.
- Distinguish within-species variation from a boundary between species.
Watch out for this
A student says: "Any two organisms that look alike must belong to the same biological species." What is wrong with this?
Show the answer
Similar appearance does not establish the ability to interbreed and produce fertile offspring. Reproductive evidence is central to this concept.
Check your understanding
Which evidence best supports a boundary under the biological species concept?
Show the answer
They are reproductively isolated and do not exchange genes through fertile offspring under natural conditions.
This addresses the reproductive criterion.
Exam-style questions
Check your understanding
Which is NOT a problem in applying the biological species concept?
Show the answer
Animals that interbreed in the wild
Interbreeding wild populations are exactly what the concept is designed for.
Quick recall
Is looking different enough to establish two species?