Lesson 5 of 5 / Variation, selection and species
Decide what makes a biological species
Is looking different enough to establish two species?
In this lesson: Explain the biological species concept and apply its reproductive criterion with appropriate limits.
About 5 min
The key ideaA 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.
Is this true? "Any two organisms that look alike must belong to the same biological species."
Similar appearance does not establish the ability to interbreed and produce fertile offspring. Reproductive evidence is central to this concept.