Lesson 4 of 6 / Evolution, species and phylogeny
Species and reproductive isolation
How can populations stop exchanging genes?
In this lesson: Explain the biological species concept, its limits and allopatric/sympatric speciation.
About 8 min
The key ideaReduced gene flow allows divergence; speciation is established when reproductive isolation separates lineages, not merely when a barrier appears.
Explore the idea
Separation is the start of the question
The starting population contains variation and exchanges genes through interbreeding. Colours distinguish variants; they do not define different species.
Before fertilisation, barriers can prevent mating or gamete fusion. After fertilisation, low hybrid survival or fertility can restrict gene flow. A biological species concept is harder to apply to fossils, asexual organisms and populations whose breeding cannot be tested.
Original conceptual populations. The two clusters are a drawing aid and do not themselves imply geographical isolation. The cross means strongly reduced gene flow, not that every real species boundary is perfectly absolute. The contact outcome is evidence supplied to the model, not a guaranteed result of waiting.
Explanation
The biological species concept groups natural populations whose members can interbreed and produce viable, fertile offspring, while being reproductively isolated from other such groups. It focuses on gene flow rather than appearance alone.
The concept is difficult to apply to fossils, asexual organisms and geographically separated populations whose mating cannot be observed. Hybridisation between otherwise distinct groups also complicates sharp boundaries. A classification should state which evidence and species concept are being used.
In allopatric speciation, a geographical barrier reduces gene flow. Mutation, selection and other population processes can then produce divergence. If reproductive barriers persist when contact resumes, separate species may have formed. Geographical separation alone does not instantly create a new species.
In sympatric speciation, reproductive isolation develops within the same geographical region. Behavioural differences such as mating signals or timing, and physiological differences affecting compatibility, can reduce interbreeding. Host preference or ecological differentiation can contribute; in some plants polyploidy provides a further mechanism, though the central required idea is isolation without a geographical split.
Pre-fertilisation barriers can prevent mating or fertilisation; post-fertilisation barriers can reduce hybrid survival or fertility. Explain the barrier's effect on gene exchange rather than merely naming it.
Step by step
- 1
Identify the gene-flow barrier
Geography, behaviour or physiological incompatibility?
- 2
Explain divergence
Separate populations accumulate different inherited changes.
- 3
Test persistent isolation
Would gene exchange resume if contact occurred?
Worked example
Work through the evidence
Two bird populations live together but respond only to different courtship songs and rarely mate across groups. Which mode could this support?
One way to explain it
Sympatric divergence through behavioural reproductive isolation, if the barrier is persistent and supports separate gene pools.
Why this answer works
- There is no required geographical separation.
- Song difference matters because it changes mating and gene flow.
Is this true? "Two populations become different species the moment a river separates them."
The barrier reduces gene flow, but speciation requires sufficient reproductive divergence, not just location.