The formation of a geographic barrier such as a river causes fragmentation in a population. Gradually, genetic drift and natural selection cause genetic differences to accumulate. During a drought year the river dries up and the two populations begin to occupy the dry riverbed. The two populations do not produce viable offspring.

Why?

Question

The formation of a geographic barrier such as a river causes fragmentation in a population. Gradually, genetic drift and natural selection cause genetic differences to accumulate. During a drought year the river dries up and the two populations begin to occupy the dry riverbed. The two populations do not produce viable offspring.

Why?

BlackTom AI · Accepted Answer

Post-zygotic isolation — specifically hybrid breakdown — explains why these reunited populations do not produce viable offspring.

1) Relevant knowledge points and concepts
- Allopatric speciation: a geographic barrier (the river) separates a population, allowing genetic drift and natural selection to cause divergence in isolated groups.
- Reproductive isolation: mechanisms that prevent gene flow can be pre-zygotic (prevent mating or fertilization) or post-zygotic (reduce viability or fertility after fertilization).
- Hybrid breakdown (a form of post-zygotic isolation): hybrids between diverged populations may be viable/ fertile initially but their offspring (or hybrids themselves) have reduced fitness, inviability, or sterility due to incompatible genetic interactions (Dobzhansky-Muller incompatibilities).

2) Step-by-step reasoning
- Step 1: River creates geographic isolation; populations evolve separately by drift/selection, accumulating genetic differences.
- Step 2: River dries and secondary contact occurs; the two populations now meet in the dry riverbed.
- Step 3: Despite contact, crosses between individuals yield nonviable offspring. Because mating/contact occurred but offspring fail, the barrier to gene flow is acting after fertilization — a post-zygotic barrier.
- Step 4: The pattern of nonviable or low-fitness hybrids fits hybrid breakdown or related post-zygotic mechanisms caused by incompatible alleles accumulated during isolation.

3) Calculations
- None required for this conceptual question.

4) Why the correct answer is correct
- The populations had geographic isolation (allopatric), then upon secondary contact they cannot produce viable offspring. That points to reproductive incompatibilities expressed after zygote formation — classic post-zygotic isolation, specifically hybrid breakdown/inviability due to accumulated genetic differences.

5) Why the other options are incorrect
- "The river caused sympatric speciation": incorrect because sympatric speciation occurs without geographic separation; here a river created geographic isolation (allopatric).
- "Hybridization has already occurred": incorrect; hybridization implies successful interbreeding and viable offspring, but the scenario reports nonviable offspring.
- "Pre-zygotic isolation due to migration": incorrect because pre-zygotic mechanisms prevent mating or fertilization (e.g., behavioral, temporal, mechanical), whereas these populations do meet and attempt reproduction but produce nonviable offspring — a post-zygotic effect.

6) Conclusion
The correct answer, Post-zygotic isolation due to hybrid breakdown, is consistent with divergence during geographic isolation and failure to produce viable offspring upon secondary contact, and matches the provided answer.

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