Questions: Reproductive Isolation: Mechanism Accumulation During Divergence

Question 1

An ancestral population has genotype AABB. Population 1 evolves to AAbb; Population 2 evolves to aaBB. Hybrids (AaBb) are inviable. Which of the following best explains why?

• A. Both populations had to pass through a low-fitness intermediate, and the inviable hybrid reflects that historical bottleneck
• B. The a and b alleles combine in hybrids for the first time, and this novel combination was never tested by natural selection in either lineage
• C. Genetic drift in isolated populations always produces deleterious alleles that are expressed in hybrids
• D. Postzygotic barriers require that one allele be directly deleterious, and natural selection failed to remove it

Answer: (B)

This is the Dobzhansky-Muller model. The key insight is that neither 'a' (in AAbb) nor 'b' (in aaBB) is deleterious in its home genetic background — each allele evolved and was fixed in a context where the other was absent. The incompatibility arises only when a and b are brought together in the hybrid for the first time — a combination that was never tested by selection in either lineage. No fitness valley was crossed because each step was neutral or beneficial locally.

Question 2

Why do prezygotic barriers typically evolve earlier during allopatric divergence than postzygotic barriers?

• A. Prezygotic barriers are simpler genetically and require only a single-locus change to become complete
• B. Mate recognition traits are often under strong sexual selection and can diverge rapidly; postzygotic barriers require incompatible genetic interactions to accumulate across two genomes
• C. Natural selection directly favors prezygotic barriers in allopatry as soon as populations separate
• D. Postzygotic barriers can only arise after prezygotic barriers have already reduced gene flow

Answer: (B)

Mate recognition signals (songs, colors, pheromones) and habitat preferences are often under strong sexual and ecological selection, allowing rapid divergence even in isolated populations. Postzygotic incompatibilities, by contrast, require specific incompatible alleles to arise independently in two separate populations — a process that depends on particular mutations accumulating and interacting badly in hybrids. This takes longer because it is not directly selected for in allopatry. Note: reinforcement can later directly select for stronger prezygotic barriers once hybrids are unfit, but this is a secondary contact phenomenon, not an allopatric one.

Question 3

The Dobzhansky-Muller model requires that at least one population pass through a period of reduced fitness as incompatible alleles accumulate.

• T. True
• F. False

Answer: False

This is precisely the problem that the Dobzhansky-Muller model solves. The 'fitness valley' problem asks: how can incompatible alleles be fixed by natural selection if intermediate genotypes are unfit? D-M resolves this by showing that each allele is fixed in a genetic background where it is compatible — allele 'a' fixes in a population that still has B, so the aB combination is functional. Only when a and b are brought together in a hybrid does the incompatibility manifest. No population ever experiences reduced fitness during the accumulation process.

Question 4

Reinforcement — natural selection strengthening prezygotic isolation — can only work if postzygotic barriers are already partially in place.

• T. True
• F. False

Answer: True

Reinforcement requires that hybridization be costly: selection acts against individuals that waste reproductive effort on unfit hybrids, favoring those that avoid cross-population mating. But this selection only exists if hybrids are actually less fit — which requires postzygotic barriers to already be present. Without postzygotic barriers, hybrids are viable and fertile, and there is no selection pressure against hybridization. Reinforcement is thus a secondary-contact mechanism that depends on a prior stage of genetic divergence producing hybrid dysfunction.

Question 5

Explain how two populations can accumulate postzygotic incompatibility without either population experiencing reduced fitness, using the Dobzhansky-Muller model.

Model answer: In the Dobzhansky-Muller model, incompatibility arises from independently evolved alleles that are each functional in their home genetic background. Imagine the ancestral genotype is AABB. Population 1 evolves a new allele 'a' in a genome that still has B — the aB combination works fine, so 'a' can be fixed by drift or selection. Population 2 independently evolves 'b' in a genome that still has A — Ab also works fine. When these populations hybridize, the hybrid carries both 'a' and 'b' for the first time. If 'a' and 'b' interact badly (incompatible protein products, disrupted developmental pathways), the hybrid is inviable or sterile — yet neither population ever had to survive a low-fitness intermediate.

The Dobzhansky-Muller model resolves the 'valley crossing' problem by showing that postzygotic incompatibility is an emergent property of independent evolution in two lineages, not a result of selection for incompatibility or any population passing through reduced fitness. The incompatible combination (a+b together) simply never existed in either population's evolutionary history — it is a novel combination that natural selection never had the opportunity to evaluate.