Two populations evolve in isolation. In population A, a neutral mutation changes allele 'A' to 'a' (giving genotype aaBB). In population B, a different neutral mutation changes 'B' to 'b' (giving AAbb). Hybrids produced when populations meet are sterile. Which best explains this?
ANatural selection in each population favored alleles that would prevent future interbreeding
BThe 'a' and 'b' alleles interact poorly because they have never co-occurred before, even though each is neutral in its own background — a Dobzhansky-Muller incompatibility
CIsolation caused both populations to develop prezygotic barriers before the populations met
DBoth mutations must have been harmful in their respective populations to produce sterility
This is the Dobzhansky-Muller model. Neither mutation needed to be harmful — each was neutral in its own genetic background. The incompatibility only emerges when the alleles meet for the first time in a hybrid. This shows reproductive isolation can evolve as a byproduct of ordinary genetic divergence, without selection ever directly favoring barriers to interbreeding.
Question 2 Multiple Choice
A biologist finds that two closely related frog species breed at the same pond but at completely different seasons — one in early spring, the other in late summer — and never interbreed. A third pair of frog species does hybridize, but the hybrid tadpoles die during development. Which statement correctly categorizes these barriers?
ABoth are postzygotic barriers — both reduce reproductive success after mating attempts
BThe first is postzygotic (temporal isolation); the second is prezygotic (hybrid inviability)
CThe first is prezygotic (temporal isolation); the second is postzygotic (hybrid inviability)
DBoth are prezygotic barriers because neither produces a viable adult hybrid
Temporal isolation acts before mating ever occurs — the species never encounter each other's mates — making it prezygotic. Hybrid inviability acts after a zygote forms but the embryo fails to develop, making it postzygotic. The classification hinges on whether the barrier prevents zygote formation (prezygotic) or reduces fitness after the zygote exists (postzygotic).
Question 3 True / False
Postzygotic reproductive barriers act after a hybrid zygote has formed, reducing or eliminating the fitness of the hybrid or its descendants.
TTrue
FFalse
Answer: True
True. Postzygotic barriers by definition act after fertilization. They include hybrid inviability (embryo fails to develop normally), hybrid sterility (hybrid survives but cannot reproduce, as with mules), and hybrid breakdown (F1 hybrids are viable but F2 or backcross offspring show reduced fitness as incompatible gene combinations segregate out). In each case a zygote formed; the barrier reduces fitness thereafter.
Question 4 True / False
According to the Dobzhansky-Muller model, at least one of the mutations involved in a reproductive incompatibility should have been harmful (or selected against) in the population where it first arose.
TTrue
FFalse
Answer: False
This is precisely what the Dobzhansky-Muller model refutes. Both mutations can be neutral or even beneficial in their own genetic backgrounds. Because the two populations are isolated, the alleles never co-occur until secondary contact, so there is no selection against either allele individually. The incompatibility is a novel property of the *combination* — it emerges only in hybrids. This elegantly explains how reproductive isolation evolves without any selection directly favoring it.
Question 5 Short Answer
Why is population isolation a necessary prerequisite for Dobzhansky-Muller incompatibilities to accumulate?
Think about your answer, then reveal below.
Model answer: In a continuous interbreeding population, all alleles co-occur and are exposed to selection together. A new mutation must be compatible with all alleles already present in the population, or it will be eliminated. Isolation allows allele 'a' to fix in one population and allele 'b' to independently fix in another, without the two alleles ever co-occurring and being tested in combination. Only when isolation ends and hybrids form do these alleles meet for the first time. A Dobzhansky-Muller incompatibility cannot accumulate in a single population because incompatible combinations would be exposed to selection before they could spread.
The model requires at least two independently evolving lineages. Each lineage can accumulate neutral substitutions freely in its own genetic background. When isolation breaks down, the novel combination of alleles — never previously tested by selection — can cause hybrid dysfunction. This also explains why the number of potential Dobzhansky-Muller incompatibilities grows rapidly with divergence time: each new substitution in one lineage can potentially be incompatible with any subsequent substitution in the other.