A population of fish is divided when tectonic uplift raises a land bridge, separating a large lake into two isolated bodies. After 50,000 years, the land erodes and the populations come into secondary contact. Which outcome would provide the STRONGEST evidence that allopatric speciation has been completed?
AThe two populations have visibly different coloration patterns
BThe two populations occupy different depth zones after secondary contact
CThe two populations interbreed freely and the populations merge back into one gene pool within a few generations
DThe two populations fail to produce viable or fertile offspring when they attempt to mate
Allopatric speciation is complete when reproductive isolation is sufficient to prevent gene flow upon secondary contact. Different coloration (option A) or habitat partitioning (option B) indicates divergence but doesn't prove reproductive isolation — the definition of distinct species. Free interbreeding (option C) shows speciation has NOT occurred. Failure to produce viable/fertile offspring — prezygotic or postzygotic isolation — is the direct evidence of completed speciation. The geographic barrier enabled divergence; reproductive isolation is the criterion.
Question 2 Multiple Choice
Which of the following is NOT a necessary requirement for allopatric speciation to proceed?
AGene flow between the separated populations must be prevented during the period of divergence
BThe separated populations must accumulate genetic differences through drift, selection, or both
CThe geographic barrier must permanently prevent any future secondary contact
DSufficient time must pass for reproductive isolation to develop
Allopatric speciation does NOT require the barrier to be permanent. In fact, secondary contact is common and informative — it tests how much isolation has accumulated. Many well-studied speciation events involve a period of isolation followed by secondary contact, where partial or complete reproductive isolation is observed. The barrier only needs to halt gene flow long enough for divergence to accumulate. Once reproductive isolation is established, the two species can coexist even if the barrier disappears.
Question 3 True / False
In allopatric speciation, the geographic barrier is directly responsible for generating the genetic differences between the separated populations.
TTrue
FFalse
Answer: False
The barrier's only role is to stop gene flow. The genetic divergence itself is produced by the evolutionary forces that operate independently in each population: mutation introduces variation, genetic drift changes allele frequencies (especially in small populations), and natural selection adapts each population to its local environment. Sexual selection may drive divergence in mating signals. The barrier is a necessary condition for divergence (without it, gene flow would homogenize the populations) but not the mechanism of divergence. This distinction matters: the same barrier duration produces more speciation in small populations (faster drift) than large ones.
Question 4 True / False
A hybrid zone — a geographic region where two formerly allopatric populations mate but produce offspring of reduced fitness — indicates that reproductive isolation is partial, not complete.
TTrue
FFalse
Answer: True
A hybrid zone is direct evidence of incomplete reproductive isolation: the populations can still interbreed (some gene flow is possible), but hybrid fitness is reduced, which maintains the distinction between them. This is a 'snapshot' of speciation in progress. Over time, the hybrid zone can narrow as selection against hybrids reinforces isolation, or it can widen and the populations can merge if isolation is insufficient to maintain separation. The existence of hybrid zones supports the view that speciation is a continuous process, not a single event.
Question 5 Short Answer
Why is secondary contact considered the critical test of whether allopatric speciation has occurred? What outcomes are possible, and what does each tell us?
Think about your answer, then reveal below.
Model answer: Secondary contact tests reproductive isolation directly — the defining criterion for speciation. Three main outcomes are possible: (1) The populations interbreed freely and merge, indicating speciation did not occur — not enough isolation accumulated during separation. (2) The populations form a hybrid zone with partial fertility or fitness reduction, indicating speciation is in progress but incomplete. (3) The populations coexist without interbreeding, indicating complete reproductive isolation and completed speciation. Secondary contact is the critical test because physical appearance and genetic divergence are insufficient — species are defined by reproductive isolation, and the only way to test that is to put the populations back together.
The time during allopatry determines which outcome occurs: short separation in large populations → likely merging; long separation in small populations with strong divergent selection → likely completed speciation. Reinforcement — natural selection against hybrids strengthening pre-mating isolation — can accelerate the completion of speciation after secondary contact. This is why the zone of contact is often the most evolutionarily active region of a species' range: it is where selection for reproductive isolation is strongest.