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Difficulty: HardAdaptive Radiation and Mechanisms of Speciation

A continuous population of ancestral organisms undergoes allopatric speciation via vicariance following a geological event. Arrange the following evolutionary events in the correct chronological sequence from the initial ancestral state to the complete establishment of distinct species.

  1. 1A single continuous interbreeding population occupies a uniform geographical habitat.
  2. 2Geological movement forms a physical barrier that divides the original population into geographically isolated subpopulations.
  3. 3Independent genetic drift and divergent natural selection alter allele frequencies in each isolated gene pool.
  4. 4Prezygotic reproductive isolation mechanisms, such as modified courtship behaviors or altered mating seasons, evolve independently.
  5. 5The geographical barrier breaks down, but secondary contact yields no interbreeding due to established reproductive barriers.

Answer

The correct chronological sequence of vicariant allopatric speciation events is: (1) A single continuous interbreeding population occupies a uniform geographical habitat, (2) Geological movement forms a physical barrier dividing the original population, (3) Independent genetic drift and divergent natural selection alter allele frequencies, (4) Prezygotic reproductive isolation mechanisms evolve independently, and (5) Breakdown of the barrier upon secondary contact yields no interbreeding.
The correct sequence follows the classic allopatric speciation pathway via vicariance: an initial unified population is split geographically by a physical barrier, preventing gene flow. Over time, independent evolutionary forces (mutation, genetic drift, and natural selection) drive divergence in each isolated gene pool. This accumulation of genetic differences results in intrinsic reproductive isolation mechanisms (such as prezygotic behavioral or temporal shifts). Finally, when secondary contact occurs after the removal of the barrier, the populations can no longer interbreed, confirming that speciation is complete.

Step-by-Step Solution

1
Identify the starting condition of the population.
The process begins with a single continuous ancestral population in gene flow equilibrium.
Allopatric speciation requires an initial intact gene pool before physical separation occurs.
2
Identify the physical trigger of vicariant speciation.
Geological disruption creates a physical barrier splitting the population into isolated sub-units.
Vicariance stops interbreeding and eliminates gene flow between the newly separated groups.
3
Trace the microevolutionary divergence occurring during geographic separation.
Mutations, genetic drift, and local natural selection cause independent divergence of gene pools.
Without gene flow to homogenize allele frequencies, isolated populations diverge genetically.
4
Determine the emergence of intrinsic reproductive barriers.
Prezygotic barriers (e.g., behavioral mating cues or temporal shifts) arise as a byproduct of genetic divergence.
Reproductive isolation mechanisms must form to prevent gene flow even if spatial overlap resumes.
5
Evaluate the test of speciation upon secondary contact.
The physical barrier dissolves, but secondary contact reveals complete reproductive isolation.
The persistent inability to interbreed and produce fertile offspring confirms that two distinct species now exist.

Key Concept

Vicariant allopatric speciation sequence and secondary contact
Estimated Time:2m 0s
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Topics

Adaptive Radiation and Mechanisms of Speciation

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