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Difficulty: MediumAdaptive Radiation and Mechanisms of Speciation

Arrange the following sequential events representing the process of sympatric speciation via allopolyploidy in plants in the correct chronological order from first to last.

  1. 1Interspecific hybridization occurs between two distinct diploid plant species, producing an infertile hybrid.
  2. 2A chromosome doubling event (nondisjunction) occurs within the germline or early zygotic mitotic divisions of the hybrid.
  3. 3A fertile allopolyploid (amphidiploid) individual is produced, capable of self-fertilization or mating with similar polyploids.
  4. 4Immediate post-zygotic reproductive isolation is established because backcrossing with parental diploids produces sterile triploid offspring.

Answer

The correct chronological sequence is: (1) Interspecific hybridization producing an infertile hybrid, (2) Chromosome doubling via nondisjunction, (3) Formation of a fertile allopolyploid individual, and (4) Establishment of immediate reproductive isolation from parental species.
Allopolyploidy is a key sympatric speciation mechanism in plants. It begins when two different species cross (interspecific hybridization) to generate an infertile hybrid. Subsequent nondisjunction causes chromosome doubling, restoring fertility by providing matching homologous pairs for meiosis. This creates a fertile allopolyploid that is instantly reproductively isolated from its diploid ancestors because any backcross results in sterile triploid offspring.

Step-by-Step Solution

1
Identify the initial genetic event bringing separate species together.
Interspecific hybridization between two diploid species creates a hybrid containing one set of chromosomes from each parent.
Before polyploidy can occur, gametes from two distinct plant species must fuse.
2
Identify the genetic mutation that restores fertility to the sterile hybrid.
Spontaneous chromosome doubling (nondisjunction) duplicates each chromosome.
Univalent chromosomes in the initial hybrid cannot pair during meiosis, leading to sterility until doubling creates homologous pairs.
3
Determine the resulting organismal state after chromosome duplication.
Creation of a fertile allopolyploid lineage.
With homologous chromosome pairs restored, normal meiosis produces viable, fertile gametes.
4
Determine how the new lineage becomes a distinct, isolated species.
Immediate reproductive isolation from the original parent populations.
Mating between the new 4n4n polyploid and original 2n2n parents produces 3n3n triploid offspring, which are sterile due to unbalanced meiosis.

Key Concept

Sympatric Speciation through Allopolyploidy
Estimated Time:1m 30s
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Topics

Adaptive Radiation and Mechanisms of Speciation

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