In a meadow population of a wild flower, an instantaneous non-disjunction event produces a… — JAMB UTME

In a meadow population of a wild flower, an instantaneous non-disjunction event produces a tetraploid lineage ( $4n$ ) living alongside the original diploid ( $2n$ ) population. Cross-pollination between diploid and tetraploid individuals produces triploid ( $3n$ ) seeds that germinate into plants unable to undergo normal meiosis, rendering them completely sterile. Which speciation mode and type of reproductive isolating barrier are demonstrated in this scenario?

Sympatric speciation involving a post-zygotic reproductive barrierAnswer
B
Allopatric speciation involving a pre-zygotic reproductive barrier
C
Adaptive radiation driven by the inheritance of acquired somatic traits
D
Sympatric speciation involving continuous phenotypic variation without genetic isolation

Answer

Sympatric speciation involving a post-zygotic reproductive barrier

Because the diploid and tetraploid plants share the same geographical habitat without spatial barriers, speciation proceeds sympatrically. Furthermore, because fertilization successfully occurs to produce triploid (

3n

) seeds, but the resulting mature plants cannot produce functional gametes due to abnormal meiotic pairing, the isolation mechanism acts after zygote formation (post-zygotic hybrid sterility).

Step-by-Step Solution

Identify the geographical context of the population.

Both the original diploid (

2n

) and newly formed tetraploid (

4n

) plants inhabit the same physical meadow without geographic isolation, which defines sympatric speciation.

Sympatric speciation occurs when a new species evolves from a single ancestral species while inhabiting the same geographic region.

Analyze the nature of the reproductive isolation.

Cross-pollination successfully yields triploid (

3n

) zygotes and seeds, but the resulting offspring are sterile due to unequal chromosome segregation during meiosis.

Because fertilization occurs and zygotes form, but the hybrid offspring are sterile, this mechanism is classified as a post-zygotic isolating barrier (specifically hybrid sterility).

Key Concept

Polyploidy as a mechanism of sympatric speciation and hybrid sterility as a post-zygotic barrier

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