Q.38 Which one of the following is a post-zygotic barrier in speciation?
(1) Behavioral Isolation
(2) Habitat Isolation
(3) Temporal Isolation
(4) Hybrid Sterility
Post-Zygotic Barriers in Speciation: Hybrid Sterility as the Key Mechanism
Speciation occurs when populations evolve reproductive isolation, preventing gene flow. The question “Which one of the following is a post-zygotic barrier in speciation?” tests this core concept in evolutionary biology. The correct answer is (4) Hybrid Sterility. Understanding prezygotic vs. postzygotic barriers is essential for exams like NEET, CSIR NET, and graduate-level genetics.
Prezygotic vs. Postzygotic Barriers: Quick Overview
Prezygotic barriers block fertilization before zygote formation (e.g., mating failure). Post-zygotic barriers act after zygote formation, reducing hybrid fitness. Post-zygotic ones strengthen speciation by eliminating unfit offspring, driving divergence.
Correct Answer: Option (4) Explained
(4) Hybrid Sterility
This is the classic post-zygotic barrier. Hybrids form but are sterile, unable to produce viable gametes. Chromosomal mismatches during meiosis cause this—e.g., horse (64 chromosomes) + donkey (62) = sterile mule (63 chromosomes). No gene flow occurs, reinforcing speciation. Famous examples include human-horse hybrids or Drosophila fruit flies.
Why Not the Other Options? (All Prezygotic Barriers)
These prevent zygote formation, making them prezygotic:
-
(1) Behavioral Isolation
Species differ in mating rituals (e.g., bird songs, courtship dances). Females reject wrong signals, avoiding mating. Prezygotic—no zygote forms. -
(2) Habitat Isolation
Populations occupy different habitats (e.g., apple vs. hawthorn flies). They rarely meet to mate, so no zygote. Ecological separation acts prezygotically. -
(3) Temporal Isolation
Breeding times differ (e.g., one species flowers in spring, another in summer). Gametes never meet, blocking fertilization prezygotically.
| Barrier Type | Examples | Timing |
|---|---|---|
| Prezygotic | Behavioral, Habitat, Temporal | Before zygote |
| Postzygotic | Hybrid Sterility, Hybrid Inviability | After zygote |
Real-World Relevance and Exam Tips
Post-zygotic barriers like hybrid sterility explain why lions and tigers produce sterile ligers. In plants, polyploidy often leads to fertile hybrids, accelerating speciation. For MCQs, memorize: sterility/inviability = postzygotic; prevention of mating = prezygotic.
This distinction clarifies Darwin’s natural selection in reproductive isolation.
