Q.88 Which one of the following is a “post–zygotic” isolating mechanism of speciation?
(A) Behavioral isolation
(B) Fertilization failure
(C) Hybrid sterility
(D) Seasonal isolation

Correct Answer: (C) Hybrid sterility

Hybrid sterility represents a classic post-zygotic isolating mechanism in speciation, as it occurs after zygote formation when hybrid offspring fail to produce viable gametes.​

Option Analysis

• (A) Behavioral isolation: This pre-zygotic mechanism prevents mating due to differences in courtship rituals or signals, blocking zygote formation entirely.​

• (B) Fertilization failure: A pre-zygotic barrier where gametes fail to fuse, such as through gametic incompatibility, halting reproduction before zygote development.​

• (C) Hybrid sterility: Post-zygotic, as seen in mules (horse-donkey hybrids), where the zygote forms and develops but the adult hybrid cannot reproduce due to genetic incompatibilities.​

• (D) Seasonal isolation: Pre-zygotic temporal barrier where species breed at different times, preventing encounters during fertile periods.​

In speciation processes critical for CSIR NET Life Sciences, the post-zygotic isolating mechanism of speciation acts after zygote formation to reduce hybrid fitness, reinforcing species boundaries through evolutionary divergence. Unlike pre-zygotic barriers that stop mating or fertilization, post-zygotic ones like hybrid sterility ensure gene flow ceases despite initial hybridization.​

Pre- vs. Post-Zygotic Barriers

Pre-zygotic mechanisms prevent zygote creation:

• Behavioral isolation: Mate choice differences (e.g., bird songs).​

• Fertilization failure: Gamete mismatch (e.g., pollen tube issues in plants).​

• Seasonal isolation: Timing mismatches (e.g., frog breeding seasons).​

Post-zygotic mechanisms impair hybrids:

• Hybrid inviability: Embryos die early.

• Hybrid sterility: Adults form but are infertile (e.g., ligers).​

• Hybrid breakdown: Later generations weaken.​

Role in Speciation

Post-zygotic barriers like hybrid sterility arise from Dobzhansky-Muller incompatibilities, where diverged genes interact negatively in hybrids, driving allopatric or sympatric speciation. For CSIR NET aspirants, recognize hybrid sterility (mule example) as the key post-zygotic isolating mechanism of speciation, distinguishing it from pre-zygotic options in MCQs. These mechanisms maintain biodiversity by limiting introgression.​