What is reproductive isolation? Describe any two types each of pre-mating and post-mating isolating mechanism.

Answer: Reproductive isolation is the prevention of interbreeding (gene exchange) between populations of different species by biological factors called isolating mechanisms. It keeps species distinct and helps in the formation of new species.

Pre-mating (prezygotic) isolating mechanisms (prevent mating/fertilization):

1. Habitat (ecological) isolation: the populations live in different habitats in the same area and so do not meet.
2. Seasonal (temporal) isolation: they breed or flower at different times or seasons.
3. Ethological (behavioural) isolation: differences in courtship behaviour, calls or displays prevent mating.
4. Mechanical isolation: differences in genitalia or flower structure prevent transfer of gametes.

Post-mating (postzygotic) isolating mechanisms (act after fertilization):

1. Hybrid inviability: the hybrid zygote or embryo dies before maturity.
2. Hybrid sterility: the hybrid survives but is sterile, e.g. the mule (horse × donkey).
3. F₂ breakdown: F₁ hybrids are fertile but the F₂ offspring are weak or sterile.