Q.Write down the mechanism that prevents interspecific crosses.

Interspecific (between-species) crosses are prevented by a set of reproductive isolating mechanisms that either stop mating/fertilisation altogether (prezygotic) or ensure any resulting hybrid cannot reproduce successfully (postzygotic), and these mechanisms are what keep species distinct.

Prezygotic isolating mechanisms (act before fertilisation, preventing mating or fertilisation between species):

• Habitat/ecological isolation: closely related species live in different habitats within the same general region and so rarely encounter each other.
• Temporal isolation: species breed at different times of day, season, or year, so their reproductive periods do not overlap.
• Behavioural isolation: courtship displays, mating calls, or pheromones are species-specific, so individuals of one species do not recognise or respond to the mating signals of another.
• Mechanical isolation: differences in the structure of reproductive organs (genitalia in animals, floral structures in plants) physically prevent mating or successful pollen transfer between species.
• Gametic isolation: even if mating occurs, the sperm and egg (or pollen and stigma) of different species are biochemically incompatible and fail to recognise each other or fuse.

Postzygotic isolating mechanisms (act after fertilisation, if it occurs, preventing the hybrid from contributing genes to future generations):

• Hybrid inviability: the hybrid zygote either fails to develop at all or dies before reaching reproductive maturity. …