A reproductive isolating mechanism is a structural, functional, or behavioural characteristic that prevents successful reproduction from occurring between different species. This helps in accumulating genetic variations in species. If reproductive isolation does not exist, variant forms freely interbreed with the normal forms and this would lead to intermixing of their genotypes.
These mechanisms are of two types:
1. Pre-Mating Mechanisms:
Premating isolating mechanisms are anatomical or behavioural differences between two species that prevent the possibility of mating.
i. Habitat isolation occurs when two species occupy different habitats, even within the same geographic range, so that they are less likely to meet and attempt to reproduce.
ii. Temporal isolation occurs when two species live in the same location, but each reproduces at a different time of the year, preventing a successful mating.
iii. Behavioural isolation occurs when there are differences in mating behaviour between two species.
iv. Mechanical isolation is the result of differences between two species in reproductive structures or other body parts, so that mating is prevented.
2. Post-Mating Mechanisms:
Post-mating isolating mechanisms are the result of developmental or physiological differences between the members of two species after mating.
i. Gamete isolation is the physical or chemical incompatibility of gametes of two different species. If the gametes lack receptors to facilitate fusion, they cannot form a zygote. An egg may have receptors only for the sperm of its own species.
ii. Zygote mortality is a mechanism when the zygote dies soon after its formation.
iii. Hybrid in viability – The offspring of parents of two different species is known as a hybrid. It dies before reaching sexual maturity.
iv. Hybrid sterility – The hybrid fails to reproduce sexually. For example, the mule is a sterile hybrid between a male donkey and a mare, a hinny is a sterile hybrid between a stallion and a female donkey.
Plants are bisexual and can establish a reproductively isolated species very rapidly. Polyploidy and hybridisation are important speciation mechanisms in plants.