3. Ethological isolation: Ethological isolation: It is a behavioural isolation, in which potential mates meet but cannot mate, due to differences in courtship displays or other specific signals that are necessary rituals before mating. The signals may be of the following three types, which stimulate the opposite sex for mating: (a) Visual stimuli: Visual stimuli: Feather displays and dancing in male birds is necessary to attract the female, e.g. peacocks, pheasants and birds of paradise. The colour and shape of the feathers as well as display pattern is so unique for each species that mating between two different species is not possible. Collection of the nest material and construction of the nest as by the weaverbird male is also a very specific display that cannot be imitated by the other species. (b) Auditory stimuli: Auditory stimuli: Songbirds like cuckoos, mynas, nightingales, parakeets etc. use auditory signals to attract the opposite sex. Sometimes the singing goes on for several days before the pair can actually come together for mating. Auditory communication is used by a large number of animals, viz. frogs, toads, cicadas, gibbons, monkeys, jackals etc. (c) Chemical stimuli: Chemical stimuli: This includes odour of the animals that attract the opposite sex for mating. For example scent of musk deer and musthing in elephants attract the females. In insects, particularly Lepidoptera, females produce highly specific pheromones that can be detected by the highly specialized antennae of males from a distance of about 2 kilometers. 4. Mechanical isolation: Mechanical isolation: In this case the above isolating mechanisms are not present and therefore mating is attempted but is not successful due to mechanical problems such as differences in the structure of genitalia. Dufour (1844) described “Lock and key mechanism” in the genitalia of insects. In the species of Drosophila the genitalia are so different that copulation is mechanically not possible.

## B. Post-mating mechanisms

These reduce the success of interspecific crosses. In case premating mechanisms fail to prevent mating then several post-mating mechanisms prevent the success of mating and hybridization. There are 4 such mechanisms, which are given below: 1. Gamete mortality: Gamete mortality: Mating and sperm transfer takes place but egg is not fertilized. In Drosophila vaginal wall swells killing spermatozoa should interspecific crosses take place. If mating takes place between Bufo fowleri and Bufo valliceps, sperms cannot penetrate the egg membrane of each other, leading to mortality of gametes. 2. Zygote mortality: Zygote mortality: Egg is fertilized but the zygote dies. Eggs of many species of fishes may be present in the spawning grounds and some may be fertilized by the sperms of different species forming zygote but such zygotes fail to develop due to differences in chromosomes. 3. Hybrid Hybrid unviability: Zygote develops and hybrid is produced but is physically weak and inviable due to physiological disturbances in the body. It fails to survive for long and prematurely dies. Such cases have been recorded in different species of ducks. 4. Hybrid sterility: Hybrid sterility: Hybrid is viable, physically strong and physiologically sound but is sterile due to differences in chromosomes and different gene arrangements. Mule is a cross between male donkey and female horse and Hinny between female donkey and male horse and both are sterile, albeit physically strong. Sometimes all isolating mechanisms break, leading to fertile hybrids, which are generally not reproductively isolated from the parents and can produce fertile offspring by Introgression (hybrids backcrossing with parents to produce fertile offspring). This will be instant speciation.