difference in the breeding seasons prevents interbreeding. E.g. Toad, Bufo americanus breeds much early in the spring; whereas Bufo fowleri breeds very late in the season. They are able to maintain their species identity because of the differences in the breeding seasons.
iii. Sexual or ethological isolation/ Behavioural isolation – Prevents mating due to the difference in their sexual behavior. The species are not separated from one another either in time or in space. The mating calls of two closely related species of frogs, Hyla versicolor (grey tree frog) and Hyla femoralis (pine wood tree frog) are different which prevents interbreeding.
iv. Morphological isolation or mechanical isolation – This type of isolation is due to the differences in their external genitalia that is seen in two different species. The size difference between the toad species Bufo quercicus and Bufo valliceps , prevents their interbreeding. v.
Physiological isolation –Though mating may occur, the gametes are prevented from fertilization due to mechanical or physiological factors. E.g. The sperms of Drosophila virilis survive only for about a day when introduced into the sperm receptacle of Drosophila americana while the sperms of Drosophila americana live for a longer time. vi.
Cytological isolation – Fertilization does not take place due to the differences in the chromosome numbers between the two species, the bull frog Rana catesbiana and gopher frog Rana areolata .