(a) Prezygotic isolating mechanisms and
(b) Postzygotic isolating mechanisms
(a) Prezygotic isolating mechanisms include habitat isolation, seasonal isolation, ethological isolation and mechanical isolation. In all these types there is failure of zygote formation. Organisms may occur in different habitats; or their breeding periods differ; or their behavioral patterns are different or mating is not possible because of difference in size of reproductive organs.
(b) Postzygotic isolating mechanisms include gametic
mortality, zygotic mortality, hybrid inviability, development hybrid sterility
or segregational hybrid sterility. It means either the gametes die, or
if they meet, a zygote fails to develop, or if the embryo grows, the reproductive
organs do not develop due to abnormal chromosomal combinations.
When species are geographically separated, they fail to interbreed and are called sympatric species. If they remain in the same habitat, but due to genetic changes through recombination or mutation fail to interbreed, they become allopatric species.
Thus, due to isolation, an interruption in gene flow between
sister populations takes place. If such sister species are separated for
long periods, the differences become pronounced enough to prevent interbreeding.
Thus, reproductive isolation develops and a new species is formed.
Thus, the origin of species is a process regulated by natural selection
and isolation.
