c. Isolation

Isolation is the separation of the population of a particular species into smaller units in a way that prevents interbreeding between those units. Any barrier that blocks gene flow, or the exchange of genes, between such isolated populations is called an isolating mechanism, and because isolating mechanisms can promote genetic divergence between the separated groups, they are often what allows speciation to occur. Isolating mechanisms are broadly of two kinds — geographical isolation and reproductive isolation.

I. Geographical Isolation : It is also called physical isolation. It occurs when a single original population is split into two or more groups by a physical, geographical barrier — a river, an ocean, a mountain range or a glacier, for instance — that then prevents interbreeding between the separated groups. Because the separated populations are now exposed to different environmental conditions, they tend to acquire different new traits through mutation over time, gradually develop distinct gene pools, and no longer interbreed even if the original barrier is later removed — new species can form in this way, as seen in Darwin's finches.

II. Reproductive Isolation : Reproductive isolation arises from changes in genetic material, in the gene pool, or in the structure of the reproductive organs themselves, and it too prevents interbreeding between populations, without necessarily requiring any geographical separation. Reproductive isolating mechanisms are further divided into two categories:

Types of Isolating Mechanisms :

A. Pre-mating or pre-zygotic isolating mechanism : This mechanism prevents fertilization and zygote formation.

i. Habitat isolation (Ecological isolation) : members of a population live in the same geographic region but occupy separate habitats, so potential mates do not meet.

ii. Seasonal or temporal isolation : members of a population live in the same geographic region but are sexually mature at different years or at different times of the year.

iii. Ethological isolation : because of specific mating behaviour the members of the population do not mate.

iv. Mechanical isolation : members of two populations differ in the structure of their reproductive organs. …