The division of a single population into two or more groups because of some barrier for interbreeding is called Isolation. There are a number of processes by which two related populations living in the same area, can remain distinct These have been called Isolating mechanisms by Dobzhansky. According to him, the isolating mechanisms are classified into the following types.
i) Geographical Isolation: Two parts of one population are separated by some geographical barrier and arc prevented from interbreeding. Large bodies of water are barriers for land-dwelling animals. High Mountain ranges, deserts, dense forests and extremes of temperature serve as affective barriers. Such populations are completely ‘out of touch with each other genetically’ so that new mutations, genetic drift and the action of natural selection, in one population have rio effect on the other population. Thus, a new population may be developed.
ii) Environmental Isolation: Population living under different environmental conditions remains isolated from one another and are prevented from interbreeding. Environmental isolation depends upon differences in food habits and other physiological requirements of the animals.
For example, an insect which inhabits only coniferous frees, is environmentally isolated from an Insect which inhibits only the deciduous trees.
iii) Seasonal Isolation: The breeding season of two groups of animals or plants do not coincide..Th American toad. Bufo Americans, and the Fowler’s toad, B. fowleri have similar distribution and form fully fertile hybrids in the laboratory crosser. But in nature, they remain distinct because B. americanus breeds early in the season and B fowleri breeds late.
iv) Mechanical Isolation: The anatomy of the reproductive organs different from each other that copulation between males of one population and females of the other, is impossible.
The genitalia of a male will fit into those of a female of the same species as a key fit into a lock, but will not fit the genitalia of females of other species. Observations have not confirmed this theory.
v) Physiological Isolation: Reproductive isolation may exist in those cases in which matings - between the males and females different populations take place. Patterson has shown that in some interspecific matings in Drosophila, the sperm fails to survive in the receptacles of the female of other species.
vi) Hybrid Sterility: Normal vigorous hybrids are formed but they are sterile and further exchange of genes is completely blocked. The Mule is a classical example of hybrid sterility.
Origin of Isolating Mechanisms
According to Muller, reproductive isolation is due to differences in genes that arise during the origin of sub-species and species in population.
According to Dobzhansky, reproductive isolation is the result of natural selection. Hybrids are either sterile or poorly adapted and are, therefore, eliminated by natural selection.
Isolation and Species Formation
Two populations become seperated from each other by means of geographic environmental barriers; each acquires new mutations and is acted upon by forces like genetic drift, natural selection, etc.
When a gene pool becomes divided by some geographic environmental factors, the allotrophic populations become differentiated so as to give rise to reproductive isolation. After the development of reproductive isolation, the populations may again come! into contact, still they remain distinct and &e said to be sympatric. Thus, geographic environmental isolation is this species formation. The development of the reproductive isolation brings new species formation.