Geographic Isolation :
It plays an important role in allopatric isolation. In geographic isolation, two populations of same species are separated by some physical or geographical barrier or they occupy different geographic areas. The barriers may be maintained ranges, desert, thick forests, land bridges or water bodies etc. Because of these barriers individuals of these separated population have no chance to meet and interbreed. This greatly reduces the exchange of genes between them and single common gene pool split into two gene pools. As a result, new mutation, genetic drift, and action of natural selection occur independently in an isolated population. This lead to Genetic Divergence
Examples :
- Snails – Distribution of land snails in Hawaiian island, is an example of geographical isolation. The volcanic ridges of the island are separated into a valley which has abundant vegetation at the bottom but their top is barren. Each valley has a different variety of land shails and degree of divergence between them is directly proportional to the distance between valleys.
- Ratula Indica – Indian giant squirrel Ratula Indica lives in the forest. Its variety living in deciduous forests of Gujrat is yellowish in color, that living indeciduous forest of Maharastra is light brown and living in every green forest in Mysore is dark brown. Here different forests acting as a barrier.
- Salt water acts as barrier – Amphibians (Fresh water living)
Isolation due to Distance :
Members of widely distributed species which occupy large territory without any natural barrier, have been found to be reproductively isolated. The simple reason for their isolation is their inability to cover great distances. Example: Mammals found in the Holarctic area of North America are unable to interbreed only because they find it impossible to cover such long distances.
- Altitude – Drausophilla Persimilis Prefers higher altitudes. While D. Pseudobscura prefers lower altitudes; so effectively isolated.
Climatic Isolation :
In absence of any geographic barrier, the abrupt climatic differences within the range of distribution of species are found to produce isolation of its members into units. The climatic factors may be temperature. Moisture, mineral concentration and chemical composition of the surrounding. These directly influence the metabolism and produce physiological difference in organisms
Example :
Climatic isolation seen in water of North and South Cape Code.
Seasonal Isolation :
Difference in breeding season prevent mating and interbreeding among individuals of different population or of different species. Seasonal isolation is very common in plants and frequently occur in insects and certain other invertebrates.
Example :
Blair (1941) found seasonal isolation in toads. American tood Bufo Americanus and Bufo fowleri, when reproduce in captivity in laboratory produce hybrids. But inspite of having similar geographic distribution, their hybrids are rare nature. Because B. Americanus breeds early in rainy season and B. Fowleri breeds late.
Habitat Isolation or Ecological Isolation :
The Habitat isolation is caused due to difference in habits and habitat of the organisms such as food and physiological requirements and place of living. This term was used by Moore and is synonymous to ecological isolation. Population which is isolated environmentally develop habitat isolation specially during their reproductive period. These isolation enhance other isolating mechanisms between sympatric species.
Example :
Two ecologically distinct population of stickleback fish (Gasterosteus) in Belgium. One of them lives in freshluater round the year, while other returns to sea in winter and migrates into estuaries in spring and summer. Latter is unable to live in one habitual due to change in chlorine content in blood Hybrids between two forms can be obtained by artificial insemination but in nature, their occurrence is very rare.
Ethological/Behavioral solution :
The prerequisite for sexual union of gametes is that sexes meet to enable fertilization to occur. The act of mating is completed only when there is appropriate exchanges of stimuli. Thus, ecological isolation refers to parries to mating.
Example :
- Difference in size prevents hybridization between oad tood and Gulf Coast toad. Former smaller in length while latter always bigger. Here, size prevents interbreeding.
- Some species produce specific sounds during breeding season, it play an important role in courtship of frogs, toads and birds. In Gryllus compestris, female attracted by stridulating male of their species.
Mechanical Isolation :
The complex structure of genitalia in many animals does not permit copulation among different species. Genitalia in insects are developed on Lock and Key Principle- (i.e. male and female genitalea are so exactly fitted to each other that even slight deviation in structure of either. Renders copulation impossible.
Physiological Isolation:
Certain species are established on basis of some physiological difference development between them.
Example :
In certain species of Drausophilla mating among members of different species is not possible because vaginal mucous membrane swells up after copulation. Swelling lasts for few hours, if mating is among members of same species, but continues for days, if mating is interspecific. As a result, fertilized egg. Is perished in parental body.