Anything that divides a species into subgroups, which do not freely interbreed, is said to be the isolated members of the species. Dobzhanky (1937) used the term isolating mechanisms in his book “Genetics of the origin species”.

Isolating Mechanism:

This is broadly classified into two groups:

1. Geographic Isolation

2. Reproductive Isolation

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Geographic Isolation:

When populations are separated by geographic barrier, it is called geographic isolation. Wagner (1868) was the first who stated that geographic isolation is a factor for formation of new species. The populations of two different localities are separated by geographic barrier like mountain, landmass, water body, forest desert etc. the high mountain range, deserts, dense forests and extremes of the temperature also acts as effective barrier. e.g. finches of Darwin are geographically isolated in different areas of the Galapagos islands. Such isolated populations are completely cu off another genetically. Thus, genetic divergence develops in the isolated population.

3. Reproductive Isolation:

An isolating mechanism that prevents the gene exchange between Mendelian populations is called reproductive isolation. It is sub-divided into two groups: (i) Pre-zygotic isolation (ii) post-zygotic isolation.

(i) Pre-zygotic Isolation:

This mechanism prevents inters specific crosses (i.e. fertilization and zygote formation).

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It is of following types:

(a) Ecological or Habitat Isolation:

It occurs when species occupy different habitats in the same territory.

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(b) Seasonal or Temporal Isolation:

It is found between populations in which the members reach sexual maturity or flowering at different times of the year.

(c) Ethological Isolation:

In some difference in the sexual behaviuor (mating, dance, song, light, scent) prevents interbreeding.

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(d) Mechanical Isolation:

In animals the structural difference in the external genetalia prevents interbreeding. In flowers, structural difference also prevents pollen transfer.

(e) Physiological Isolation:

In this mechanism, two different species mate but fertilization is prevented by Physiological differences between them.

(ii) Post-zygotic Isolation

(a) Gametic Mortality:

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When there is mating between two different species, to the gametes are killed because of antigenic reaction in the genital tracts.

(b) Zygitic Mortality:

When fertilization is successful by the interbreeding of two species the zygote dies.

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(c) Hybrid Sterility:

During inter-specific interbreeding viable hybrids are produced but they are sterile.

Role of Isolation in Speciation:

Geographic isolation is essential factor for development of genetic diversity and subsequent origin of species. The allopatric speciation in undisputed as speciation start at the complete geographic isolation. Sympatric speciation (speciation occurring in single population) can only occur under restrictive conditions and it has been controversial. (Bush 1994).