Isolation
Isolation is the segregation or separation of populations by certain barriers, which prevent interbreeding. As a result gene flow between populations is prevented. Each population on isolation, develops genetic divergence independently leading to the formation of new species. The factor which brings about isolation is called 'isolating mechanism'. The main types of isolating mechanisms are as given below.
Geographical Isolation
It is the isolation in space of the two populations of the same species, due to geographical barriers such as the sea, deserts, mountains and rivers, for land plants and animals.
Geographical isolation plays an important role in 'allopatric speciation'. Allopatric species are the related groups of individuals occupying different geographical areas. A single common gene pool gets split into two gene pools due to the barriers such as a river, or a mountain. Each such isolated population is affected by separate environmental factors. This leads to development of genetic divergence. Once the genetic divergence is achieved, the two populations cannot interbreed even if the barrier disappears. Hence, the two isolated populations can be recognised as two separate species. The southern elephant seal Micounga leonica occurs in the cold waters of the southern coasts of South America, South Africa, Australia and New Zealand. A close relative of this is the northern elephant seal Micounga argostiastics. It is found in the cold waters along the coast of western North America. The two forms are very much similar to each other. However, the two forms are separated by about 5000 km of tropical seas. Hence, the two forms cannot interbreed.
It is the isolation brought about by genetically determined agency, which prevents interbreeding. Following are the various mechanisms by which reproductive isolation can be brought about.
Premating or Prezygotic Isolation
These come into play in the early stages of the reproductive process itself and prevents the formation of a fertilized egg (zygote). They can be as follows:
Ecological or Habitat Isolation
The differences found in the habitats occupied by the two populations of the same species, may prevent interbreeding between them. Habitat isolation may not give any opportunity to meet and mate, to the isolated populations.
Seasonal Isolation
It is the isolation brought about due to the differences in the breeding season of the population.
Ethological or Behavioural Isolation
In some species, differences in the sexual behaviour may prevent inter breeding between the populations.
Mechanical or Morphological Isolation
It is the isolation brought about by the morphological differences, particularly with reference to reproductive organs (external genital organs).
Physiological Isolation
The physiological differences between individuals may also sometimes bring about isolation.
Gametic Mortality Isolation
In some cases of inter specific mating, the gametes get destroyed in the genital tract due to antigenic reactions.
Postmating or Postzygotic Isolation
These are isolating mechanisms that operate after mating occurs in the individuals. Following are some of them:
Cytological Isolation
It is the situation where after mating, fertilization fails to occur due to differences in the chromosomal number.
Zygote Mortality Isolation
In some cases, even if successful fertilization occurs following an interspecific mating, the zygote may not survive. It may die at any stage of development.
Hybrid Inviability Isolation
Here the hybrid organism resulting from an interspecific breeding fails to survive.
Hybrid Sterility Isolation
Sometimes viable hybrids may be formed but may become sterile, failing to produce young ones. e.g., Mule.
Hybrid Breakdown Isolation
It refers to the inviability or adaptive inferiority of the hybrids in several filial generation or hybrids in back cross.