Isolating mechanisms" available in nature.

Discuss the different “isolating mechanisms" available in nature.

The development of differentiation in sympatric populations which are those inhabit the geographical area, will not interbreed even when kept together. Such a situation results due to varied factors which are collectively caused isolating mechanisms.

Isolating Mechanisms :

The isolating mechanisms may be purely spatial or may involve specific mechanisms covering reproductive isolation, which will operate in sympatric population. Setbbins in his recent book, Processes of Organic Evolution, classified reproductive isolation mechanisms into two types:

(a) Prezygotic mechanisms :

These prevent contact between species at the time of active reproductive period or else the fertilization is checked even if pollination may be effectively brought about. These may be: (1) Spatial or ecological isolation, (2) Seasonal or temporal isolation, (3) Ethological isolation i.e., different courtship patterns in animals and (4) Mechanical isolotion i.e., Incompatible reproductive structures.

(b) Postzygotic mechanisms :

In these mechanisms, fertilization is brought about but the hybrids are either inviable or weak or the hybrids are sterile or in still other cases the F, be fertile but F, progeny will be sterile. Description of Different Types of Isolating Mechanisms : The different types of isolating mechanisms are described as follows:

(1) Prezygotic Isolation Mechanisms : These mechanisms prevent contact between species at the time of active reproductive period or else the fertilization is checked even if pollination may be effectively brought about. These may be of different types :

(a) Spatial or Ecological Isolation :

It is possible that populations of the same species are isolated by geographic barriers of millions of years but still the isolated populations cannot diverge from each other so as to become distinct species, In many cases, geographical isolation is accompanied with ecological barriers, because under ordinary condition different areas would have different climatic, biotic and edaphic factors. Such differentiation may ultimately lead to speciation.

(b) Seasonal Isolation :

In many cases, isolation is brought about due to different flowering times, so that no hybridization may be possible. Seasonal isolation, therefore would be effective only when combined with other kinds of isolation. For instance, Pinus radiata and P. attenvata are kept apart due to geographical, ecological and seasonal isolation

This type of isolating mechanism is found only in animals and deals with different courtship patterns. The conspicuous plumage and melodious songs are some of the examples. Similarly, the singing of toads and frogs is also associated with this kind of isolating mechanisms. In Drosophila, it is shown that males are quite specifically to remove inhibitions, so that females will copulate with males of her own species.

(d) Mechanical isolation :

This type of isolation is provided due to differnces in the genital organs of different species in animals, so that copulations may not be easily brought about. This would be more effective where insect pollination is brought about and for this purpose often elaborate flower structures are found. As for example in orchids, the elaborate structure of the flower resembles the female insect so that the male insect is attracted. During an attempt of pseudocopulation, the male insect delivers pollen on stigma.

2. Postzygotic Isolating Mechanisms :

In certain cases although fertilization may be brought about between different species, bout the hybrids in the F1 or in the F2, generation will be eliminated due to one of the following three Teasons:

(i) The F1 hybrid produced may be inviable or week.

(ii) The F1, hybrid may be viable and vigorous, but may be sterile.

(iii) The F1 progeny of the vigorous F2, hybrid may be weak and sterile.

In each of these three situations, the genomes of two different species are unable to adjust in the same background. Such a phenomenon for the lack of better explanation is described as genic disharmony.