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  Isolating mechanisms

Isolating mechanisms:

The species might be stated as “a group of organisms which are reproductively isolated from other such classes”. Consequently the maintenance of a species as a distinct class is due to numerous isolating mechanisms. They are:

1. Geographical isolation:

It is a general kind of isolation. The isolation among populations is caused due to geographical barriers like rivers, mountains, forests, oceans or deserts. Such natural barriers prevent interbreeding among them.

Therefore mutations created in one population will lead to the formation of new species. The existence of closely associated species of frogs in Southern India and Srilanka is a classical illustration. Such fresh water animals are prevented from interbreeding due to a narrow sea. Since of isolation for a quite long time they have evolved into different species.

2. Premating isolations – such mechanisms defend interspecific crosses:

a) Ecological isolation – Members of the populations take place in various habitats in the same general area.

b) Seasonal or Temporal isolation – Mating or flowering periods take place at various seasons.

c) Sexual, Psychological or Ethological isolation – It is a behavioral isolation where males and females of similar species get attracted to each other.

d) Mechanical isolation – Physical non-correspondence of the genitalia or floral portions.

e) Gametic isolation – Spermatozoa, or pollen tubes of one species are not attracted to the ovules or eggs of other species.

3. Postmating or postzygotic isolations:

Such isolating mechanisms while permitting fertilization might prevent the hybrid zygote from further development.

a) Hybrid inviability – The hybrid zyotes are inviable.
b) Hybrid sterility – The hybrids develop though they stay sterile. They are not capable of producing a normal complement of functional sex cells.
c) Hybrid breakdown – F1 hybrids are normal and fertile, though F2 has many weak or sterile individuals.


Speciation:

The species are natural, biological unit. Among the different taxa, a species is not man-made. It is a natural realism. The procedure of evolution operates at the species level only. It is since of these reasons, in evolution much significance is given to the ‘Origin of Species’. There are many types of species.

Allopatric species – Species occupy various geographical regions.

Example: species of frogs in India and Srilanka. The two land regions are separated by the Gulf of Mannar. Sympatric species – closely associated species living altogether in one common locality, yet sustain their species identity

Ex: Ranahexadactyla, R.tigrina and R.cyanophlictis living altogether in a pond.

 

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