Isolation and isolation mechanisms-Lecture notes on evolution

A species may be defined as “a group of organisms that are reproductively isolated from other such groups”. Thus the maintenance of a species as a dintinct group is due to several isolating mechanisms. They are
1. Geographical isolation
It is a common type of isolation. The isolation between populations is caused due to geographical barriers such as mountains, rivers, oceans, forests or deserts. These natural barriers prevent interbreeding between them. Thus mutations formed in one population will lead to the formation of new species. The existence of closely related species of frogs in Southern India and Srilanka is a classical example. These fresh water animals are prevented from interbreeding due to a narrow sea namely Gulf of Mannar. Because of
isolation for a fairly long time they have evolved into distinct species.
2. Premating isolations – such mechanisms prevent interspecific crosses.
a) Ecological isolation – Members of the populations occur in different habitates in the same general region.
b) Seasonal or Temporal isolation – Mating or flowering periods occur at different seasons.
c) Sexual, Psychological or Ethological isolation – It is a behavioural isolation where males and females of the same species get attracted to each other.
d) Mechanical isolation – Physical non-correspondence of the genitalia or floral parts.
e) Gametic isolation – Spermatozoa, or pollen tubes of one species arenot attracted to the eggs or ovules of another species.
3. Postmating or postzygotic isolations – These isolating mechanisms while allowing fertilization may prevent the hybrid zygote from further development.
a) Hybrid inviability – The hybrid zyotes are inviable.
b) Hybrid sterility – The hybrids develop but they remain sterile. They are incapable of producing a normal complement of functional sex cells.
c) Hybrid breakdown – F1 hybrids are normal and fertile, but F2 containsmany weak or sterile individuals.