Classifications of reproductive isolating mechanisms are: A. Pre-mating mechanisms and B. Post-mating mechanisms.
A. Pre-Mating Mechanisms:
These prevent the copulation and fusion of gametes of inter-specific crosses by:
1. Geographic isolation:
In this, organisms are separated by geographical barriers such as water bodies, mountains, deserts etc. which prevent interbreeding between them.
So processes like natural selection occurs independently on both sides and leads to the formation of new species or sub species e.g. Darwin’s finches on Galapagos Islands.
2. Spatial isolation:
In this, individuals isolated by long distance are unable to breed due to their inability to cover long distances.
3. Seasonal or Temporal isolation:
Due to the effect of factors like photoperiod, temperature and humidity, the breeding season of the animals of one species is different from that of other species, so are reproductively isolated e.g. American toad (Bufo Americans) breeds in early part of rainy season, while Fowler’s toad (B .fowleri) breeds in later part of rainy season, so are reproductively isolated.
4. Ecological or Habitat isolation:
Mating inability between the organisms of closely related species or subspecies occupying the same region but different habitats, e.g. two species Rana i.e. Rana grylio (Pig frog) and Rana areolata (Gopher frog) live in different habitats. So the two species show ecological isolation.
5. Ethological isolation:
Reproductive isolation due to difference in courtship or sexual behavioural patterns between members of closely related species e.g. males of different species of Sticklebacks have different mode of nest building and courtship dancing behaviour. Similarly, Pintail duck and Mallard duck also show ethological isolation.
6. Physiological isolation:
Mating inability due to functional incompatibility in their mating e.g. swelling of vaginal mucous membrane of Drosophila in case of inter-specific mating.
7. Mechanical isolation:
This type of isolation occurs due to the differences in morphological structures such as external genitalia e.g. In Drosophila and Glossina.
8. Genetic isolation:
Mating inability between the members of related species due to interspecific sterility due to chromosomal incompatibility.
B. Post-Mating Mechanisms:
If the efficient premating mechanisms fail, then post mating mechanisms prevent hybrid formation.
These are of five types:
1. Gametic mortality:
In case of external fertilization, the sperms may fail to fertilize the eggs due to unfavourable climatic conditions. In case of internal fertilization, the sperms of different species may be immobilized or killed in female genital fract.
2. Zygotic mortality:
It results in death of the zygote e.g. in sheep and goat.
3. Hybrid inviability:
In this, hybrid zygotes fail to develop as the embryos formed by inter-specific crosses abort during development e.g. hybrids between the Rana pipiens and Rana sylvatica do not survive beyond the early gastrula stage.
4. Hybrid sterility:
In most of the organisms, hybrids are sterile due to incompatibility of chromosomes e.g. mule (2N = 63) – produced by crosses between male ass (2N = 66) and mare (2N = 60) and hinny – produced by crosses between she-ass (Janet) and stallion (male horse).
There are number of examples where interspecific crosses may produce fertile interspecific hybrids in captivity as no reproductive barrier has been developed between these species even after their long isolation from each other e.g.
(i) Fertile ‘tigons’- a hybrid between African lioness (Panthera leo) and Asian tigers (P. tigris) but do not breed in nature due to different geographical distributions.
(ii) Fertile hybrids between mallard duck (Anas bascas) and pintail duck (A. acuta) but do not breed in nature due to different mating and nesting behaviour.
(iii) Fertile hybrids between polar bear (Ursus maritimus) and Alaskan brown bear (U. arctos) but do not breed in nature due to different habitats (ecological isolation).
This shows that reproductive isolation is not a universal phenomenon.
5. Hybrid breakdown:
In certain cases hybrids are fertile, but progeny is weak, abnormal or sterile. This phenomenon is called hybrid breakdown e.g. interspecific hybrids of cotton (Gossypium).
At the genetic level, the offsprings of interspecific hybrids are usually sterile due to incompatibility between the chromosomes of two different species and their failure to pair in meiosis.