Two major theories have been proposed to explain the origin of reproductive isolation:
1. Muller’s view
Muller regarded that reproductive isolation is an incidental by product of genetic divergence, which takes place during the origin of subspecies and species in allopathic populations. (When the population of a species occupies different environments or becomes isolated into subgroups by geographical isolation, each isolated subgroup behaves as an independent population it is called allopathic population).
To simplify, it can be said that evolving populations adapt themselves to different environments. Therefore, a reshuffling and restructuring of the genes, chromosomes and entire genotype takes place.
2. Dobzhansky’s view
Dobzhansky suggested that reproductive isolation arises as a result of natural selection. He noted that the hybrid of the species is often less adapted or may be even sterile or enviable.
These will tend to be eliminated by natural selection. The hybrids and those genes favouring hybridization will slowly be eliminated from the population. Thus, natural selection serves to reduce the wastage of gametes and less-fit hybrids.