Origin of reproductive isolation

• It may originate either accidently or due to natural selection because of weak and defective hybrids between races.
• Allopatric speciation: When two races have diverged sufficiently due to geography.
• Sympatric speciation: Due to disruptive selection in same locality.
• When two races exists either allopatric or sympatric, they may be expected to have accumulated different gene combinations making them highly adapted to their respective environment.