Reproductive isolation mechanism

• It is classified into two types :

1. Premating: those operating before occurrence of fertilization.
2. Post mating: those operating after fertilization.

I. Premating isolation mechanism

a. Seasonal or habitat isolation:

• Potential mates from the two species don’t meet because they flourish in different seasons or habit.
• For eg: Tradescantia cenaliculata ( deep shade) and subasperma ( sunlight).

b. Behavioral or sexual isolation:

• Involves differences in courtship patterns of different species.
• As a result females of a species prefer males of same species and inter specific mating are rather rare.
• For example: melanogaster and D. simulans (often called sibling species due to morphologically similar) may show extremely effective sexual isolation.

c. Mechanical isolation:

• Mating between individuals of the two species are attempted but fertilization can’t be achieved because their genital don’t fit together.

II. Post mating isolation mechanism:

a. Gametic mortality:

• Either egg or sperm get destroyed in interspecific crosses.
• For example: In plants, pollen grains may not germinate on foreign stigma or pollen tubes may fail to grow in foreign style.

b. Zygotic mortality :

• Fertilization takes place but either zygote doesn’t develop or hybrid individuals have poor viability.
• Eg: Hybrid seeds obtained from cross between Linum perenne and austriacum  fail to germinate unless rescued through embryo culture.

c. Hybrid sterility :

• In this hybrids are normal but sterile.
• For example, F1 hybrids males from cross between pseudoobscura and D. persimilis  are completely sterile.