Question

1. What are pre-zygotic and post-zygotic reproductive isolation? What are the barriers classified in each?

Answer 1

These isolating mechanisms which serve to limit gene flow between species are categorized in terms of pre- or
post- mating mechanisms or pre-zygotic and post-zygotic reproductive isolations,

1) Pre- zygotic isolating mechanisms

This prevent union of gametes -> zygote
i) mates do not meet (seasonal or habitat isolation)

ii) mates meet but do not mate (ethological or behavioral isolation)

iii) mates meet but no sperm transfer (mechanical isolation)

2) Post-zygotic isolating mechanisms
This causes varying degrees of hybrid sterility or fitness

i) sperm transferred but dies before fertilization

ii) zygote dies

iii) zygote produces an F1 adult that has reduced viability (survival)

iv) hybrid is viable, but partially or completely sterile (fecundity) or the F2 generation is deficient.

An organism that mates with a different "semi-species" may produce low viability offspring and this individual would have lower fitness compared to an organism that discriminates against such semi-species and mates with members of its own semi-species. The evolution of species discrimination mechanisms are clearly an advantage over one that only possess post-zygotic isolating mechanisms.