Pre-zygotic barriers prevent mating or egg fertilization if members of different species try to mate.

Post-zygotic barriers zygotes may arise, but develop into inviable or infertile offspring

Reproductive barriers

a.  Habitat isolation - rarely encounter each other

b.  Behavioral isolation – courtship behaviours

c.  Temporal isolation (different breeding seasons)   d.  Mechanical isolation – anatomical incompatibility e.  Gametic isolation – cannot fertilise

Prezygotic barriers

A Prezygotic isolation example

• Two closely related Mimulus flower species• Different morphologies• M. lewisii bumblebee pollination• M. cardinalis hummingbird pollination

Courtship ritual as a behavioral barrier between species.

Forget it!

a.  Reduced hybrid viability (aborts) b.  Reduced hybrid fertility

(eg donkey + horse = mule (sterile hybrid)         

c.  Hybrid breakdown (future generations with reduced fertility)

Postzygotic barriers

Postzygotic – hybrid sterility• mother horse, father donkey sterile mule • mother donkey, father horse sterile hinny

Postzygotic – hybrid inviability

eg. sheep-goat embryos inviable

A summary of reproductive barriers between closely related species