1. Reproductive barriers
           
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

2. Prezygotic 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

3. A Prezygotic isolation example
Two closely related Mimulus flower species
Different morphologies
M. lewisii bumblebee pollination
M. cardinalis hummingbird pollination

4. Courtship ritual as a
behavioral barrier between species.

5. Anatomically unlikely!

6. Forget it!

7. Postzygotic barriers a. Reduced hybrid viability (aborts)
b.  Reduced hybrid fertility
(eg donkey + horse = mule (sterile hybrid)         
c.  Hybrid breakdown
(future generations with reduced fertility)

8. Postzygotic – hybrid sterility
mother horse, father donkey sterile mule
mother donkey, father horse  sterile hinny

9. Postzygotic – hybrid inviability
eg. sheep-goat embryos inviable

11. A summary of reproductive barriers between closely related species