PREZYGOTIC AND POSTZYGOTIC BARRIERS


Website:
http://mansfield.osu.edu/~sabedon/campbl24.htm#behavioral_isolation
http://bioactive.mrkirkscience.com/19/ch19intro.html
http://ohoh.essortment.com/reproductiveiso_rcgu.htm

There are specific forms of reproductive isolating mechanisms, which serve to prevent interbreeding between two different species. There are prezygotic barriers, which interfere with fertilization, before the gametes are united, and postzygotic barriers, which are after the unification of gametes. Prezygotic barriers range from temporal isolation, habitat isolation, behavioral isolation, mechanical isolation, and gametic isolation. Temporal isolation is due to reproduction at different times of day. Habitat isolation is basically different species that live in the same area, but breed in different habitats. Thus, this can interfere with reproduction prior to fertilization. Behavioral isolation can occur due to the signals performed before mating, and if those signals are done incorrectly, this can stop the courtship. Mechanical isolation is basically the incompatibility genital organ structures, which prevents fertilization. Gametic isolation occurs because the gametes are unable to combine, and are thus incompatible.

There are also postzygotic barriers. Hybrid inviability is basically when the hybrid dies at a very early stage of development. Hybrid sterility is when the hybrid is able to survive, but it is unable to reproduce successfully. Hybrid breakdown is when the offspring of the hybrid are unable to reproduce successfully. An example of hybrid sterility is basically the mule, which is a hybrid between a female horse and a male donkey. This mule is sterile, and thus it is unable to reproduce successfully. It is essential to remember that postzygotic barriers occur after fertilization occurs. In conclusion, prezygotic and postzygotic barriers are forms of reproductive isolating mechanisms.

external image Mule280.jpg
http://www.ruralmissouri.org/Images/RuralMissouri/Sept04/Mule280.jpg

evolutionary tree diverging into horse and giraffe lineages
evolutionary tree diverging into horse and giraffe lineages
horse
horse
Giraffe
Giraffe
http://post.queensu.ca/~forsdyke/haldane1.htm