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Reproductive Isolation - Prezygotic - Postzygotic

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Reproductive isolation is a mechanism that keeps species from mating with others. Prezygotic isolation prevents the fertilization of eggs while postzygotic isolation prevents the formation of fertile offspring. Prezygotic mechanisms include habitat isolation, mating seasons, "mechanical" isolation, gamete isolation and behavioral isolation. Postzygotic mechanisms include hybrid inviability, hybrid sterility and hybrid "breakdown."

In evolution, one of the major concepts is what is a species? And a species is a group of organisms that are reproductively isolated from each other. Now, what does that mean?

Well, reproductive isolation depends on various mechanisms that keep one species from being able to succesfully mate with other species. Now these fall into two categories, these reproductive isolating mechanisms. They're are pre-zygotic reproductive isolating mechanisms, the ones that prevents the zygote from happening the fertilization of the egg by sperm. As compared to the post zygotic reproductive isolating mechanisms. Those are okay, we got the sperm to the egg. This is what prevents the formation of a fertile viable hybrid offspring. And just in case I didn't say it before, hybrid is a cross between two different species.

Now pre-zygotic reproductive isolating mechanisms fall into several different categories. There is habitat isolation. If the two species live in different locations and their habitats never coincide, that will keep them isolated, this is one of the reasons why we consider lions and tigers to be two separate species even though, some of you that have paid attention to some of the freakier parts of your text book may have seen pictures of ligers or tiglons. Because, if you have a really artificial situation, you have a zoo. And somebody happens to keep the tigers and lions in the same cage or in close enough cages, sometimes in a weirdly different situation you can wind up with tigers and lions successfully mating with each other and producing this tiglon or liger.

Mating seasons or sometimes they'll call it temporal isolation. If your mating season is say the spring, then you wil mate with your species during the spring. You will not mate with the species whose mating season is say the fall. You walk up to them, "you don't want to mate?" No they don't. They're completely disinterested. Meanwhile, fall comes around, they come up to you. "You want to mate?" No. Not the time for it. So that's another way of keeping two perhaps closely related species separate from each other.

Mechanical isolation. I'll put that in quotes and I'll put some other things into quotes. This is when the mechanics prevent it. Essentially when the male key doesn't fit the female's lock. And if you want to do some interesting reading, go on to the internet and see if you can find out about some of the different shapes of the keys and try to imagine in your head, how do they manage to keep that into the lock? But basically that's what's going on there.

Then there's gamete isolation. That's when the gametes are incompatible with each other. Where the sperm, even if it somehow manages to get its way to the egg, it doesn't have the right enzymes or receptor proteins to be able to effectively work in its way into the egg. Now there's one more that I forgot to put up and that's behavioral, and this is perhaps one of my favourite ones to talk about.

Behavioral isolation involves things like courtship rituals. If you want to see behavioral isolation in action, go to a high school prom. Watch the dance. A lot of the behaviors that you see the kids going on there are actually part of the behavioral isolation that keeps the human species separate form other species. If a spider came up to you and started wiggling its front legs to you, you're very unlikely to go hubba hubba. But it's trying to show off its courtship rituals that to another spider maybe very alluring. But to you not a spider. On the other hand if you walk up to a spider and say "hey, how you doing?" It's going to scuttle away, afraid you're going to slap it. Because you're using the wrong courtship rituals. Now, behavioral isolation, these rituals, these ways that we engage in mating behaviors, a lot of times they actually have more than just, "hey let's make sure that you're the right species." They often would demonstrate how wonderful you are, what kind of combination in the genes that you have and how impressive they are. Don't you want to share them in the next generation with me.

For example there is this little kind of bird. And what happens is that in the morning, when it's the right mating season, the female birds will all line up and they'll watch and the males will come out. Now the males have these what's called a comb, that's if you've ever seen an old cartoon with roosters like rooster combs. The comb is that red floppy thing that's on the head of the male bird. Well, the males want to inflate it with blood, and then they'll do a little dance. And the male who's got the nicest dance and the biggest comb will get the attraction of all the females. What's interesting is that the females if one of them goes forward and the other ones don't, she'll quickly scuttle back. But if a whole bunch of them go forward then all of a sudden that one male sudenly go whoa and lots of females are approaching him. And if you doubt that humans are affected by this, have you ever watched the flavour of love or rock of love? Why are all those women so interested in being with the bachelor of the week who's on one of those dating shows.

Now scientists would watch this and they wonder what's going on there and then they tested to see what it was that was happening. What they did is that they took a rubber glove, a red rubber glove. Now this bird's about maybe this big. They took a red rubber glove, cut off the fingers, stuff them with something and they put on a head of a male who had a small little comb. He walked up barely able to stand but he started wobbling around with his gigantic comb and all the females were and almost killed him. Well after they did some investigation they found out that that particular bird has a parasite. These worms that get into the blood vessels of the comb and its most active at dawn, and so by inflating their combs if they can inflate their comb very high that tells the females look at my immune system ladies look at how effectively it's protecting me whereas if you're infested with worms and you try to inflate your combs it's like this and the females are not interested. So behavior isolation doesn't just help keep us reproductively isolated its ways of us demonstrating our awesome genes. Look at again the high school prom the dance what this dancing demonstrate? Look at my skills, my nervous system can coordinate with my muscle system very well.

Post-zygotic. Finally you've somehow managed to get the egg and sperm together of two different species but what prevents that from developing? Well a lot of times you wind up creating a hybrid of those two species that is simply not viable. Whether it dies even before being born or maybe it's weak and unable to survive long on its own not very competitive.

A lot of times you wind up even if it is viable and survives it winds up being sterile. A good example of this is the mule. If you take a donkey and horse and you cross them you'll get a progeny. You'll get an offspring of it and its called the mule. And mules are great animals. Thats why during the prospecting era here in California, the stereotype was the forty niner bring along his materials packed up on his mule. Mules are very strong, great animals but they are sterile because they are this hybrid. So donkeys and horses are two separate species because of this hybrid sterility.

A last one that's a little bit unusual is this is concept called hybrid breakdown and thats where okay, the first generation of the hybrid maybe it can survive and maybe it can even have some offspring, but the next generation after that the children of the children and the grand children and so on they start having greater and greater problems. And typically these sorts of things are due to mismatches in the chromosomes of the two separate specie parents.

There you go. Those are the reproductive isolating mechanisms.