 I have six forms of reproductive isolation that you get to learn and they're all unique. Keep in mind that most of the time in order to initiate or develop these forms of reproductive isolation, geographic isolation probably is going to happen first. But then when the salamanders came back together, there are several options for why they might have been isolated from each other reproductively. They might have been like, I really might like to make a baby with you, but for some reason I can't. Maybe it's ecological isolation. Ecological isolation. You are just going to know that that says isolation on every single one of these. Ecological isolation means we live in different places. If we're talking about like grasshoppers, one population of grasshoppers, they live in the same place, but one of them lives in like the grass and the other one, but of course lives in the tree. So the grasshoppers in the grass are not going to come in contact with the grasshoppers in the tree because they live in different ecological niches. Does that kind of make sense? I'm sure the answer is yes because I can hear you yelling at me. Okay, another possibility, another form of another reason why we can't make babies is because of temporal isolation. Temporal isolation is a time issue. So a time issue might be the grasshoppers. Maybe they all live in the grass now because do grasshoppers ever live in trees? So now if they, even if they're not ecologically isolated from each other, they live in the same place, but one population likes to make babies in the morning and the other population likes to make babies at night. And maybe they would have been totally compatible, but their timing is off and they're not flexible. So temporal isolation means you're going to have a time issue. You can't make babies because you don't like to do it at the same time. It's always a problem. All right, another form would be behavioral. Behavioral isolation would be something like the grasshopper would behavior like their song that they sing, their flashiness, their little dance that they do, or even just like the way that they sit and chill in front of the TV on the couch, whatever it is, there's a behavioral thing that is not compatible. Like, dude, I'm sick of you sitting on your butt on the couch. Extremely unattractive to me. I think that I am feeling reproductively isolated from you and then you don't make babies with that person. So behavioral would be some kind of like behavioral thing that makes it so that they can't make babies with each other. Aw. That's a bummer. Okay, let's try another one. If, keep in mind that all these other things do work. So let's say that, dude, I love your couch-sitting behavior. That's awesome. But this is the one. We can have reproductive isolation that is mechanical. And you might think, dude, really mechanical. You might think structural. Like, truly, the parts don't fit. So they can't copulate. So this grasshopper has parts that don't fit with that grasshopper's parts. You might, you like how the behaviors are. You like how everything smells. You like the song. It's all good. But literally the pieces don't fit together. So even if you want to, you can't because the parts aren't going to let it happen. Okay. How about sometimes the parts will fit. And then there's something called gametic. Ooh, that's gametic isolation. And that means the sperm don't like the egg. So they're not going to combine. Sperm and egg are actually incompatible with each other. Maybe the sperm can't find the egg. Maybe the egg is like, oh my gosh, nice try. Did you know that eggs, when they get fertilized, there's like a spark of electricity that kills all the other or like blasts all the other sperm away from, except for the one that got through. So maybe the spark of electricity happens too soon and the egg can't, you know, make the sperm happen. Or maybe too many sperm get in and then the egg gets fertilized by like 80 sperm and that's not going to work. So I mean, you can see that we could have all sorts of crazy issues going on with the gametes and then sometimes gametes fuse and then you have something or you possibly have something like a baby gets made but it's called hybrid inviability. Inviability or infertility. These are two different things. So a hybrid is a critter gets made. Two different species, the sperm and the egg, they combine, this is awesome. But either the fetus dies before it can be born or the fetus is infertile. And who's this an example of, you guys? That's my friend, the horse and the donkey. Remember the one of them's a boy and the other one's a girl, which is usually the case if you want to make a baby. One of them's a mule and the other one's a henny. I can never remember which one was the boy horse and which one was the girl mule, whatever. Bottom line is if the baby can't make babies, it doesn't count. The baby has to be fertile as well. These are mechanisms of speciation. These are things we can see. These are things we can test. These are things that we can actually go, oh, that's how it happened. That's how that worm developed its own little characteristics and now can no longer make babies with that other worm over there and whoa, we've got two species of worms now. There's evidence that this has happened. The first piece of evidence that speciation takes place is the fossil record, so let's take a look at that thing. That thing.