 So speciation, in order for speciation to occur somehow through some process, to one population of critters has to separate and become reproductively isolated from the other population. We had to start out with one group that can totally make babies with each other and then something has to happen to separate them most of the time. The first thing that happens is geographic isolation, geographic land isolation. Something happens in a spatial sense that separates a population of critters. An example of this, this is a true example of a salamander species that originally started in, looks like Oregon here and this population migrated south and as they migrated south they were separated by the Central Valley and they didn't, the Central Valley in this case is an example of a geographic thing that separates them. They couldn't cross the Central Valley, too hot, too dry, too wide, salamanders just couldn't hang. So they weren't mixing those two separating populations. Now they're going to be experiencing different pressures, they're going to be experiencing different environments, different variations are going to be more successful in different environments and they're going to be, now that they can't make babies with each other, not because they physically can't but because they're not in the same place to make it happen, now they're going to experience different pressures on their gene pools. Can you visualize that? And ultimately these two populations continued to migrate south and eventually they came back back here in Southern California. They actually migrated so far down that they passed through the Central Valley and now they can hang out with each other and they're like, what up Dog Pounds, let's hang out. Guess what? Could they make babies? No. They might have wanted to, they might have even tried but it didn't work. The geographic isolation enabled reproductive isolation, means they can't make babies anymore. If they can't make babies, then their gene pools aren't going to mix and that's going to keep them separate and keep them becoming more and more different because they are being acted on by different selective pressures and they have different genetics to act upon. Now there are lots of different kinds of reproductive isolation that we can talk about and reproductive isolation could happen first but I think what you'll see is that it's kind of unlikely that reproductive isolation would happen in a population that used to be reproductively compatible all of a sudden them being not compatible anymore is kind of unlikely. It seems like they would have to be separated and then kind of change amongst themselves and then that reproductive isolation would kind of result from that. Okay, so let's go ahead and list out some possible forms of reproductive isolation because there's lots of possibilities here.