Hi. In this video, we're going to talk about species, what they are, and how they form. Now, speciation is the process by which an ancestral species splits into two or more species. And we can see that there are really two types of evolution at play or two kinds of broad categories. You can have microevolution, which is the change in allele frequencies in a population, and macroevolution, which is broad patterns in evolution well above the species level. So, speciation kind of serves as a link between micro and macro evolution. As the change in allele frequencies in a population grows, we can eventually have a speciation event, which ultimately will lead to patterns of broad evolution over long periods of time.
Now, we're normally going to be talking about species according to the biological species concept, which is basically the idea that species are defined by reproductive barriers. So what does this mean? Well, you can have something called reproductive isolation, which are basically mechanisms that prevent two species from mating and producing viable offspring, and we'll get into the details of that in just a moment. However, it's also worth noting that you can also form hybrids which are offspring that result from inter-species mating. However, these offspring tend to not be viable, and we'll talk about what all that means in a moment.
So here you can see a sort of nice example of speciation. You have this initial population of fruit flies, you separate them and give them different foods and let them go through several generations of breeding. And then when you mix those two populations back together, well most of the yellow flies that had the starch available as food, they're going to want to mate with each other, those maltose flies are going to want to stick with each other and mate. You might have a few that will mate with each other, but you have the beginnings of speciation. These are starting to diverge.
Now when we talk about reproductive isolation, there are different types of barriers that must be overcome. And we group these into pre-zygotic barriers, as in mechanisms that prevent zygote formation, and post-zygotic barriers which prevent the zygote from functioning properly. So prezygotic barriers include gametes that won't fuse, behavioral differences in courtship, and what's called mechanical isolation, which is basically the idea that organisms don't have the right parts to mate. Think of this as a sort of round hole square peg scenario. Now, post-zygotic barriers are basically when hybrids are sterile or do not survive. They die very quickly because, again, these are not viable offspring. So if the offspring are sterile or do not survive and die very quickly, that means they are not viable.
And over here, you see a nice example of post-zygotic barriers. These organisms are called ligers. Yes, this is a hybrid between a lion and a tiger. It's Napoleon Dynamite's favorite animal. And guess what? Ligers are totally sterile. You can make ligers, but you can't have ligers mate with each other and make more ligers. You can only form the hybrid through a mating of a lion and a tiger. Now, let's turn the page and talk about some other detail.