字幕列表 影片播放 列印英文字幕 Captions are on! Click "CC" at bottom right to turn off. So if you've seen our videos before you may know that one of us is a cat person. Well the other one of us is a dog person, and when we were younger, we used to wonder…wouldn't it be cool to have one animal that is half dog and half cat. A dogcat. There was a cartoon we used to watch as a kid about that, although because of our tv reception issues, it never came in very clear. Hence, our childhood of instead watching nature shows. As cool as a half cat, half dog animal would likely be---we need to understand that cats and dogs are different species. What is a species? Well organisms that are in the same species can interbreed AND their offspring can reproduce. That means---the huge variety of domesticated dog breeds---they are all the same species. Even though they can look very different. Same with domesticated cats. All the different cat breeds are STILL the same species. You can watch our classification video to learn about the hierarchy levels in taxonomy, but in this video, we're just going to focus on the level of species itself. Now can two different species breed and have offspring? Yes---for example, let's consider the magnificent Zonkey. Yes, it's a thing. It's a hybrid actually---a cross between two different species---a donkey and a zebra. Pretty rare but it can happen. They typically are going to be sterile though---so even though the donkey and the zebra had the baby zonkey---that Zonkey will not be fertile. Donkeys and zebras are different species so this fits into that species rule. Speciation, which means the development of a new species---can occur when populations are reproductively isolated in some form. Why? Well first, check out our natural selection video which talks about natural selection as a mechanism of evolution and how change over time can lead to a new species. Our focus right now is to see how isolation can happen in the first place which can give rise to speciation. If we're going to get a little more fancy, we can talk about two main types of speciation: allopatric speciation and sympatric speciation. And here comes our disclaimer---in our short video clip, we are only giving some examples of isolation and how speciation can occur. In allopatric speciation, there is a geographic barrier that separates the populations. So while natural selection is acting on these populations, there is also some big geographical barrier---like a river, a mountain---that keeps them from being able to interbreed and share the same gene pool. Eventually over a period of time there can be change over time in the separated populations in their separated areas from mechanisms like natural selection or genetic drift. Over time, these populations can have significant genetic differences that may not allow them to interbreed even if they were brought together. Populations separated geographically can form different species. You might think you would always need a geographic boundary to separate populations, but in sympatric speciation, the speciation happens in the same area. Yet, there's something else isolating them. What is it? Well it can be a lot of things. We'll talk about just a few of them now. Let's start with prezygotic barriers—that means---barriers that occur before you can even make a zygote. A zygote is a fertilized egg so a prezygotic barrier is not even allowing fertilization to happen. So of these prezygotic barriers, let's start with this first one here: behavioral isolation. This is when species can have different behaviors, even very slight differences, that can isolate them. For example, birds having different songs---some only having very slight differences---can prevent the males from attracting females of other populations. And these birds can look very similar. Appearances are deceiving. One of the common biology examples is the Eastern and Western meadowlark. Surprisingly, you can find them in the same area. They look SO similar…but they are isolated by their behavior. Mates are attracted with a different song. Temporal isolation---species could breed at different seasons, years, even different times of the day. They may look very similar but if they don't have the same breeding season, then you're going to have an isolation. Habitat isolation. See even assuming that organisms live in the same area---that doesn't mean their habitats are exactly the same. You could have two species of amphibians living in the general same area but if one prefers an aquatic environment and one prefers a terrestrial environment, that will be a habitat isolation. Now you can have postzygotic barriers too. That means that mating and fertilization actually occurred because you have the zygote---a fertilized egg---but there is some barrier that separates the species even still. Remember our zonkey example? Perfect example: the offspring is not fertile. That's one barrier that separates donkeys and zebras. Sometimes offspring that are produced between two different species are very weak and do not survive long. And sometimes if species interbreed, the offspring is NOT able to develop in even very early embryonic stages because there is a genetic incompatibility. All of these are postzygotic barriers that can happen in the same environment. Now in our short video, we want to point out three things. First, this is just a very few set of examples of isolations that can lead to speciation. We encourage you to explore the huge list of other isolation types that can lead to speciation. Second, species can be impacted by more than one type of isolation. Third, please understand that isolation itself is not the mechanism for the actual change over time. Change over time, which can occur in gene pools of populations, can be due to mechanisms like genetic drift or natural selection. For example, let's take the case of natural selection. Remember that variety naturally exists in the populations. However, genes in a gene pool that result in high fitness---meaning more offspring---can INCREASE in the frequency of the population which can cause the population to change over time. Isolation is what separates the gene pools of species so the mechanism ACTING on the populations is acting on them…separately. Well that's it for the Amoeba Sisters and we remind you to stay curious!