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Have you ever wondered how we classify different organisms into different species? Well, before we look at that, let's go over the difference between asexual reproduction and sexual reproduction. In asexual reproduction, one organism, like a single bacterium, will divide into two daughter cells that are both genetically identical to the original cell. In sexual reproduction, two members of the same species will reproduce together in order to form genetically unique offspring. Now, in general, we say that organisms that reproduce asexually usually have low genetic diversity, whereas sexually reproducing species have high genetic diversity. So what is a species? Now, this can be a very difficult question to answer. For sexually reproducing organisms, we can say that two organisms, like this cat and this human, are members of different species. They're unable to have offspring together. However, for asexually reproducing organisms, like bacteria, protists, and archaea, it's a little more confusing. These species don't mate with other organisms. So we have a difficult time classifying them into different categories. And we call this the species problem. But in this video, we're going to spend time just looking at those sexually reproducing organisms. And these are separated into different species by different forms of what we call reproductive isolation. And this is the idea that there are many forces that stop two different organisms from having offspring together. And we can divide these forces into two separate categories, pre-zygotic forms and post-zygotic forms. Pre-zygotic isolation refers to all the different forces that prevent two organisms from having offspring together that occur prior to the formation of a zygote. And remember that a zygote is a single cell that is made up of the genetic material of both organisms that have reproduced together. Post-zygotic forms of isolation we'll get into a little bit later. So the first type of pre-zygotic isolation is temporal/habitat isolation. And temporal isolation refers to the fact that not all organisms mate at the same time. Some may mate at night, while others mate during the day. Some may mate in spring, while others mate in winter. If two organisms do not find mates at the same time, then they are temporally isolated. Habitat isolation refers to the place where the organisms mate. Some may prefer mating in the forest, while others prefer mating in the mountains. And if two organisms don't find mates in the same place, then they are also isolated. If time and place aren't a problem, then the next barrier is behavioral isolation, which refers to mate selection and how organisms go about attracting a mate. Now, not all organisms will attract a mate the same way. Perhaps one animal, like a bird, will attract a mate by singing a song, whereas this bunny rabbit may do a little dance to attract a mate. So we have behavioral isolation. And now we have mechanical isolation, which deals with the physical inability of two organisms to mate, even if they wanted to. Now, a great example of this is a huge animal like an elephant being unable to mate with a tiny mouse. If two organisms do mate successfully, they may still encounter gametic isolation, which is when fertilization between the two gametes to form a zygote is impossible. Now, once the zygote has been formed, we can move on and look at post-zygotic forms of reproductive isolation. And the first form is zygote mortality. And this occurs when even if the two gametes from the two organisms can fuse successfully and form a zygote, that zygote would have a high mortality rate and be unable to develop into a mature offspring. Next we have hybrid inviability, which occurs when a zygote is able to grow into a mature offspring, but that offspring will have a high mortality rate and won't be able to grow into a mature adult. Finally, we have the last form of reproductive isolation, which is hybrid sterility. And this is when the offspring can grow into a mature adult. But that mature adult is not able to mate and have offspring of its own. So if two sexually reproducing organisms are not isolated by any of these barriers, then we can generally say that they are members of the same species. So what did we learn? Well, first we learned about the species problem and how classifying different organisms into different species can be quite difficult. We have a pretty good definition for sexually reproducing organisms, but not really for asexually reproducing organisms. And next we learned about reproductive isolation and how we can say that two sexually reproducing organisms are reproductively isolated if they are unable to freely produce fertile offspring together.