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Video transcript

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 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 are unable to have offspring together however for a sexually 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 prezygotic forms and post-zygotic forms prezygotic 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 reproduced together postzygotic forms of isolation we'll get into a little bit later so the first type of prezygotic isolation is temporal / habitat isolation and temporal isolation refers to the fact that not all organisms mate at the same time some inmate at night while there's made during the day some a maiden spring all others made in winter if two organisms do not find mates at the same time than 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 are in a problem then the next barrier is behavioral isolation which refers to mate selection on how organisms go about attracting a mate now not all organisms will attract made the same way perhaps one animal like a bird will attract mate by singing a song whereas this bunny rabbit may do little dance to attract a mate so we have behavior 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 you can move on and look at postzygotic forms of reproductive isolation and the first form is I go to mortality and this occurs when even if the two gametes from the two organisms can fuse successfully and form a zygote but as a goat 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 you know we have a pretty good definition for sexually reproducing organisms but not really for a sexually 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
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