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Reproductive isolation

Species are groups of organisms that can reproduce together; reproductive isolation keeps species separate. Pre-zygotic isolation prevents mating or fertilization (through temporal, habitat, behavioral, or mechanical isolation), while post-zygotic isolation affects zygotes or hybrids (via zygote mortality, hybrid inviability, or hybrid sterility). Although sexually-reproducing species are easier to classify, the species problem makes classifying asexually-reproducing organisms difficult. Created by Ross Firestone.

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  • leaf blue style avatar for user Kauê
    What about horses and donkeys: animals from two different species but capable of producing a mule (or a hinny)? Shouldn't they be of the same species by the definition at around ?
    (5 votes)
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  • aqualine ultimate style avatar for user Zachary Radulski
    What is love? My friend Haddaway asked so many times, so I was curious as to what it is.
    (6 votes)
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    • blobby green style avatar for user sipaarabu587
      Well, love for close family is a cocktail of hormones used to make you have a close bond, mostly evolving from the usefulness of working well with others in your band/tribe you would be in(which was normally made of close relatives). Love for a person outside of your family is a different mixture of hormones (made of more happy hormones like endorphins or dopamine) used to make the two of you mate and reproduce to spread your genes. This evolved from the how people who had this "love" did spread their genes more, unlike the non-mate loving counterparts. So, in general, love is bunch of hormones made to make you survive or mate, like any other adaptation.
      (3 votes)
  • male robot hal style avatar for user Camile
    In the case of bactéria, the capacity to transfer some parts of their genome isn't a source of diversity (maybe bigger than sexual reproduction could produce)?
    (2 votes)
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  • blobby green style avatar for user epouvantails.phd
    So what exactly is the difference between gametic isolation and hybrid sterility? Thanks!
    (2 votes)
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    • male robot hal style avatar for user Ras_Mekonen
      Gametic isolation occurs when the two organisms/species mate successfully but can not form a zygote (pre zygotic mechanism)> In the case of hybrid sterility, the zygote is formed and grown into a new hybrid organism but the hybrid is not able to reproduce. For example, mule which is a hybrid of mare and jack (two different species).
      (2 votes)
  • blobby green style avatar for user arielalalalala
    So is reproductive isolation the barriers that unable a species to breed successfully with other species either with pre-zygotic factors or post-zygotic factors? Is it preventing the interbreeding between the same species or different species?
    (2 votes)
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    • leaf green style avatar for user Shane McGookey
      Reproductive isolation is defined as "the inability of a species to breed successfully with related species due to geographical, behavioral, physiological, or genetic barriers or differences."

      Both pre-zygotic and post-zygotic barriers lead to reproductive isolation and thus prevent mating between different (but perhaps related) species.

      Note that this must be the case, as we conventionally consider two organisms to be of different species if they are unable to successfully produce viable, fertile offspring with each other (i.e., if they are reproductively isolated).
      (2 votes)
  • stelly yellow style avatar for user Quantum Cat
    If a species reproduces asexually, does that mean all organisms in that species are the same? How can there be any genetic diversity if they never breed with each other? Could all organisms in that species be descended from one organism thousands or millions of years ago?
    (2 votes)
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    • male robot donald style avatar for user Tybalt
      You're correct; if an organism reproduces asexually, their offspring will basically be clones of the parent. But because the offspring are clones, there will be little to no genetic diversity in the population. Mutations are still possible in these populations and can lead to diversity; however, they will not happen as quickly or make changes as drastically as what is seen with sexual reproduction. Unless an organism can reproduce extremely quickly in this manner, there will be little, if any, diversity in the population.

      It is possible for all organisms living on Earth right now to have arisen from one species of organism billions of years ago. Scientists call this ancestor the LUCA, or the Last Universal Common Ancestor. Every organism shares a couple of genes with every other organism on the planet, meaning that there must have been a common ancestor to everything. The LUCA probably would have looked like a small bacterium. It gets a little messy, however, if you account for horizontal gene transfer, where organisms like bacteria share their genes with each other rather than passing them down through reproduction. If, say, bacteria passed around some of their genes between plants or archaea, then you could have something that looks like common ancestry when it was really just something giving their genes to another organism. It is a very complex topic, so I encourage you to do some further reading on horizontal gene transfer and the LUCA.

      Does this help?

      (As a side note: prokaryotes that reproduce asexually, like bacteria, do not take much time at all to reproduce. E.coli, for instance, only takes 20 minutes to double! This can lead to entirely new mutant strains or even new species popping up within a matter of weeks. You don't see that happen too often with eukaryotes, because of their slower reproduction times.)
      (2 votes)
  • blobby green style avatar for user Janny Guerra
    what would be responsible for the divergence of two species from a common ancestor?
    Natural selection or reproductive isolation?
    (2 votes)
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  • blobby green style avatar for user Julia Kulyomina
    Question I've always had... Grolar bears. A mix between a polar bear and a grizzly bear. I believe that they are more common now that the polar bear territory is further south. There have also been cases of grolar bears successfully reproducing (in the wild). What does that make grolar bears, grizzly bears and polar bears.
    (2 votes)
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  • aqualine tree style avatar for user Star Birds
    Excuse me, so does reproductive isolation deal with the same species or different species - my question is, at , is the teacher referring to the same species, two organisms or two different species and different organisms? Thanks!
    (1 vote)
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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 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.