Let's explore how new species evolve from old ones. Created by Mahesh Shenoy.
- [Instructor] Hi, here's a question for you. Do you think humans evolved from apes? If that's true, then why are apes still around? Why haven't all the apes evolved into humans? What do you think? Well, the simple answer is, no. Humans did not evolve from apes. Instead, the right way to think about it is that both humans and apes evolved from a common ancestor. These guys were neither apes nor humans and this is just drawn by an artist, okay. We don't know exactly what they look like. But this can raise a very big question. How do new species like these evolve from an old species? Well, I'm glad you asked that question because that's exactly what we're gonna talk about in this video. So, this process when new species evolved from old ones, we call that speciation. Okay, so let me just write that down. That's basically what I wanna talk about. I'm gonna talk about something called speciation. Speciation and this is where new species, new species, new species form or evolve from old ones or you can say existing ones, whatever you can say old ones now. But before we continue, one question you might be having is hey, what's a species, right? Well, in short we say two members belong to the same species if they can reproduce and make a fertile offspring. Fertile meaning even this offspring is further able to reproduce. So that therefore we will say these guys belong to the same species. They're all dogs. On the other hand, if two members are unable to reproduce at all or make a fertile offspring then we will say they belong to different species. So, for example this chimpanzee and this dog, they belong different species because they cannot reproduce at all and of course if you want more details then we have a dedicated video called species which explores this in great detail. You feel free to go back and check that out. Anyways, coming back to our question, how do new species get formed? Well, the major requirement for this is something called reproductive isolation. Reproductive isolation. So, what does this fancy term mean? Well, it basically means that we need to make sure that there are at least two groups of the same species that are not reproducing with each other. That's the whole idea where in reproductive isolation. If you can maintain two groups which do not reproduce with each other then we will see as time passes by they will both evolve separately into two different species, okay. And now if you're wondering how does that happen, let's look at a particular example. We will not look at humans and the apes, that's basically because it's very complicated. Instead, we look at a much simpler example. Let me tell you a story. This is a story of a group of red colored beetles living on a bush, okay. Now, let's say some of these beetles migrate to a different place altogether. Now, this can happen in many ways. Maybe a wind carries them away or maybe they sit on in some animal and that takes them to a different place or maybe I don't know, they sneak into an aeroplane and that takes them or do to a different country altogether. Anything can happen. So, let's say these beetles go to a different place altogether, very far away and then they repopulate and we have a new set of population. Now, imagine that these two population of the same species, right, they're all red beetles, same species. They cannot reproduce with each other simply because they are too far away to travel back and forth, all right. Then we will say they are reproductively isolated because they cannot reproduce with each other. Now, we will see as time passes by, as more and more generations come, natural selection and genetic drift will start working differently in these different environments and they will eventually grow into two different species. Okay, first let me tell you how these two places are very different, okay. First of all, these bushes are pretty much gray in color because they are covered with smoke let's say and that could be because this is staying this bush is very close to a large factory nearby which is producing a lot of smoke so that makes this completely gray. And secondly, in this environment, let's say there are a lot of crows that eat on the beetles but in this environment let's assume there are no crows but instead there are lots of lizards that eat on these beetles. Now, with the state set, let's see how the two population evolve differently. So, as these beetles are continuously reproducing and continuously dying, let's say one fine day some mutation happens in the genes that's responsible for color, okay. Because of this some bugs are formed, some beetles are having blue color let's say, some beetles are born with gray color let's say and some other beetles are born with green color. This this can happen to to random changes in DNA and that keeps on happening all the time, all right. And let's say for the sake of simplicity, the exact same mutation happens here also, all right. So, again here also we have some green, we have some blue and we have some gray beetles. So, if you come back over here, what'll happen? Well, the green ones have an advantage because they are sitting on a green bush, harder to see and therefore more chances of survival, more chances of reproducing. And so as time passes by, as generations goes by we will now see the green beetles number to start increasing, all right. So, after many generations we might see the majority of the beetles are green in color. We call this natural selection and this also we have talked a lot about in our previous videos on natural selection. So, the green color gets naturally selected over here. But what do you think happens in this particular environment? Well, over here green doesn't have an advantage. Instead, it's the gray one that has an advantage, right which means after a few generations over here, it's the gray one that gets naturally selected. All right, let's do one more mutation. Let's say another mutation causes some beetles to be born with large wings. Okay, I'm just making things up over here. Beetles already have wings, small wings with which they can you know hop a little bit but let's say these large wings allow the beetles to fly to large distances, okay. Some beetles are born with large wings, let's say and maybe some beetles are born with no wings at all, okay, pure mutation this is all right. And of course the same thing happens here as well. Some beetles are born with large wings, some beetles are born with no wings at all, okay. Now, let's see what happens in these two environments. Let's come back over here now. Over here don't you see that this beetle has an advantage like over these lizards because if they can fly higher that means lizards can't eat them, right. That means again higher chances of survival therefore after a few generations they will get naturally selected and we will see most of the beetles have wings now. On the other hand, what do you think happens here? Well, here notice if the beetles are flying, they can be easily picked up by these crows. These crows are excellent. They are also flying, right. So, can you see flying beetles are at a disadvantage over here. In fact, the beetles which have lost their wings they have an advantage because the ones that have lost their wings will maybe will start crawling you know at the bottom and maybe because of that they will have a higher chance of surviving. So, over here these guys will not be naturally picked. Instead, the ones which have no wings that get naturally picked and after a few generations you find here we have beetles with no wings. Since this is so much fun, let's throw in one last mutation. Let's say one last mutation causes some beetles to be born with horns. Now, usually things don't happen so drastically but we are making up an example, so why not? Okay and let's imagine that one day some fire you know that the bush is under fire because of that almost all the beetles died but luckily the ones that survived we're mostly the horned ones, okay. The horned ones let's say had absolutely no advantage but just by pure luck they survived that accident and after a few generations now we find most of the beetles are having horns. This evolution by pure luck we have seen before is called a genetic drift, okay. These horned ones were lucky to survive that accident and so they pass their genes. Maybe on this side also some beetles are born with horns but maybe they were not so lucky. Let's say the beetles which are born with horns immediately got eaten by this lizard and they never got to pass their genes and so there are no horned beetles in this. And this is where we we'll stop. Now, look at these beetles, how different they are. They have different colors, these ones fly, these ones don't, they have horns, so many different features and we did this for a few generations. Imagine this happening for thousands of generations. How different, how much different features they would have accumulated over the years, right. They could be so different now that if now one of this beetle tries to reproduce with this one, they will just not be able to do it. The DNA of these beetles might be so different than these beetles that is just impossible to form a baby. In other words, we now have two different species of beetles and this is how one species evolves into two or more other new species, speciation. And of course this was an oversimplification but you get the idea, right? So, let's quickly write down what we saw. The first thing we saw was that the two population got separated from each other. This technically we call geographical isolation. Basically means the two population are confined to two separate locations, geographically isolated. That made sure that they couldn't reproduce with each other. That made sure of reproductive isolation. Turns out there are other ways to make sure of reproductive isolation which we'll not talk about but keeping them separate, geographical isolation is the best way to make sure that they do not reproduce with each other. Okay, after that what happened? Then we saw that in both these environments natural selection and genetic drift worked differently. Because of this as generations passed on, their DNA started changing and becoming more and more different from each other and eventually after a few thousand years it became so different that they were incompatible to for fertilization and so we had two new species. And just to be super clear, if you're wondering why it's necessary that these population do not reproduce with each other, think about what would have happened if they were constantly visiting each other and reproducing then whatever variations were selected over here, that variation could flow over here, those genes could flow over here. Similarly, whatever variations were selecting over here, those genes could flow over here. This gene flow would make sure that the DNA of this population and DNA of this population stays pretty similar to each other, right. Then we wouldn't have new species. So, you see it is super important that the genes selected over here do not flow over here, genes over here do not flow over here. Gene flow shouldn't happen, right. Only then as time passes by, the DNA starts becoming more and more different. Only then we will have speciation after lots of generations. So, reproductive isolation is super-important without which it's almost impossible to have speciations. And so we can put all of this together and we can now say that these two species of beetles evolved from one common ancestor, the red beetles right. And I'm pretty sure use now agree that it's wrong to say this evolved from this or this evolved from this. That's not that I will say that, right. Both of these evolved from one particular species. In a similar manner, humans and apes evolved from one ancestral species. And of course things over here we're definitely way more complicated than I told you but you get some idea now, right. It turns out that this took about seven to eight million years and there may have been even more species in between, more speciation in between which I have not drawn over here but you get the overall picture of how new species are formed now. Okay, so that's pretty much it. Let's quickly see if we can recall what we learned here. Can you recall what speciation is? What do you think is the most important requirement for speciation? And now can you explain to yourself or maybe to your friends very briefly, how speciation occurs? If you have difficulty in answering any of these questions, no worries you can always go back and just revisit the video.