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Introduction to evolution and natural selection

Natural selection is a mechanism of evolution and explains how species adapt to their environment over time through variations in traits. Examples include the peppered moth adapting to industrial pollution, yearly flu virus changes, and antibiotic-resistant bacteria. Understanding natural selection is essential for studying living systems. Created by Sal Khan.

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  • female robot grace style avatar for user Michelle Tan
    Just curious, are viruses living?
    (568 votes)
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    • leafers sapling style avatar for user Peter Collingridge
      Viruses are a borderline case but generally are not considered to be alive. Although they contain DNA (or RNA), they lack the machinery to replicate themselves. They also don't have any form of metabolism (so don't feed).

      An analogy would be a piece of paper that has "Please photocopy" written on it. It can replicate itself, but only in the presence of a human with a photocopier, so you wouldn't think it were alive. Virus are essentially little packets of DNA that encode the instructions "Copy this DNA (and packaging)".

      Having said that, "giant viruses" (such as Mimivirus, Megavirus and Pandoravirus) have been discovered recently which are bigger than some eukaryotic cells and contain more genes than some bacteria (>1100), including genes for whole metabolic pathways. These discoveries blur the line between viruses and life even further. However, they still need to infect another cell to run these pathways and to replicate.

      Finally, remember that "life" is just a label we attach to objects in the universe because it's convenient; there is not necessarily a sharp dividing line between what is and is not alive, and we (as a society) can choose to draw the line where we feel it is most convenient).
      (848 votes)
  • piceratops ultimate style avatar for user Charles Xu
    I have a quick question. Why do species vary? For example why are some people taller or shorter than others? If everyone came down from basically the same thing why is there so much variation? Even just so many different hair colors?
    (106 votes)
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    • leafers ultimate style avatar for user Meredith
      This is a good question :) I'd like to explain it from both a technical and practical point of view:
      Technically, lets say that maybe once upon a time, there were only brown-eyed humans. Then, one day, while a baby was being created in the mother's body, the genetics made a mistake while creating the baby, creating a little "oopsie" in the genetic material. We usually think of "mutants" as horrible half-humans with disfigurations, but originally, things like blue eyes were mutations in the human genome reflected in their phenotype (appearance).
      Practically, variation is really great for all living things. Say there's a whole field of purple and white flowers, and one day a disease comes along that, because the two flower colors are slightly genetically different, can only kill the purple flowers. If the species had no variation, they would have all died, but the white flowers were saved due to their differences.
      Hope I helped answer your question! :)
      (220 votes)
  • blobby green style avatar for user Roni Miller
    Why aren't there animals that are in the middle of different stages of development?
    (46 votes)
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    • leafers sapling style avatar for user Peter Collingridge
      I'm not quite sure what you mean by "in the middle of different stages of development". I guess from the context, you mean in the process of evolving into something else. In which case, all animals are in the middle of different stages of development, we just don't know what they will evolve into yet.
      (155 votes)
  • piceratops sapling style avatar for user Bjorn Shearer
    What causes these "random mutations"? Has this been answered or it is still a mystery of the universe?
    (73 votes)
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    • old spice man green style avatar for user Nathan Davis
      There are 3 ways a mutation can occur unfortunately I wouldn't feel comfortable going too indepth as I don't know it will enough to explain each step.

      The first chance is when the DNA begins to 'unwind' when it first starts making a copy of itself. The molecules don't completely seperate from the original and the copy will reflect the mistake.

      The 2nd chance it has will be when the DNA strands being to rejoin or rewind. The DNA is flexible and if the strands are bending at the wrong way at the wrong time, when it rejoins, the wrong base pairs will join together. Kind of like a zipper... unzip and zip enough times sooner or later it will break.

      3rd is a virus. The original DNA is fine, a virus comes along and just injects itself into the dna itself. When the DNA replicatates because the virus is now part of the DNA, it will be replicated during normal replication.

      Its been awhile so I am not 100% sure if i am accurate with the descriptions but I do know that those are the 3 ways that mutations occur. First when it splits, then when it puts itself back together, or sometime between those stages by an external influence.
      (68 votes)
  • male robot hal style avatar for user Tahsin
    Whats an RNA ?
    (31 votes)
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  • blobby green style avatar for user Sam
    Would it be accurate to think of evolution as being similar a game of telephone? As a word is passed on from ear to ear, it slowly changes, until it might be a completely different word at the end. Is evolution similar where, as DNA is passed on throughout generations, it randomly mutates until a descendant looks nothing like its ancestor? And then, of course, natural selection gets rid of all the "bad" mutations.
    (37 votes)
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    • old spice man green style avatar for user Nathan Davis
      Yes, You can say in that game of telephone, that 'natural selection' would favor a word that very easy to pronounce and not very prone to being misheard as a different word. So if you started with a word like 'orange', the likelihood of a 'mutation' when the word is being passed from person to person would be very small as nothing rhymes with or even sounds like the word 'orange'. Starting with a word like 'door' can very easily evolve or mutate into words like 'more' or 'for' which will again very likely to evolve again later down the change.

      Natural selection in animals favors genes that give that animal an advantage over the rest of its species to pass on their genes to the next generation. In our game of telephone, natural selection would favor words that are less likely to be misinterpreted as other words.
      (32 votes)
  • blobby green style avatar for user Daniel
    Do we (by "we" I mean science) actually know what causes the changes ?
    Because, as the bacteria example shows very well, natural selection favours antibiotic-resistant bacteria over non-resistant ones, but how did this resistant one appear in the first place ? It can't have "always" been there but in a small amount, waiting for someone else to die out and let it develop, just like homo sapiens didn't exist at all when the dinosaurs did, they actually appeared after some ape underwent changes.
    (20 votes)
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    • blobby green style avatar for user Kasun
      Very good point Daniel. The resistant bacteria did not always exists. 2 important things to know about bacteria:
      1. Due to the simple nature of bacterial DNA (or RNA for that matter), they are highly prone to gene mutations.
      2. Bacteria reproduce RAPIDLY under favorable conditions.

      So all it takes is one bacteria in the colony to have a gene mutation. (There are 3 main ways a gene mutation can happen). The result = a different amino-acid sequence will form. The different amino-acid sequence can remove the binding site of a antibiotic molecule. Now it is immune. And the bacteria will reproduce quickly.

      (Best way to understand this is:-

      - Imagine 2 lego pieces, one lego piece is on the bacteria and the other is on the antibiotic molecule.

      - If they "FIT" the bacteria is destroyed.

      - The mutation changes the lego block on the bacteria into a different shape.

      - Now the antibiotic WILL NOT FIT the bacteria. This means it has become immune. Only way to kill it is to give a different antibiotic.

      Hope this helps :)
      PS: these same mutations happen in humans too. Some of it's effects can be - Cycstic Fibrosis. Sickle Cell Anemia etc etc. It is an amazing field of study :D.
      (26 votes)
  • duskpin ultimate style avatar for user Aneri P.
    I was thinking a lot on this question:
    why is there so much variation? why does variation have to be so important to make the differences we have? Is variation the only thing that makes us different? or is there also there a another theory?
    Also, without all of this variation, there would be similarities and no differences, am I right?
    (8 votes)
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    • blobby green style avatar for user Rushil Kamarajugadda
      Variation is present between species because of genetic differences. As mentioned in other video; this was caused by dominant traits in specific environments. No; many other traits that have been inherited from generation to generation make us different in other ways. For example, the pygmy's in Africa are less than 3ft tall, and they are the only people who have been inheriting this very strange trait for generations. Other theories are there as well. Perhaps there will be different trait that are inherited through adaptation is genetical variations are not present. Hope this answer helps!
      (1 vote)
  • duskpin ultimate style avatar for user fireballninja
    Can you give us some mammal or plant examples of natural selection?
    (1 vote)
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    • old spice man green style avatar for user Blake Douglas
      Miyanda - your example is a good one, but a bit oversimplified. The Native Americans did have an immune system - all humans do. It was actually the fact that they had never been exposed to uniquely Old-World diseases like smallpox, and therefore had no RESISTANCE to it. In fact, a little-known detail of this period is that, just as Native Americans had no resistance to smallpox, the Europeans had no resistance to syphilis, which was unknown in Europe prior to the 1500s and apparently was brought to the continent by travelers returning from the New World. Unfortunately for the Native Americans, smallpox is far more fatal and easily transmitted, and so this "exchange" gave the Europeans a distinct advantage. Had it been the other way around, we might not be discussing the colonization of the Americas at all.
      (12 votes)
  • leaf yellow style avatar for user Mohd. Nomaan
    why do we fall ill?
    (4 votes)
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    • piceratops ultimate style avatar for user AegonTargaryen
      Because another organism bent on benefiting itself at our expense has taken root inside our body ex, the cold or the flu. Sometimes we fall ill because something in our body has stopped working or is working in a way that it bad for us ex, injury, cancer, or arthritis.
      (8 votes)

Video transcript

I think what is probably the most misunderstood concept in all of science, and as we all know is now turning into one of the most contentious concepts, maybe not in science, but in our popular culture, and that's the idea of evolution. Whenever we hear this word, I mean, even if we don't hear it in the biological context, we imagine that something is changing, it is evolving. And so when people use the word evolution in our everyday context, they think of this notion of change, that-- this is going to test my drawing ability-- but you see an ape bent over. We've all seen this picture at the natural museum, and he's walking hunchback like that, and his head's bent down and-- oh, I'm doing my best. That's the ape. Maybe he's also wearing a hat. And then they show this picture where he slowly, slowly becomes more and more upright, and eventually, he turns into some dude, who's just walking on his way to work, also just as happy, and now he's walking completely upright. And it's some kind of implication that walking upright is better than not walking upright, et cetera, et cetera. Oh, he doesn't have a tail anymore. Let me eliminate that. This guy does have a tail. Let me do it in an appropriate width. This guy has a tail, so you're going to have to excuse my drawings skills, but we've all seen this. If you've ever gone to a natural history museum, and they'll just make more and more upright apes, and eventually you get to a human being, and it's this idea that the apes somehow changed into a human being. And I've seen this in multiple contexts, even inside of biology classes and even the scientific community. They'll say, oh, the ape evolved into the human or the ape evolved into the pre-human, the guy that almost stood upright, the guy that was a little bit hunchback, so he looked a little bit like an ape and a little bit like a human and so on and so forth. And I want to be very clear here. Even though this process did happen, that you did have creatures that over time accumulated changes that maybe their ancestors might have looked more like this, and eventually they looked more like this, there was no active process going on called evolution. It's not like the ape said, gee, I would like my kids to look more like this dude, so somehow, I'm going to get my DNA to get enough changes to look more like this. And it's not like the DNA knew. The DNA didn't say, hey, it is better to be walking than to be kind of hunchbacked like an ape. And so therefore, I'm going to try to somehow spontaneously change into this dude. That's not what evolution is. It's not like-- you know, some people imagine that maybe there was a tree. There's a tree, and on that tree, there's a bunch of good fruit at the top of the tree. Maybe they're apples. And then maybe you have some type of cow-like creature, or maybe it's some type of horse-like creature that says, gee, I would like to get to those apples, and that just because they want to get there, maybe the next generation-- they keep trying to raise their neck, and then after generation after generation, their necks get longer and longer, and eventually they turn into giraffes. That is not what evolution is and that's not what it implies, although sometimes the everyday notion of the word seems to make us think that way. What evolution is-- and actually, this is the word that I prefer to use-- it's natural selection. Let me write that word down. Natural selection. And literally, what it means is that in any population of living organisms, you're going to have some variation, and this is an important keyword here. Variation just means, look, there's just some change. If you look at the kids in your school, you'll see variation. Some people are tall, some people are short, some people have blond hair, some people have black hair, so on and so forth. There's always variation. And what natural selection is is this process that sometimes environmental factors will select for certain variation. Some variations might not matter at all, but some variations matter a lot. One example that's given in every biology book, but it really is interesting is-- I believe they're called the peppered moth. And this was in pre-Industrial Revolution England that these moths-- some of the moths were-- let me see if I can draw a moth. I think you get the idea. Let me draw a couple of them. Let me draw a few peppered moths. A couple of peppered moths there. Let me draw one more. So most peppered moths, there was just this variation. Some of them were-- I guess we could call them more peppered than others. So some of them might look like this. You know, they had-- let me do other colors. Let me do a white. So it had spots like that. Some of them might have looked more like that. And, of course, they had some black spots on them. And then some of them might have been-- just barely have any spots. You just have this natural variation. Like you'd see in any population of animals, you'll see some variation in colors. Now, they were all happy, probably for thousands of years, just this natural variation. It was a non-important trait for these peppered moths. But then, all of a sudden, the Industrial Revolution happens in England, and all this soot gets released from all of these factories that are running these steam engines powered by coal. And so, all of a sudden, a lot of the things that once were grey or white, for example, maybe some tree trunks. Let me draw some tree trunks. Maybe there were some tree trunks that used to look like this. You know, maybe it looked like a-- maybe it kept a-- maybe some tree trunks used to look something like this, and a peppered moth would be pretty OK. Maybe there are some tree trunks that were pretty dark. But all of a sudden, the Industrial Revolution happens. Everything gets covered with soot from the coal being burned, and then all of a sudden, all the trees look like this. They're just completely pitch black or they're a lot darker than they were before. Now, all of a sudden, you've had a major change to these moths' environment, and you have to think what is going to select for these moths? Well, one thing that might get these moths are birds and the ability of the birds to see the moths. So all of a sudden, if the environment became a lot blacker than it was before, you can guess what's going to happen. The birds are going to see this dude a lot easier than they're going to see this dude, because this dude on a black background, he's going to be a lot harder to see. And it's not like the birds won't catch this guy. They'll catch all of them, but they're going to catch this guy a lot more frequently. So you can imagine what happens. If the birds start catching these guys before they can reproduce, or maybe while they're reproducing, what's going to happen? This guy, the darker dudes, are going to reproduce a lot more often, and all of a sudden, you're going to have a lot more moths that look like this. You're going to have a lot more of these dudes. So what happened here? Was there any design or was there any active change by any of the moths? Did any of the moths-- I mean, it looks like a really smart thing to do to become black, right? Your surroundings became black, and you wait a couple of generations of these moths, and now all of a sudden, the moths are black. And you might say, wow, those moths are geniuses. They all somehow decided to evolve into black moths in order to hide from the birds more easily. But that's not what happened. You had a lot of variation in your peppered moth population. And what happened was that when everything turned darker and darker, these dudes right here-- and dudettes-- had a lot less success in reproducing. These guys just reproduced more and more and more, and these guys got eaten up before they were able to reproduce or maybe while they were reproducing so that they couldn't produce as many offspring, and then this trait just became dominant. And then the peppered moth just became-- you can kind of view it as a black moth. Now, you might say, OK, Sal. That's one example. I need more. This is natural selection. It's purported to apply to everything. It purports to explain why we evolved from basic bacteria or maybe even self-replicating RNA, which I will talk about more in the future. I need more evidence of this. I need to see it in real time. And the best example of this is really the flu. And I'll do other videos in the future on what viruses are and how they replicate. Viruses are actually fascinating, because it's not even clear that they're alive. They're literally just little buckets of DNA and sometimes RNA, which we'll learn is genetic information, and they're just contained in these little protein containers that are these neat geometrical shapes, and that's all they are. They're not like regular living organisms that actively move and that actively have metabolisms and all that. What they do is they take that little DNA, and they inject it into other things that can process it, and then they use that DNA to produce more viruses. But anyway, we can do a whole series of videos on viruses, but the flu is a virus. And what happens every year is you have a certain type a virus, and they have some variation. I'll just make the variation by how many dots they have. And they infect-- let's say it's a human flu. They infect humans, and slowly our immune systems, which we can make a whole set of videos on as well, start to recognize the virus and are able to attack them before they can do a lot of damage. So now you can imagine what happens if, let's say, that this is the current flu. Let me do all of them. They all have these little two dots and that's how-- and we'll talk in the future what these dots are and how they can be recognized. But let's say that's how our immune system recognizes them. They start realizing, oh, any time I get this little green dude with two dots on it's, that's not a good thing to have around so I'm going to attack it in some way and destroy it before he infects my DNA and all the rest. And so you have a very strong natural selection once immune systems learn what this virus is-- and we'll talk more about what learning means for an immune system-- that they'll start attacking these guys, right? But flu, you can kind of think of them as being tricky, but they're not really tricky. They're not sentient objects, but what they do do is they constantly change. So what you have is, in any flu population, you're always having a little bit of change. So maybe the great majority of them have those two dots, but maybe every now and then, one of them has one dot, one of them has three dots, and maybe that's just a random mutation. This just randomly happened. Maybe this is one in every-- I'll make up a number: One in every million of these viruses have this only one dot instead of two dots. But what's going to happen as soon as the human immune system gets used to attacking the virus with the two red dots? Well, then this guy isn't going to have to compete with the other virus capsules for infecting people. He's going to have people's DNA all to himself. And so he or she, or whatever you want to call this virus, is then going to be more successful. So by next year's flu season when people start sneezing and are able to spread it on doorknobs and whatever else again, this guy's going to be the new flu virus. So when you see this process of every year there's a new flu virus, that is evolution and natural selection in real time. It is happening. It isn't this thing that only happens over eons and eons of time, although most of the kind of the substantial things that we see in our lives or even ourselves are based on these things that happened over eons and eons of time, but it happens on a yearly basis. Another example is if you think about antibiotics and bacteria. Bacteria are these little cells that move around, and we'll talk more about them. They actually are definitely living. They have metabolisms and whatever else. And this is just a nice note. When people talk about infections, it could either be a viral infection, which are these things that go and infect your DNA and then use your cell mechanisms to reproduce, or it could be a bacterial infection, which are literally little cells that move around and they release toxins that make you sick and whatever else. So bacteria, these are what antibiotics kill. Actually, I don't think there's a hyphen. They attack bacteria. They kill them. If you know a couple of doctors or whatever and you say, hey, I'm sick. I think I have a bacterial infection. Give me some antibiotics. A responsible doctor says no, I won't give you antibiotics just willy-nilly, because what happens is, the more antibiotics you use, you're more likely to create versions-- and I want to be very careful about the word create, because you're not actively creating them. But let's say-- and let me finish my sentence. You're very likely to help select for antibiotic-resistant bacterias. Now, how does that work? Let's say that these are all bacteria and you have gazillions of them, right? Every now and then, you get one that's slightly different, right? Now in a population of bacteria, these all will make you equally sick, and this is just some random difference in the bacteria. Maybe on its DNA some slight different changes happened, but whatever happened, these all are a kind of bacteria. You don't want to get a lot of them in your system. Your immune system can attack them and fight them off, but if you get a lot of them, then they might kill you or make you sick or whatever else. Now, if everyone just starts using antibiotics when they're not sick or when they don't really need to in a life-or-death situation, you might have an antibiotic that is really good at killing the green bacteria. But what happens if you all of a sudden kill a lot the green bacteria? Well, now the blue bacteria have the whole ecosystem that before it was competing with all these green dudes to get at all the good stuff inside of your body, but now he's all alone, and now he can replicate willy-nilly. So now he's going to replicate willy-nilly, and obviously-- once again, it wasn't like there was any design, there was any intelligent process here that said look, this bacteria should-- some bacteria said, oh, I'm going to be little bit smarter and design myself to resist this antibiotic threat. No! There's just these random changes that happen, and mutations and viruses and bacteria happen frequently and these random changes that happen, and this might be a one in one billion change, right? But all of a sudden, if you start killing off all of the people it's competing with, this guy can start replicating really fast and then become the dominant bacteria. And then all of a sudden, that antibiotic that you had developed very carefully to destroy the green dudes is useless, and you have this superbug. You might have heard the word superbug. That's what a superbug is. It's not like it designed itself somehow. It's just that we got very good at killing its competition, and so we allowed it to take over, and we can't kill it, because all of the drugs were just good at killing its competition. These bacteria just keep mutating and keep mutating, and if we use these antibiotics a little bit too heavily, we'll always be selecting for the things that won't be affected by the antibiotics. Well, anyway, I think I've spoken long enough, but this is a fascinating, fascinating topic. And I really wanted to make this my very first video or lecture if you will, on biology, because if you really went to-- you know, biology is the study of life, and we can talk about what life is, whether viruses are living, whatnot. But if you really want to study living systems, you really can't make any assumptions other than natural selection. We could go to another planet where the creatures don't have DNA, or maybe they have some other type of hereditary information stored in their cells, or they replicate some other way, or they're not even carbon based. Maybe they're silicon based. And if we went to that type of a planet in order study the biology on that planet, everything else we know about biology, about viruses and DNA, would be useless. But if we do understand this one concept, this one concept of natural selection, that your environment will select, and it's not-- you know, there's no active process here. It's just random stuff happened and they randomly select for random changes. And over large swaths of time, and these are unimaginably large swaths of time, those changes essentially accumulate, and they might accumulate into fairly, fairly significant things. We'll talk more about this in another video. See you soon.