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Natural selection and the owl butterfly

Intricate patterns, such as the owl butterfly's eye-like wing markings, evolve through natural selection. Over thousands of generations, small variations in wing patterns offer slight survival advantages, leading to the dominance of beneficial traits. This gradual process allows complex designs to emerge from random genetic mutations. Created by Sal Khan.

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  • leafers sapling style avatar for user Michael Dorsey
    Because of the short life span of the butterfly, will the evolution process happen faster then human?
    (121 votes)
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    • orange juice squid orange style avatar for user Bob
      This is true. Of course, even absent any selective pressure, there will be some changes over time just because that's the way of things, but it takes longer because without that pressure, there's no reason for any particular trait or group of traits to be selected for.

      Because they have shorter lifespans, and thus can reproduce more rapidly, there is greater opportunity for evolutionary change. So if there is some selective pressure, a greater number of generations within the same amount of time should lead to a higher rate of change.
      (95 votes)
  • orange juice squid orange style avatar for user LaFontaine
    so is it possible that the medicines we're using now might become ineffective after some thousands or millions of years?
    (30 votes)
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    • orange juice squid orange style avatar for user Bob
      Indeed. And we're currently facing some big-time problems. Because we so routinely use antibiotics, both in hospitals and on livestock farms, we're speeding up the development of antibiotic resistance in bacteria. There will come a time--and it will come soon--when our antibiotics lack their efficacy. We just better hope we have developed new drugs or techniques by then.
      (52 votes)
  • male robot hal style avatar for user Mariah Nelson
    At , Sal lists AGCCTAAAGT .... I'm sorry, maybe I missed it... But WHAT in the world do these letters represent?
    (13 votes)
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    • mr pants teal style avatar for user Matthew Belliveau
      A, T, C, and G represent the different bases that make up DNA. They are found on a single strand of DNA and they form hydrogen bonds with other bases of a second strand of DNA to create a double-helix of DNA (C pairs with G, and A pairs with T).

      In total, each cell in your body has approximately 3 billion of these base pairs.

      A = adenine
      T = thymine
      C = cytosine
      G = guanine
      (22 votes)
  • mr pants teal style avatar for user Hwankyu Song
    At Sal said that there are 2 theories explaining the eye kind of shape on the owl butterflys wing. 1 said that they can survive by scaring predators.
    2 said that they can survive because predators can only attack the wings eye shape because it looks like it has more nutrients or something.
    I think the 2nd theory has a lot of questions to explain.
    1. I dont think the butterflys can re develop their wings in the short period of time so they will all get attacked and eaten by predators, since they cannot fly away. How is the teory going to explain this?
    2. The predators would not know which part of the butterfly would be more nutritious or stuff like that. This needs to be explained clearly.
    (8 votes)
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  • blobby blue style avatar for user Zalaand Khan
    Why did he use wikipedia? Isnt wikipedia not trustworthy?
    (9 votes)
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  • primosaur ultimate style avatar for user mark.coolen
    Scientists are discovering that some changes to butterfly wing patterns, etc., are the result of genetic switches that enable and disable various parts of the butterfly genome. Since these genetic switches are already built in to the butterfly DNA as are the gene regions they control, doesn't that make this video another evolutionary just-so story?
    (4 votes)
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    • old spice man green style avatar for user Alf Lyle
      I avoid using loaded words like “fairy tale” and “myth” in my responses to questions, and I am sure you could have posed your question without saying the video contained a “just-so-story”. I actually chided another responder (whose answers are always factual and enlightening) for using “fairy tale” in one of his responses to a different question. As a federal court in the Dover case recently determined, evolution is the only scientific theory which explains the fossil and genetic evidence we have. These videos are in the Science play list. I don’t see any value in bringing the battle of intelligent design/creationism versus evolution to the Khan Academy. However, I endorse your viewing all of Sal’s videos, and where there is a claim made or where there is something you don’t understand, to ask for an explanation is perfectly valid.

      Calling the butterfly example in the video “another” fairy story implies there are other demonstrations of the correctness of the theory of evolution which are invalid (and maybe you question all of them) . However, without your citing examples (hopefully relevant to a particular video) as part of a question, this is just a rhetorical technique.
      (15 votes)
  • male robot hal style avatar for user Mr. Scott
    so between about to like he says after our age of reproduction we are more prone to disease because we have fullfiled our purpose of reproducing and our imune systems are a lot more leniant, what if someone isn't able to reproduce in the first place?
    (7 votes)
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    • leaf green style avatar for user Caiti
      It's not related to whether you actually reproduce or not, it's just related to the average ages of reproduction. For instance, human females are fertile from approximately age 13 when they begin menstruating through approximately age 50, when they reach menopause and are unable to reproduce. It does not matter whether a woman actually bears children in her lifetime, she will still reach menopause.
      (4 votes)
  • leaf green style avatar for user Ken345
    Do you think it's reasonable to assume that with the intervention of modern medicine and science that in many ways we have stopped or limited natural selection among humans? We can now allow couples to reproduce who would have remained childless in the past due to some flaws and defects in their reproductive systems. We also abort a significant number of humans around the world who might have been beneficial to the gene pool. Science also allows us to technically adapt to our environments when in the past, only natural selection would allow for better adaptation to a given environment.
    (5 votes)
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  • starky seed style avatar for user ella aficiuc
    what happens if there is a mutation and the spots on wings disappear?
    (5 votes)
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  • aqualine seedling style avatar for user cool
    that is literally a orange slice of a wing
    (8 votes)
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Video transcript

In the first video on evolution, I gave the example of the peppered moth during the Industrial Revolution in England and how, before the Industrial Revolution, there were a bunch of moths: some were dark, some were light, some were in between. But then once everything became soot filled, all of a sudden, the dark moths were less likely to be caught by predators and so all of the white moths were less likely to be able to reproduce successfully, so the black moth trait, or that variant, dominated. And then if you came a little bit later and you saw all the moths had turned black, you'd say all these moths are geniuses. They appear to have somehow engineered their way to stay camouflaged. And the point I was making there is that, look, that wasn't engineered or an explicit move on the part of the moths or the DNA, that was just a natural byproduct of them having some variation, and some of that variation was selected for. So that example, that was pretty simple: black or white. But what about more complicated things? So, for example, here I've got a couple of pictures of what's commonly called the owl butterfly. And what's amazing here, and it's pretty obvious, as I probably don't have to point out to you, is its wing looks like half of an owl's eye. I can almost draw a beak here and draw another wing there and you can imagine an owl staring at us, right? And here, too, I could imagine a beak here and you would think an owl there, too. And so the question is how does something this good show up randomly, right? I mean, you could imagine, OK, little spots or black and white or grey, but how does something that looks so much like an eye generate randomly? Now the answer is-- well, there's a couple of answers. One is why does this eye exist, or this eye-like pattern or this owl-like eye's pattern? And there, the jury's still out on that. I read a little bit about it on Wikipedia and all of these images I got from Wikipedia. In Wikipedia, they said, look, there's two competing theories here. One theory is that this, even though to us humans, the way we see things, it looks like an owl's eye, that this is actually a decoy. When some predator wants to eat one of these things, they go for the thing that looks most substantive. So instead of going for the butterfly's body, which doesn't look that substantive, they go for the big, black thing. They say, oh, that looks like it's protein rich and it'll be a good meal. So they try to snap and bite at that, and if they bite at that, sure, the guy's wing's going to be clipped a little bit and it's going to suck, but the animal itself, the actual butterfly, would survive, and maybe it can repair its swings. I don't know the actual biology of the owl butterfly. That's one theory, and then the argument against it goes, well, no, if that was the case, then you would want the black spot even further back along its-- you'd want the spot way far away from the body. You'd want it back here instead of right here, because there's still a chance, if something chomps at this little black spot, that it'll still get the abdomen of the butterfly. Now, the other theory as to why this exists-- and, you know, who knows? Maybe it's a little bit of both. Maybe both of these are true. Maybe this offers two advantages. The other theory, and this is kind of the one that jumps out at us when we see this , is, hey, this looks like an owl. Maybe this is to scare away the things that are likely to eat this dude. And it does turn out in my reading that there are lizards that like to eat these type of butterflies, and those lizards probably don't like to be around birds or owls because those owls eat them, so that might be a deterrent. And then the other example, they said is, look, they tend to be eaten by this lizard right here-- this is what Wikipedia told me-- and that this lizard tends to be eaten by this frog right there, and that the eyes of this butterfly are not too dissimilar to the eyes of this frog. And, you know, we can debate whether or not that's the case, and if this was the predator we're trying to mimic, you could make an argument that maybe we would have had more green on our wing, but that's not the point of this video. But it's a fun discussion to have as to what is useful about this eye. But let's have the question: How did that eye come about? And when I say that eye, I mean the pattern on that wing. What set of events allowed this to happen? Because when I described evolution, and we know that everything in our biological kingdom is just a set of proteins and then stuff that maybe the protein-- but mainly protein, and that protein's all coded for by DNA. I'm going to do future videos on DNA, but DNA is just a sequence of base pairs. It's a sequence of these molecules. And we represent adenine, and guanine and cytosine and thymine. Then maybe you have a couple of adenines in a row and some guanine and thymine. I'll do a lot more on this in the future, but the idea is it's just coded for by this sequence of these molecules. How do you go from a butterfly that has no eye to all of a sudden an eye that goes there? Obviously, just one change that happens from a random mutation. Maybe that G turns into an A or maybe this C and this T get deleted so everything-- that alone isn't going to develop this beautiful of a pattern or this useful of a pattern. So how do the random changes explain something that's this intricate? And this is my explanation. And obviously, I wasn't sitting there watching over the thousands or millions of years as these owl butterflies emerged, so this is just my theory of how natural selection does explain this type of phenomenon. You have a world where in some environment you have butterflies, and their wings look like-- let's say you have some butterflies that are generally like this. That's their wing, and it's a very bad drawing, but I think you get the idea, and there's just some general patterns. We've seen it before. There's variation. And the variation does show up from these little random changes in DNA. I think we can all believe that, that most of these changes are kind of benign. Maybe they just set up differently where a little pattern will show up or a little speck of pigment will show up with a slightly different color. And we even see amongst these owl butterflies, there is variation. This dude's wing is different than that guy's wing with the commonality that they do have these eye-looking shapes. And there's not just one; there's actually multiple. This guy has this other thing up here that looks interesting, and they have multiple things, but the one really noticeable feature is this eye-looking thing. So how do we go from this to an eye-looking thing? So the idea is you have some variation. One guy might look like that. Another guy, or gal, might-- just randomly, their dot might be something like that. Another gal or guy-- these wings are really badly drawn, but you get the idea. This is the butterfly. This is its antenna right there. That's its body. Another butterfly's patterns might look like this, right? And so, they're just random. But when they go into a certain environment for whatever reason, maybe one of its predators-- maybe that theory that these are supposed to look like eyes is true. And so, actually, maybe this guy just has a random pattern here. And so this guy-- and I'm not saying that it's like definitely better. They're both going to be found and killed by predators, but it's all probablistic, right? Maybe this guy has a 1% less chance of getting a predator, because when a predator just looks at him out of the corner of that eye, that little really hazy region kind of looks like an eye and the predator would be better off just not messing with it, and they'd rather go after the dude that looks like this. So it's just a slight probability. Now, you might, say, OK, what's 1% going to do? But when you compound that 1% over thousands and thousands of generations, all of a sudden, this trait might dominate because he's just going to be killed that less frequently, 1% less frequently. Now, maybe this guy has a similar trait, but his spot is closer to the abdomen. And here, it's a tradeoff, because maybe some predators get scared away by this concentration of pigment. And once again, I'm not saying that we're here yet. We're not at this very advanced, sophisticated pattern yet. We're at this random concentration of pigment that just shows up. So we see that people who have this concentration of pigment further away from their abdomen, they do well. But when it's too close, maybe some predators think that that's actually an insect and they want to eat it, so that's actually a bad trait. So what happens is this guy dominates, and so within this population, you start having a lot of variation, because he starts representing-- he's more likely to pass on these traits. And I want to make that point very clear. This isn't what happens over the course of an animal's lifetime. It's not like if somehow I experience something, or at least our current theory if I experience something, that I could somehow pass on that knowledge to my child. What it says is if my DNA just happens to have just some variation that happens to be more useful or more likely for me to survive to reproduction and for my children to survive, then that will start to dominate in the population. So then the population, you're going to have variations within that. Maybe some guys, you know, it's going to get a little bit to look like that. Maybe another one's going to look a little bit like that. Maybe it has some spots there. You can kind of view it as the variation as "exploring." But I want to be very clear not to use any active verbs here because this is all being done really as almost a common sense process, where everything changes. The changes that are most suited are the ones that are going to survive more frequently. And then the next generation's going to have more of that and then you'll have variation within that change. And then this one might be like that, and maybe this is the one. These were good compared to that, but now when you're competing amongst themselves, this one is able to reproduce 1% more than this guy or this guy. And so this guy becomes-- and maybe it's some combination of all of the above, and they mix and match. It's a hugely complex system. But then this guy represents most of the population, when I say this guy, I'm saying this guy's genetic information, at least as it pertains to his wings. And then you get variation amongst that. Maybe some of it, they have a little small dot and there's some dots around it. Maybe it's like this. Maybe one of them digresses and goes back here, but then he has trouble competing so he gets knocked out again. And then some other people have it back here. I think you get the point that this isn't happening overnight. These changes can be fairly incremental, but we're doing it over thousands of generations. So when you're talking about thousands of generations, or even millions of generations, even a 1% advantage can be significant, and when you accumulate those variations over a large period of time, you can get to fairly intricate patterns like this. So I just wanted to explain that, because this is often used as, sure, I can believe the butterfly moth or I can even maybe believe the examples of the antibiotics and the bacteria or the flu, because those are kind of real-time examples. But how does something this intricate show up? And I actually want to make a point here. We think this is more intricate because we can relate to it in our everyday lives. But if you actually look at a structure of a bacteria and how it operates or what a virus does to infiltrate an immune system or a cell, that's actually on a lot more levels a lot more intricate than a design. In fact, the whole reason why I'm using this as an example is because this is a fairly simple example as opposed to kind of explaining the metabolism of a certain type of bacteria and how that might change and how it might become immune to penicillin or whatever else. But I want to make this very clear that these very intricate things, they don't happen overnight. It's not like one butterfly was completely one uniform hot-pink color and then all of a sudden they have a child whose wings looked just like this. No! It happens over large periods of time, although there might be some little weird hormonal change that does this, but I'm not going to go there, but that is possible. But I just wanted to make this point because I think the more examples we see, the more it'll kind of hit home that this is a passive process. We're not talking about these things happening overnight. And it's actually really interesting to look at our world around us and look at ecosystems as they are today and try to think really hard about how something came to be, what it's useful for, why it might have been selected for. For example, are traits that occur after reproduction selected for? Well, probably not unless they affect the reproduction of the next cycle. For example, you might say, oh, well, the trait to be nurturing after your reproductive years, that's after reproductive years. No, but it helps your offspring reproduce. But we already see a lot of diseases, especially once we get beyond our reproductive and our child-rearing years. So once we get into our fifties and sixties, the incidences of diseases increases exponentially from when we're younger and because they're no longer being selected for, because it no longer affects our ability to reproduce, because we've already reproduced. We've already raised our children so that they could reproduce. So the only thing that happens at that point is now not being selected for. So anyway, hopefully, this video will give you a little bit more nuance on evolution, and I want to do a couple of videos like this, because I really want to make it clear that it's not making some wild claim that all of a sudden this appears spontaneously, that it really is a thing that happens over millennia and eons and very gradually.