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Natural selection and the owl butterfly

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How the "Owl Butterfly" may have gotten the spot on its wings. Created by Sal Khan.

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  • blobby green style avatar for user Sinéad Flynn
    So speaking of natural selection- If there was a intelligent couple who had two P.H.Ds and were really smart, they decided to have eight children because they thought they wanted to spread their intellectual genes. And then there's another orindary working class couple who only has one child, their average intellect passes to the child.
    Would smart people having tons of children affect future generations majority of intellect in the future?
    (262 votes)
    Default Khan Academy avatar avatar for user
    • There is still a big debate around whether DNA plays a role in intelligence, or at least how big a role. Some say nutrition of the mother during pregnancy is more important, others say upbringing and education play the biggest role. Studies of IQs of identical twins shows that they have similar IQs, which could suggest that genetics is the basis, however the counter-argument to that is that it was the similarity in nutrition that they received in the womb. But if we look at other hominids eg. a chimpanzee - even if the pregnant mother is given the correct nutrition, and the baby is raised in a stimulating but stable environment, it will never become a rocket scientist - because its genetic make-up will not allow it to progress that far. In the end I think it is best for 'P.H.Ds to breed more' - even if genetics doesn't play a role - they should still be able to provide the right environment and nutrition for the children to 'grow up smart'.
      (303 votes)
  • blobby green style avatar for user jsmithson60
    We talk about natural selection being a response to a changing environment to increase survival chances. What about as a way of changing the environment to increase survival chances? Perhaps the predators of this moth are broken into two groups; those that key into the mimicking variation, identifying the moth as prey and those that do not. Eventually, those hunting the variant will come across the owl, lizard or frog which the moth is mimicking and become prey itself, decreasing its survival.
    (23 votes)
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    • blobby green style avatar for user Chris.B.Monette
      I wouldn't surmise that cheetahs and gazelles run much faster then they did 200 years ago. Nature is a balancing act, while cheetahs force gazelles to run faster to flee, and gazelles force cheetahs to run faster to catch, the slowest gazelle is most likely to get caught. So the genes of faster animals get passed on, not necessarily "making" them faster, but only leaving the faster ones alive to reproduce. Alike with cheetahs, if you are slow you wont catch prey which may lead to death. So slower incapable animals die out. My main point is that the view needs not to be looked at something being "made" to happen. Rather the harshness of nature was too much for those incapable to deal with the struggle of life, leaving only those who are capable to pass on genetic information. So it's not so much the ability of the strong, rather the lack of ability of the weak.
      (44 votes)
  • aqualine ultimate style avatar for user Daniel
    Could a trait like the 'eye' on the wing of the butterfly become a dominant trait if it was previously a recessive trait? This may sound like a stupid question but I am curious.
    (2 votes)
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    • duskpin ultimate style avatar for user Bryan Eskew
      Not a stupid question at all.

      A recessive trait is usually due to a "defective" gene. For example, a flower may have a trait where purple petals are dominant. A trait for white petals would be recessive because one of the genes that make the purple pigment doesn't work.

      For a gene to be mutated so that it is defective, and therefore recessive, become mutated again and become dominant would be a rare event. Maybe not impossible, but I cannot think of an example.
      (5 votes)
  • female robot grace style avatar for user Pinkcatcrystal
    So, does this mean that blond hair and blue/green/gray (any color besides brown) eyes are all going to become extinct eventually?
    (2 votes)
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    • leafers tree style avatar for user dancinglizard157
      The alleles that code for such variations could very well go 'extinct' so to speak. While I am unaware of any positives or negatives associated with eye color or hair color (except for red hair, there are apparently some issues that are more likely in red-heads). If a trait has little or no influence in selective fitness then it can still become extinct through at least three ways I can think of:
      Genetic drift - randomness
      An advantageous allele arises and becomes 'fixed'
      Some huge crazy thing happens and all of the population dies
      (I say population because it's populations that evolve, not necessarily the entire species)
      Just because it could happen though, doesn't mean it will happen.
      (4 votes)
  • female robot grace style avatar for user Tessa McCoy
    At or 31, he starts talking about AGCTAAACT, what does he mean by that, I watched all of the other ones, and still don't get it, what is it?
    (3 votes)
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  • leafers sapling style avatar for user Amanda Burnett
    So its survival of the fittest, which I used to think meant that that one was stronger therefore they survived more, but it really goes deeper, the one that gets the right to mate is the fittest.
    (1 vote)
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    • piceratops ultimate style avatar for user Just Keith
      "Survival of the fittest" is actually not a good term. It should be "optimally reproductively successful" or some such phrase.

      Think of it this way, which will be more successful: a group of rabbits that can run just barely fast enough to escape their fastest predator OR a group of rabbits that can run 5 times faster than their fastest predator? If you think of how people usually think of "survival of the fittest" then you might think the group that could run 5 times faster would be the favored group. You would be wrong. It takes more food and more energy to run extra fast. Also, is there no advantage to running excessively faster than the fastest predator because as long as it is just fast enough, the rabbit lives either way. But, the group that is too fast needs more food and thus won't have as much to go around and thus cannot support as many individuals. So, the faster group of rabbits would have significantly fewer offspring. Thus, the slower (but still just fast enough) rabbits would be the most reproductively successful.

      So, natural selection favors those animals that are optimally reproductively successful, including make the better use of food by have traits that are good enough but not excessive.
      (5 votes)
  • ohnoes default style avatar for user gotdripinc
    since they have such a short life span and they reproduce so fast shouldn't evolution be easier? I mean they are one of the oldest creatures so that would mean that they adapt and evolve faster right?
    (3 votes)
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  • piceratops ultimate style avatar for user nkillarney
    How is camouflage helpful for predators with good smell, like wild dogs?
    (2 votes)
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    • piceratops ultimate style avatar for user Just Keith
      Camouflage doesn't always work. It doesn't have to. It just needs to work often enough that something else gets eaten more often than the camouflaged species. After all a camouflaged rat might be smelled just as easily as a differently colored rat, but if the differently colored rat gets eaten more often, then the camouflage has worked.

      Besides, many animals engage in scent camouflage as well. For example, some prey species have been known to rub themselves with the dung of a predator of their predator, so that their predator will avoid them.
      (3 votes)
  • marcimus red style avatar for user xozara
    I still don't really get what Natural Selection is.....can someon eplease explain it in blackandwhite for me? thanks a ton :)
    (2 votes)
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    • piceratops ultimate style avatar for user Just Keith
      In a nutshell Natural Selection is this:
      Individual members of a species that do not succeed at reproducing do not have their genes enter into the next generation of that species. Thus, whatever traits happen to improve the chance of being reproductively successful for a given species in a given environment, will be traits that enter into the next generation and will eventually become more common in the species (over the course of generations). Whatever traits harm the chances of being reproductively successful will not be common and will become less common as the generations go by because not many individuals with those traits manage to reproduce.

      Natural selection is directly connected by the environment (such things as the weather, diseases present, predators, availability of food, availability of potential mates, etc.).
      (2 votes)
  • orange juice squid orange style avatar for user Likhon
    Sal said that if intellegent couples give birth to more children, our future generation will be more intelligent than ours. My question is then why do we find some people attractive and some people people not that attractive? I can't find any direct connection between attractiveness and intelligence.(I am considering normal cases only) But it is obvious that in future the intelligent people will have better chance of surviving. Maybe my real question is what is the base of or what are the bases of attractiveness in human species?
    (3 votes)
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    • male robot hal style avatar for user Michael Zero
      Attractiveness can come in many forms. If you're like me, you like small girls with red hair. Maybe I like them because in my environment, loud hair color represents the ability to draw attention and still be successful, and that means she is good at something useful. I like them small because that means they will require less food to stay healthy. This is instinct influenced by DNA, not active thoughts of course, and is all hypothetical.
      In another environment, perhaps someone is attracted to hairy women (I am not judging). In that environment, perhaps it is cold, and thus the woman would be successful with fewer crutches like clothing, making her less of a drain on the tribe's resources.

      We tend to have a pretty universal understanding that big, strong men are attractive because they can protect the female. Small women are attractive because it means more food for us men. But we must still recognize that this is due to the environment to which we are adapted.
      (1 vote)

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.