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Course: AP®︎/College Biology > Unit 7
Lesson 1: Natural selectionNatural 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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- Because of the short life span of the butterfly, will the evolution process happen faster then human?(121 votes)
- 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.(92 votes)
- so is it possible that the medicines we're using now might become ineffective after some thousands or millions of years?(30 votes)
- 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.(49 votes)
- At, Sal lists AGCCTAAAGT .... I'm sorry, maybe I missed it... But WHAT in the world do these letters represent? 4:35(13 votes)
- 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(21 votes)
- AtSal 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. 2:27
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)- No single animal ever evolves. Species can evolve, but individuals can't. They are stuck with the genes they are born with.(13 votes)
- 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?(5 votes)
- 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.(14 votes)
- Why did he use wikipedia? Isnt wikipedia not trustworthy?(8 votes)
- Wikipedia is not an authoritative source but to get a general idea or to refresh your memory on something it is fine.(11 votes)
- so between aboutto like 12:45he 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? 13:20(7 votes)
- 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)
- what happens if there is a mutation and the spots on wings disappear?(5 votes)
- those with the mutation would eventually die out or they would increase dramatically depending on the environment(how well they can camouflage themselves to escape predators)(8 votes)
- 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)
- Yes, but it is also reasonable that within the next thousand years, we will have mastered DNA and will be able to change flaws and keep people from devolving.(7 votes)
- that is literally a orange slice of a wing(8 votes)
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.