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AP®︎/College Biology
Course: AP®︎/College Biology > Unit 7
Lesson 1: Natural selectionIntroduction 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.
Want to join the conversation?
- Just curious, are viruses living?(564 votes)
- 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).(831 votes)
- 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?(104 votes)
- 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! :)(213 votes)
- Why aren't there animals that are in the middle of different stages of development?(46 votes)
- 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.(152 votes)
- What causes these "random mutations"? Has this been answered or it is still a mystery of the universe?(74 votes)
- 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.(69 votes)
- RNA is Ribonucleic-acid. It is a polymeric molecule made up of one or more nucleotides. A strand of RNA can be thought of as a chain with a nucleotide at each chain link. It has several comparisons with the DNA, with a couple of variations.(my favorite subject=))(7 votes)
- 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.(38 votes)
- 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.(29 votes)
- 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)- 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.(24 votes)
- 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)- 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)
- Can you give us some mammal or plant examples of natural selection?(1 vote)
- 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.(11 votes)
- why do we fall ill?(4 votes)
- 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.