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Evolution and natural selection | Biomolecules | MCAT | Khan Academy


So today we’re going to
talk about a topic that’s very central to the
idea of evolution, and that’s natural selection. But before we get into that,
I want to talk about what evolution isn’t. So evolution isn’t when some
organism like this monkey magically transforms
into a human. And it’s also not
when an organism changes in some way when it’s in
trouble, like a person growing wings after jumping
off a building. And I want to clarify
that this is not what we’re referring to
when we think of evolution. And evolution is a process
that occurs to populations of an organism, not
individual members. And it occurs over
huge amounts of time, and we’re talking
millions and millions of years for even small changes. So natural selection
is one of the forces that ultimately
drives evolution, but what is natural
selection exactly? Well, why don’t we jump right
in and look at an example? Let’s say it’s 10,000 years ago
and people survived by hunting and gathering,
but they also have to worry about being chased
around by wild animals. So in order to
survive, these people need to be able to find food. But they also need to be able
to escape from predators. Well, let’s say that one
of the people of these two has a special
genetic trait and has slightly longer legs
than the other guy. Now, these longer legs
put him at an advantage, because his legs are
longer and he can run, let’s say two times as
fast as everyone else. And because of this, he’s
more likely to survive when a predator like this
bear chases him down. So what this also means is
that the guy with the long legs is more likely to reach an
age where he’s old enough to find a mate, reproduce, and
have children who would also have this special
trait of longer legs because it’s genetic. And because he’s more likely to
have kids than everyone else, over long periods of time soon
more and more of the population will have this special trait. Now, let’s look at this idea
again but a little more deeply. And let’s say there are
six people in the world and two of them have longer
legs than everyone else, And let’s say that the
ones with the longer legs have a 50% chance of
surviving and reproducing while the shorter-legged
people have only a 25% chance of
surviving and reproducing. So that means one of our
two long-legged people and one of our four
short-legged people here will reach an age
where they can reproduce, so now these people who survived
will each have four children. And naturally, these children
will resemble their parents. And the children of
long-legged people will also have long
legs, and the children of short-legged people
will have short legs. So now in our next
generation, we have four people with long
legs and four with short legs. And you can already see
that more of the population has long legs than
when we started. But let’s take it another
generation further. So half of our
long-legged people will reproduce, whereas only
1/4 of our short-legged people will reproduce. And this means that by
our third generation, we’ll have eight
long-legged kids and only four short-legged ones. Now, if we number
our generations, generations one,
two, and three, we can see that in generation
one, 33% of the population was long legged. In generation two, 50% of the
population was long legged. And by generation three, 67% of
our population was long legged. And this is all because that
special trait of having longer legs made those people more
likely to survive and reproduce than those with
short legs, and this is the crux of how
natural selection works. So why is it called natural
selection in the first place? Well, let’s say with our example
of the short- and long-legged people. Now, we use the word
selection because one trait is advantageous
over another and is selected to be passed
on to future generations more than other traits. On the other hand,
selection can also apply to a
disadvantageous trait. If we have people who
have really short legs and run really slowly,
then those people will be selected
against and won’t pass on traits to
offspring as frequently. Now, we use the word
natural because there isn’t an individual who’s
physically selecting which traits are good and
which ones are bad. It all has to do with
whoever has the greatest probability of surviving. There’s no one actually
doing the selecting except nature itself. Now finally, I just
want to point out that natural selection
does not apply to acquired characteristics. If a father teaches
his son how to hunt and this makes a child
more likely to survive, that isn’t a trait that’s
selected for us since it’s not genetic and it’s not absolutely
passed on to children. So that’s why we say that
natural selection only applies to heritable traits,
with heritable traits being any genetic trait. So what did we learn? Well, first we learned about
the concept of natural selection and how traits that
help an organism survive are more likely to get
passed on to offspring. Next we learned that
evolution, which is driven by natural selection,
occurs to populations, not individuals, and occurs
over a huge period of time. And finally, we learned
that natural selection only applies to heritable
traits, ones that are genetic and passed down
from generation to generation.

29 thoughts on “Evolution and natural selection | Biomolecules | MCAT | Khan Academy

  1. So, how do you explain the short legged people in present days? And how natural selection works these days on short legged people?:)

  2. Because not all people that had short legs died. And natural selection does not work in today's society at all really, because very few people die because of some trait their body, handicapped would get help for doing stuff, like wheelchair for some; surviving isn't difficult, and thus humans won't really evolve. But one thing i think today's humans is evolving towards is a body that could handle eating alot, without need of working out. Although it will take millions of years.

  3. This is not "extinction!" Majority today are tall. Sure the trait survives, and there are defects like achondroplasia resulting in short stature. When you think of evolution and natural selection think "majority" then ask why is it so. There's always a reason. Even heritable diseases (ex sickle cell) have had their advantage and thus evolved (allowed to exist).

  4. Natural Selection is not the only mechanism by which evolution can occur. Why no mention of genetic drift and population dynamics?

  5. Fun graphics…  awful scientific method.   no evidence… no observable repeatable facts,  makes me shudder to think that someone thinks this may be science.

  6. Please bear with me, but this is something very new to me. I'm trying to understand it better – so maybe someone can help explain this questions that I have. 🙂

    So basically, in natural selection, it is nature itself which selects who, or what, gets to live for the survival of a species. This happens to a population, not on an individual level. I believe that this can happen in the animal kingdom, but does it always apply to mankind? For instance, the long-legged ones may choose to protect the short legged ones from getting eaten by bears, so to speak. Of course, this has to do with what sort of ethical code a society would hold on to.

    I guess I am thinking more in line with how mankind is capable of something higher than just surviving for themselves. Throughout history, different men and women have taken their stand to protect the weaker ones. Of course, the humanitarians, in the course of history, were few. Perhaps, within 10-20 years, only about 5% of society would rise up to protect the weaker ones. But IF 70% of mankind would do this, would the theory of natural selection no longer be considered valid in the human world?

    I really want to understand more. Thanks!

  7. This is a bad example. If your going to use people as a example, you should have use brain power. This example is historically incorrect if your referring to Humans. To put it in plain simple terms, This was a Stupid example.

  8. All wrong….God created everything….and if u don't believe you go to HELL!!!!!!! Every knee will bow and tongue confess that he is Lord……..

  9. Ross Firestone. You are so great my man. I have been trying to understand this concept for years now and now it just all made sense.

  10. Why is it that there is no smart Darwinist that can defend Neo-Darwinism? Why is it that they are all a bunch of drop out teenagers that know nothing, and have filthy mouths? Is there one smart Darwinist that could defend Neo-Darwinism, now that it is falling apart?

  11. Any scientific theory needs to be empirically validated.

    There's ZERO evidence validating any purported Darwinian mechanism.

    The theory is a wild speculation w/o any empirical basis.

    There's NO evidence that Natural Selection works w/ the exception of adaptation.

    Scroll to the 15:15 mark.

    https://www.youtube.com/watch?v=iGCQSMsNAOc

    "Natural Selection… how does it work? Is it a creative, positive force as Darwin insisted? Is it a neutral force as some evolutionists have postulated or is it simply a purifying force which gets rid of things that don't work and are misfits and it essentially has no creative role to play? Is the creative role of Natural Selection that was postulated the kind of Deux Ex Machina which really has no solid scientific support?"

    — James A. Shapiro
    http://shapiro.bsd.uchicago.edu/

    See "Evolution: A view from the 21st century," James A. Shapiro, Prof of Biochemistry and Molecular Biology, Univ. of Chicago, (2011), page 144, "Selection operates as a selective but not a creative force."

    This means that the selection/mutation mechanism is not creative.

  12. What if the bear chooses to have a longer leg for dinner? Size matters u know, we can't speak for the bears choice of diet 🐻🐼, and btw this is coming from a short legged person 👶

  13. Today, do people who have a longer torso in comparison to their body often mean they're more malnutritioned compared with someone else with the same height but with a longer leg ratio?

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