Hello. It’s Mr. Andersen and
welcome to Biology Essentials – video 1. This is on natural selection so I’ve included a
picture here of Charles Darwin. Most people think that Charles Darwin is famous because
he somehow invented evolution. That’s not totally accurate. Why Charles Darwin is famous
is that he’s the first scientist that really gave us the mechanism that explains how evolution,
especially adaptation can occur. And so if you are trying to learn biology the best place
to start is with Charles Darwin and a better understanding of natural selection. Before
we can talk about natural selection, however, we should define what evolution is. And so
in this class in evolution, evolution is simply going to be changes, biological evolution
is any changes to the gene pool. So a gene pool is a combination of all the genes in
a population or we call those different varieties alleles in a population. And so according
to scientists the gene pool should remain at equilibrium. In other words, the frequency
of the different alleles will never change unless you violate one of these five constraints
of Hardy-Weinberg Equilibrium. And it happens all the time, so evolution is constantly occurring
and it can occur if you have a small population, non-random mating, mutations, migration. So
these five things can cause evolution but we are going to talk about those later. Today
I want to talk about selection. Because selection is when you live or die, it’s called differential
reproduction success, when you live or die based on the appearance that you have. In
other words you’re made the way you are and as the environment changes you’re selected.
Either you have high fitness and you’re able to survive and pass your genes on or you have
low fitness and you die. And if you have enough fitness over time, that can eventually lead
to adaptions in a population. So the smallest unit that can actually evolve is a population
and it’s simply changes in the gene pool. Why do we start with natural selection? Well
of these five things here, it’s the only one that can lead to adaptation or organisms that
are better adapted to their environment. And so let me give you an example of that. Let’s
say you have a bacterial infection. Let’s say you are infected by a number of bacteria,
let’s call them seven and you decide to treat the bacteria. So let’s say you take some antibiotics,
penicillin for example. And they have different varieties of resistance to that antibiotic.
And so when you take the antibiotic on day one it destroys or lysis or pops all of these
bacteria, those that have low resistance. So these ones are selected, in other words
the ones that have a high resistance are selected for. The ones that have a low resistance are
actually going to die. These ones then will reproduce through binary fission and we’re
going to have a new population of organisms that have a better resistance to antibiotics.
Now there are two ways that we can actually get variety in a population. The only was
we can get new novel characteristics in other words like the ability to be resistant to
antibiotics is through mutation. Everything that’s been added to the first strand of DNA
in that first living thing has been added through a mistake or a change in the DNA and
that’s called a mutation. The other thing that can actually create variety is reproduction.
And so in bacteria they use asexual reproduction but they have mechanisms by which they can
change those mutations or vary those mutations or pass them on. In us its just sex and sex
is going to take those mutations and then in our offspring create a variety of different
types. And so this is just theoretically how natural selection occurs. But let me give
you maybe the most famous example of how natural selection occurs in the wild. And right here
is a picture of a peppered moth. A peppered moth has two different varieties, it has the
dark phenotype or physical characteristic and the light phenotype. Now we know this
about moths, that the light phenotype is actually homozygous recessive (dd). In other words
that is a recessive trait. And the dark appearance you can be either homozygous dominant (DD)
or you can be heterozygous dominant (Dd) for that and you’re going to have the dark appearance.
And so if you look at this wood right here where it appears today you can see that this
one, the light phenotype or the light physical characteristic is camouflaged. In other words
it fits in. If you are a bird flying over and you are looking for moths to eat you’re
not going to see that light appearance. You’re going to see that dark appearance and you
are going to go eat that moth. And so you are going to select that dark appearance.
You are going to kill that dark appearance and its genes are going to die with it. And
so at this point the light moth is going to have a higher fitness. It’s more likely to
survive, reproduce and pass its genes on generation after generation. So natural selection has
created this appearance. Now why would we even have the dark moth. Well maybe they can
fit in on some of these dark splotches or maybe they can help them and that’s actually
what happened. And so in the Industrial Revolution in the 1800s coal powered plants started to
push coal dust out into the environment and so the environment started to get darker.
In other words as that coal dust started to accumulate on the trees the trees got darker
and darker and darker. When they got darker these moths that were light in appearance,
now they started to pop out and those are the ones that were going to be preyed on by
the birds themselves. And so what happened was a change in the gene pool and natural
selection or evolution occurred. So let’s look at some actual numbers. In 1848, 98%
of the individuals were light in appearance and only 2% were the dark. And so we an actually
figure out what the gene pool frequencies were. And so over here I have 100 genes and
all of them are light right now. So let’s figure out how many of them were light. And
so right here to understand this you have to have an understanding of Hardy-Weinberg
equilibrium. And so here’s our famous equation. If you don’t know this you should probably
look at the video on Hardy-Weinberg Equilibrium. P squared stands for the individuals that
are homozygous dominant. 2pq stands for the individuals that are heterozygous and q squared
tells us what individuals are, the frequency of the individuals that are homozygous recessive.
And so we can actually use this whenever we are doing these problems. We usually start
with the recessive and we can figure out the allele frequency. And so 98% of the individuals
we know that they are little d little d. That they’re light in appearance. And so I can
set q squared equal to 0.98. I can take the square root of both of those, and I’ve done
this earlier so I know that q is roughly .99. In other words q tells us the allele frequency.
And so that means that back in the day, in 1848, 99 out of 100 were of the light allele
frequency and only 1 out 100 were of the dark frequency. And so that’s a simple Hardy Weinberg
problem. Now let’s see what happens over the next fifty years. Well if we check back in
the next 50 years we find that the population has changed quite a bit or it has evolved
quite a bit. And so it’s almost reverse. So now we have 5% of those that are the light
appearance because they are being prey on birds. And 95% of those are on the dark appearance.
Let’s see what happened to the actual allele frequency. And so we know that q squared at
this point is not 0.98 it is .05. And so if I take the square root of that, q now equals
roughly 0.22. In other words the gene pool has changed. The gene pool has changed dramatically.
We used to have 99% or 0.99 as our q value but now it is only 0.22. And so we still have
a lot of those dark around. We still have a lot of those light around but it’s changed
over time. And the reason it’s changed over time is due to selection. In other words the
environment changed and when the environment changed then they adapted, or the population
adapted. What can that lead to? Well it eventually can lead to something called an adaptation.
So an adaptation, if we talk about an adaptation, is the best example or the best word for that
is a process. We tend to think lots of times that an adaptation has to be a physical characteristic,
but it could be your behavior that you have. And so what is an adaptation? It is a process
that is selected for. In other words let me give you an example of that. In the Rift Valley,
it is a great place to study evolution right now, in the Rift Valley we have these lakes
and the lakes are inhabited by a type of fish called a cichlid. What’s unique about a cichlid
is that they have this jaw out here that we can see. It’s not quite that big, but the
have another jaw back here. It’s called a pharyngeal jaw. And what that allows them
to do is to use that for different processes for feeding, for feeding on different foods
and by doing that they are able to, they are able to exploit a number of different niches
in that environment. And so what we have seen is an explosion of cichlid populations in
these lakes in Africa. It started with just probablyone or a few different types of cichlids
and they have adapted to fill all of these different roles. And so they’re perfectly
adapted for that environment. How did they become perfectly adapted? It’s just through
a process of natural selection. As that environment starts to change, and they are starting to
change, as we get invasive species or fish that weren’t there to begin with, then we’re
going to have pressure, change in the environment and those species are going to have to adapt.
And so this is how we get species. It’s just evolution taken to its extreme where we get
macro evolution or big changes so species can’t interbreed anymore. And so that’s a
lot, but we’re just getting started. And so what’s natural selection? Natural selection,
if I could give you one example, the best definition for what natural selection is,
natural selection is simply differential reproductive success. In other words, we’re each made a
little bit different. If that allows us to survive and pass those genes on, we have high
fitness, that eventually can lead to adaptations. But again there’s no goal towards this perfection.
It’s simply random changes that are selected by the environment. And that’s why Charles
Darwin is famous. So thanks.