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.