Hello everybody. Today we're going to discuss a little bit about evolution, a
little bit about Charles Darwin and some of his ideas and the proof that
we have to show that evolution is in fact happening. So, simply put, evolution
is gradual change or diversification of a population over several generations.
A very important thing to remember is that INDIVIDUALS don't evolve. POPULATIONS or
groups of individuals evolve over time but the individuals themselves don't
change their genetic makeup. There are four major mechanisms of evolution.
We call these mechanisms of change and they are mutations, migrations, genetic
drift and natural selection. We're going to go through all of these and
understand how it is that each of these different things could change the number
of genetic variations in a population. Let's imagine that we're looking at this
moth right here, and normally it has very light-colored wings. Its wings are
sort of an off-white color, but let's imagine that there is a mutation in its
DNA that just arises by mistake. So it might be a frameshift mutation or a
point mutation. Let's imagine that this mutation caused the butterfly's wings to
change color so now the butterfly has different colored wings than it
originally had now it has very dark colored wings so now just by random
chance we have a different number of dark mobs and we had originally
originally we didn't have any and now we have a couple okay so now let's talk
about migration so for this example we're going to imagine that over here on
the left so right here we have an island that has a population of the
light-colored moths and it's separated from the mainland by a really large body
of water so normally there aren't any other moths live on this island but
let's imagine that we have a really big hurricane that comes through and it
blows a bunch of these mobs over here over onto the island and so now you have
changed the frequency of dark moths due to migration or species moving from a
different place
so now even though no genetic mutation had occurred to cause the white moth to
become a dark moth we had new ones that came in for whatever reason from a
different place so that changed the frequency of the dark moths versus the
white moths so for genetic jerk I think I'm gonna need a little bit more room
here because we're going to start off with a population that has equal numbers
of dark and light blogs so one of the unfortunate things that then happens to
this population of mods is that somebody accidentally steps on two of these mobs
right here and so neither of them survives long enough to reproduce there
is one white moth who survives long enough to reproduce and it reduces to
offspring so like this and then each of these two here assuming these three dark
mobs each produce two offspring and so now we have two white mobs and six dark
mods and then if all of those moths survive to reproduce in only two
generations we've now created a population that has three times more
dark mods than we have light-colored mods and so just this random chance
could suddenly cause what we call genetic drift in the population now
we're going to come to the fourth and final way that evolution happens this is
probably the best known which is called natural selection and now we're going to
see why I originally chose to have light-colored and dark colored moths is
this example because we're going to go through one of Charles Darwin's most
famous observations so Darwin was famous for observing a specific species in
London called the peppered moth or the pepper moth and originally it looked
kind of like this it was sort of greyish white in color and one of the things
that was great about having this moth be exactly this color is that it blended in
with a certain kind of tree bark that was very common in London at the time so
here is the pepper moth against the bark of a tree and I had to make it big so
you could actually see it but the pepper moth is right here and then this its
head right here but you can see that it's extremely difficult to see the
pepper moth and unless you knew it was there be very difficult to see it
especially from far away every once in a while there'd be a mutation and a dark
colored pepper moth would be produced just by rain
chance so you can do this pepper moth would stand out pretty significantly
against the light colored bark of the trees and so it probably wouldn't
survive long enough to reproduce okay so I've made myself a little table here and
it's important to put this into context in terms of time and place so in the
year 18 11 it's really really uncommon to see these dark colored pepper moths
because they stand out so vividly against the light colored bark of the
trees that they would almost certainly be eaten by predators and so maybe a few
would survive long enough to reproduce but they would have to be extremely
lucky whereas with the light colored pepper moths they would blend in almost
perfectly against the bark of the trees and so almost all of them would survive
long enough to reproduce because they would eat so now we fast forward about
almost 40 years and we're going to take a new survey in the year 1840 eight so
take a minute and think about what sort of things were going on in this time gap
right here so between the early 1800s and the mid eighteen hundreds so if
anybody thought of some of the inventions that were invented during
that time gap we can think about things like the first steam-powered locomotive
but also the rise of machines and for the first time machines were employed to
make parts for other machines and at this time the only form of power known
was other water power but increasingly coal power so the Londoners were burning
lots of coal to keep their houses warm but now also using coal to do things
like run factories and run locomotives and coal produces a huge amount of soot
and a huge amount of smog and so during this time period between 1811 and 1848
the trees of London got pretty gross and pretty covered in soot and smog and so
now instead of having these light-colored tree bark we had trees
with really dark bark and really dark leaves so now the light-colored mods
would show up very clearly against the bark of the dark suit covered trees and
so it was very rare to have any of them survive long enough to reproduce whereas
suddenly it became extremely advantageous to be these darker colored
moths right here and over just a few generations of mods
between 1811 and 1848 suddenly there were almost no light colored pepper mods
anymore and now almost all of them were dark coloured this is an example of what
we call natural selection and the phrase natural selection wasn't actually used
until much later but this is the first recorded case where somebody observed
this happening now remember the mods themselves are not changing color a
light colored moth doesn't suddenly decide to become a dark colored moth
we're just seeing the number of dark colored moths increase relative to the
total population so earlier on we would only have seen a small percentage of the
mobs be dark colored whereas later on we would see a large percentage of the mods
being dark colored so let's go back to this idea of evolution for a minute
remember that this is gradual change in a population not change in individuals
and it's going to work by virtue of these four different mechanisms mutation
migration genetic drift and natural selection one of the final things that's
really important to remember about evolution is the idea of descent with
modification so it's all well and good to know what evolution is but it doesn't
help us a whole lot if we don't understand what it means for us so let's
look at this tree for a minute down here where this dot is is a common ancestor
for all of these individual organisms so fish and lampreys and reptiles and
mammals they all at one point did have a common ancestor however at one point you
had a small modification that occurred which caused the invention of the
vertebral column so when organisms started to have a backbone and then from
there we invented things like jaws and walking legs and having eggs and having
hair all of those were invented by mistake they weren't something that was
invented purposely however it allowed for the organisms to survive in a
different way and maybe to survive a little bit longer and ultimately those
organisms formed their own species however even though now Turtles cannot
mate with leopards and are totally different species some things are
actually really interesting if you look at the skeleton of these organisms if
you look up here we can see that we have what are called homologous structures so
you're probably pretty familiar with the structure of your
own arm your humerus your ulna your radius your car pulls your metacarpals
your phalanges but if you look at the bat the whale and the cat you can see
that they actually have very similar structures even something like a whale
that doesn't actually have fingers that it manipulates still has finger like
bones and Wales actually have wrist bones because whales and humans have a
common ancestor way way way way back if you look at something like a bat oops
you can see that the bat also has those same finger like structures even though
it doesn't use them in the same way it's a small modified structure that looks a
lot like human fingers and that's because if you look way back down here
over at this column right here you can see that we actually had a common
ancestor way back when so finally you can also see that we all start off
looking very similar so here's the lamprey over here which you also saw on
the chart earlier and then you also see things like chickens and cats and bats
and humans we all look very similar up until the middle of our development when
we start to see recognizable structures that's it for now make sure you bring
your questions to class