hey biology students this is our second
video in our theory of evolution lecture
series and the title of this video is
mechanisms of evolution I chose this
cartoon for our opening slide because in
our last video we were talking a lot
about the evolution of the giraffe and
how there were some false ideas about
how that worked and I thought this was
cute so the little monkeys up in the
tree and he says so tell me why did you
spend all those years of all Ving a
long-necked to get at these leaves when
all you really needed to do was climb
the tree pretty cute so in the last
lecture video we spent some time talking
about two mechanisms of biological
evolution recall how do we define
biological evolution we define it as it
has changed that occurs in the inherited
characteristics of a population over
time so that's the definition that were
using for evolution and we learned about
in the last lecture video Charles Darwin
and his famous book on the origin
species and how he developed the theory
of evolution by proposing a mechanism
for how populations can evolve how we
see these changes accumulate in
populations and his mechanism is called
natural selection and recall that in his
book he actually begins by helping
people understand that really we have
been doing something very similar to
natural selection in an artificial
mechanism called artificial selection
and what that means is that the breeder
or the human you know us we're breeding
animals we're breeding plants for a
purpose we are actually selecting for a
particularly desirable trait for example
we talked about breeds of dogs so here
we have four different variations of
dogs in terms of their coat color and
appearance
and so what happens is the breeder
decides one of these of this very of
these different variants of dogs which
of these do I find the most desirable
that I would want to breed future dogs
to have that particular trait well let's
say that as the breeder we decide that
we would like to have more dogs with
spots where the background of the coat
color is white and then they have either
brown or black spots so what do you do
well of course you have to find a mate
for this dog so we say there's a genetic
cross for their offspring if this is a
genetically controlled trait of coat
color and pattern of the coat is
genetically controlled by the DNA then
of course their puppies should be
spotted and here we see some puppies
that have black spots and some have
brown spots so maybe you would further
select with I want more brown spotted
puppies so remember we talked about
there might be inbreeding so you might
have these siblings mated and then there
are babies maybe you would have brown
spots and over many generations we would
actually of all this population of dogs
with a particular trait in mind the
second mechanism of evolution that we
discussed in the last video was natural
selection so this is what Charles Darwin
identified and he identified that the
environment could select for a desirable
trait there could be sweaty called
selective pressures or selecting agents
in the environment and these could be
things like predators or shortage of
food or disease that could select for
particular traits to be passed on within
populations so remember in the last
video we used the bunny simulation and I
showed you where we had white bunnies
and we had brown bunnies and they were
pretty much equal numbers in the
population until we had
selecting pressure this being a predator
in the form of a wolf or a fox and so
the wolf would be the selecting pressure
and what happens is that wolf is is
actually selecting for the coloration
and the bunnies because the brown
bunnies are better camouflaged compared
to the white bunnies so the Wolves eat
the white bunnies and we have more brown
bunnies reproducing and over time what
happens
well perhaps even that white bunny
coloration pattern goes extinct and
instead we see brown bunnies in the
population and we would say this
population has evolved as a result of
the selecting pressure of predation so
in this video we are describing five
mechanisms for biological evolution so
there's more explanations for why we see
changes in the characteristics of
populations over time artificial
selection and natural selection are only
two potential mechanisms for how
populations can evolve
so Charles Darwin when he he coined this
term natural selection which applies in
many many scenarios in nature however
natural selection to him didn't make
sense in the context of the peacock so
in this picture you're seeing the male
peacock which the male peacock has this
brilliant blue and green coloration
pattern with these huge feathers we call
it the the train and so it has this huge
train of feathers and the female we
actually call the female the peahen is
sort of this drab color sort of grey or
brownish colors and it's not the
brilliant beautiful coloration pattern
of the male the male peacock and the
peacock Charles Darwin writes about the
 peacock in his writings and he
was actually pretty annoyed by the
peacock because the evolution of the
peacock didn't make sense in the context
of natural selection to him this would
be a huge disadvantage in terms of your
survival to have this very heavy train
of feathers that you would have to carry
around as a male peacock and that would
he said would you know that would be
advertising to predators to come in and
eat me because I wouldn't be able to fly
the peacock leo would be carrying all
these heavy feathers and they would not
only be brightly colored and stand out
in the environment but if they were
chased by a predator they probably would
be likely to be eaten compared to you
know a lighter weight bird so this
confused Charles Darwin until he started
to look around in nature and he noticed
a pattern
and the pattern was that in nature
especially in certain in certain animals
compared to others this really stands
out and what he noticed was that the
males in the species tended to be more
brightly colored than the females and so
this is really true in birds and this
helped him develop a third mechanism for
how evolution can work and he he called
this sexual selection and what this
implies is that the females in the
species can actually choose which males
they want to mate with and they often
choose the males that are more brightly
colored so look around in birds in
particular and you'll see this we call
sexual dimorphism medical word sexual
dimorphism
what that means is that the females in
the male's look really different where
the the males are usually brightly
colored compared to the females which
are usually more camouflaged for their
environment this is also true in insect
population so we see in these in these
uh butterflies we can see that on the
Left we actually would be able to
predict this this would be the female
because she is brown colored and more
likely to blend in where the males are
always going to be the brighter color
and this is because the females are more
attracted or more likely to mate with
the males if they're brightly colored
one times the males also have some sort
of characteristic song that they make
when they're trying to attract a mate or
even little mating rituals or little
mating dances that they do to attract a
mate and this is really interesting in
nature because when it comes to the
biology of reproduction remember males
make a lot more gametes there's a lot
more sperm that's made by the males and
a lot fewer eggs that are made by
females so the females are more choosy
in their mates because they have fewer
fewer eggs and they want to ensure the
survival of their offspring so here's
the other interesting point about this
is that when scientists in particular
have have looked at peacock populations
and they look at peacock populations and
they notice that the males with the
longer feathers the the larger the train
of feathers the more likely they are to
attract a female peahen and to mate but
what they find in these studies is that
actually paternity matters in terms of
the survival
of the offspring is that the males
with the larger trains with the bigger
feathers they actually produce healthier
offspring isn't that interesting
the studies support that the females by
choosing the males with a particular
trait so the larger the longer feathers
that is actually advertising biological
fitness health and the possibility of
your offspring inheriting that trait and
surviving better than they would if the
female mated with a peacock with with
smaller feathers so there actually is a
reason behind the females choosing the
males with particular coloration
patterns but it becomes a very important
selecting pressure that the females are
going to be the selecting pressure and
choosing a particular trait in the males
that is going to ensure the success of
their future offspring okay let's now
look at the fourth mechanism for
biological evolution and it's called
genetic drift so we've looked at three
mechanisms of selection where there is a
desirable trait that is selected now
here's genetic drift genetic drift is
not selection okay it is based on random
events and random chance and this is a
little bit less common okay to see than
some of the other mechanisms of
evolution but it can certainly affect
the inherited characteristics and within
populations and cause changes to those
populations it just does it in a
different way than those three
mechanisms of selection instead usually
acting on small populations compared to
large
okay that's where you're more likely to
see a genetic drift event occur it's due
to random random events and really
there's two types of genetic drift that
we described and so the two types of
genetic drift that we describe we call
the founder effect and the bottleneck
effect okay so now let's describe what
those two types of genetic drift involve
okay so founder effect this is defined
as a small group of individuals that
leave an existing population and
establish a new population in a new
geographical location so they break away
from an original population a small
group break away from the original
population and they are now the founders
of a new population and we see
divergence in the characteristics within
those two populations now from the
original compared to the new population
how about an example let's go back to
the Galapagos where Charles Darwin spent
most of his time remembering these are a
collection of islands off the coast of
Ecuador in South America there are a
number of very unique species in the
Galapagos and this is actually true of a
lot of we see the founder affect a lot
in island populations where the
organisms the plants and the animals
that reside on the islands resemble the
plants and the animals from the mainland
but with variations because there was
this founder effect that the islands get
get colonized by small groups of plants
and animals that either fly there from
the mainland swim from the mainland or
blown by the wind from the mainland and
it's random chance who ends up being the
individuals that land on these island
populations and that
that they have that gives rights to new
groups of organisms for example the
Penguins on the Galapagos so there are
penguins that are called banded they're
called banded penguins that reside off
the coast of South America and also in
Africa as well they're called banded
penguins and so they reside off the
coast of South America and that would be
the mainland penguins penguins but the
Galapagos actually has a unique species
of Galapagos they're called Galapagos
penguins and this this species of
penguins is actually the only penguins
that can survive north of the Equator so
all other penguins either reside you
know other banded penguins can reside
off the coast of South America and
likely that's where the Galapagos
penguins evolved from they evolved from
a founding population of banded penguins
that likely swam to the Galapagos and
diverged from the population abandoned
penguins off the coast of South America
and in fact they have genetically
diverged they're a genetically unique
group they're an entirely different
species of banded penguins and they
actually have their own adaptations to
survive in warmer water they have
different mating calls and you know
there's definitely differences that has
led to a divergence in what we call
speciation and the explanation is likely
the founder effect but again random
chance it was random which group of
penguins actually migrated to the
Galapagos and a
established a brand-new population of
penguins that are the only known
penguins that live in the northern the
northern hemisphere here's another
example of random events that we call
genetic drift it's called the bottleneck
effect this is the second type of
genetic drift here we see a decrease
that occurs in the size of a population
due to a random event and usually this
is a naturally occurring event so like a
flood or a tsunami or a forest fire
something along those lines
and so the imagery here of this bottle
is showing us that we have this original
population with all these marbles green
and orange and red marbles in the
population representing different
alleles in that population and there's
some sort of bottlenecking effect where
we get a sudden decrease in the size
randomly randomly eliminating the red
marble from the population so what's
gonna happen is that only those with the
green and the orange marbles survive
this bottlenecking event by random
chance
not because of selection not because
they're better adapted in the
environment so this is not selection
this is simply random chance okay by
random chance the red marble is not it's
not select or sorry it's not surviving
in this population and is eliminated
right so in future generations we would
just see a green marble and the orange
marble we would only see those
particular genetic traits and this is
not due to the Fitness what we call the
biological fitness of the organism this
is simply due to random chance okay so
an example this is sort of an
interesting one so there's a collection
of islands off the coast of Indonesia
and the federal States of Micronesia and
this one particular island is called
Pingelap it's hard to even read let's
that map but this little island called
Pingelap so it's an Indonesian island
Pingelap it's an island and so what
happened was in the 1700s there was a
typhoon a very very severe storm on the
it around the island of Pigelap and
what happened was the population
decreased to around 20 people can you
imagine so only 20 people so that's what
we would call a bottleneck where
suddenly there is a decrease in the
population size as a result of random
chance so what happened was the people
who survived the typhoon or at least at
least one member of that population we
know of he had a rare mutation it was a
complete color blindness so color
blindness can be variable some people or
can't can't just see red or green
complete color blindness where
everything you see would be in grayscale
so if you saw this macaw bird you
wouldn't see any color at all it would
just you would be seeing in black and
white okay
that is extremely rare actually in the
population for that to occur however in
the island of Pingelap because somebody
in this in this bottleneck event
by random chance happened to have the
gene for complete color blindness and
likely there was probably some
inbreeding within these people because
all of the current descendants the
people who currently live on Island a
Pingelap descended from from this
bottleneck effect from the this small
group of people that were left after the
typhoon and one of those people had this
complete colorblindness allele and
so now on the island of Pingelap about
10% of the population has complete
colorblindness where all they see is in
greyscale which is again an extremely
unusual mutation so it's a good example
of a genetic bottleneck event which
randomly reduce the number of people in
the population by random chance somebody
had this complete colorblindness allele
and then pass that on to future
generations where we see this again
change in the characteristics of that
population we see a biological evolution
event occurring here in this population
of Pingelap but it's not due to
selection
okay so that brings us now to our fifth
mechanism of biological evolution fifth
explanation for why we see changes in
inherited characteristics in populations
over time so this fifth mechanism is
called gene flow and gene flow refers to
migration so migration of individuals
into or out of population but very
important to understand is that it
results in crossbreeding or breeding
between new populations so people are my
you know individuals doesn't have to be
people but you know individual organisms
are migrating into a new location where
there is already an established group of
individuals and then they breed with
those individuals and they introduce
their new alleles into that established
population by migrating in and then that
can change what we call we call the gene
pool or this collection of alleles that
exists in the population that can change
the gene pool of that population and
over many generations we can see
biological evolution occur we can see
changes in the inherited characteristics
of those those populations that have
experienced gene flow most students have
trouble understanding the difference
between genetic drift and gene flow
those terms sound very similar but gene
flow the way I think about it is that
gene flow is more in a ways more
purposeful so if you are sort of
purposefully migrating into a new area
so you're flowing in or flowing out okay
where genetic drift you think about
something drifting like driftwood down a
river that has sort of a random sound to
it like okay it's just drifting and who
knows where it's going kind of thing so
that's the emphasis is that genetic
drift is more due to random events and
that gene flow is more about
in a way as somewhat the term at least
sounds more purposeful and has to do
with with breeding between populations
new populations so a good example of
that is actually to take a little page
in this in the history of humanity if
you would with me
there is evidence of gene flow between
us we are modern humans and you may know
this but our species name is sapiens so
we are we are Homo sapiens and this is
modern humans but if you know anything
about human history you may know that we
are not the only species of humans that
has ever existed on that book on this
planet there's plenty of fossil evidence
that supports that and now there's even
DNA evidence and what they can do is
they can take DNA from fossils of a
close cousin of us in the human family
the Homo neanderthalensis that's their
scientific name but you may know them as
the Neanderthals pictured here on the
screen is artists or representation of
what the Neanderthals might have looked
like based on the fossil evidence that
we have the reconstruction of their
facial features that they have actually
larger brained
than modern humans but shorter in
stature a really broad eyebrows and that
sort of thing what we can see from this
map is that there were waves of human
migration hundreds of thousands of years
ago and we're seeing three different
groups represented in this map we're
seeing the yellow being another species
of of historical humans called Homo
erectus then we're seeing the Homo
neanderthalensis we're seeing the
Neanderthals in sort of this darker
yellow and they they migrated out of
africa so it's
understood that modern modern humans
based on fossil evidence originated in
Africa and we're all African by the way
in our history and then we get these
waves of migration Out of Africa and so
there was an early wave of migration of
the Neanderthals in the into Europe and
the first fossils of them were actually
found in the Neander Valley in Germany
which is where they get their name the
Neanderthals and then there were waves
of Homo sapiens so modern humans Homo
sapiens evolved in Africa and migrated
that's the red lines that you see we're
seeing the red lines the migration so up
to even just about 40,000 years ago
Neanderthals and modern humans coexisted
right now obviously we're the only
species of humans on the planet but that
has not been always the case so we bred
interbred with Neanderthals and there's
evidence of that I thought you might
want to take a look at what scientists
are learning about that so I want to
show you it just a very short clip I
found from YouTube of some scientists
that are trying to understand what what
the results of this gene flow between
the under thousand and Homo sapiens
might indicate let's take a look
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
so those are the five
mechanisms of biological evolution of
how populations can change in their
inherited characteristics over time and
I hope that was helpful
