Mechanism of Evolution Video Lecture - NEET

you
hi it's mr. mazurka Watson in this video
i'm going to be talking to you guys
about mechanisms of evolution or in
other ways how can we get evolution to
occur so I just wanted to start off by
kind of defining what we mean by
evolution most of the time when we talk
about evolution we're talking about a
type of evolution that is referred to as
macro evolution macro meaning big so
when you think about evolution you're
probably thinking about species giving
rise to new species and things changing
over the course of long scale time so
here's an example of how the dinosaurs
came to be even though they all share a
common ancestor over the course of time
that gave rise to a bunch of different
species or types of dinosaurs this is
macro because this takes a very long
time to occur it's not like all these
dinosaurs were born overnight what gives
rise to macro evolution are these small
scale changes in the gene frequency so
if you have a species small little
changes can add up over time to then
eventually give rise to new species so
these smaller scales us changes are
things that we refer to as micro
evolution micro meaning small so imagine
I have a species of insect here the
small changes that are happening in its
population over time are eventually
going to lead to a type of macro
evolution so we say that micro evolution
leads to macro evolution so we talked
about these mechanisms we're mostly
going to be focused on those small scale
changes in allele frequency in the
population so what really is evolution
in these terms well imagine we have that
insect let's say within that species of
that insect we have some blue and some
green and in this example are gonna have
equal amounts we have five of each type
while evolution will be occurring if
over the course of time let's say that a
certain one is thriving more than the
other so in this case blue seems to be
more prominent than the green ones over
a course of let's say a few years now if
you look at this you already know about
natural selection which I'm sure you do
you'd probably look at this and say sure
evolution is occurring because the blue
ones are better adapted to their
environment so those are the ones that
are surviving they're reproducing more
than the green what we're really going
to be focused on though is what other
things can make this happen
so when we define evolution though in
these terms what we're talking about is
that evolution is any change in the
frequency of alleles in a gene pool
anything that causes certain alleles to
become more frequent while other ones
become less frequent we're gonna say
that evolutionary change is happening
more small-scale so it's type of micro
but it is evolution nonetheless so again
we're gonna be focused on how can
evolutionary change or these changes in
allele frequency how can they occur
outside of natural selection you already
know about natural selection what are
the forces can cause this sort of change
to happen the first one we're gonna look
at is genetic trip so in my class what
we did was a lab using these different
colored beams we had black ones and
white ones and we use that to simulate
different types of moths like moths and
dark moths and what students did to
start was they took a population size of
150 of each and they randomly selected
out 50 lucky survivors the other 50
would die and they did this for a course
of let's say three different trials to
make three new generations and what most
groups saw is that the population didn't
really change change much in other words
it didn't evolve they still had after
three times of doing this just around
fifty fifty of white ones two black ones
that's because when you take a large
sample size a good probability is going
to be that it's gonna be half of each as
long as we start off equal like we did
but then when students did is then they
took a small sample size it said okay
instead of 50 surviving only five are
going to survive this time and this is
where they started to see evolutionary
change happen what you should have seen
is that if you choose five it is more
likely that you can choose out five of
one color or five of the other or maybe
three and two but either way it is more
likely that we can get that change in
allele frequency to happen so this is
called genetic drift because this change
that you saw is not due to excuse me not
due to natural selection not due to
adaptations but rather just due to
random chance you randomly chose out
five to survive and those five ones
might have all been black or all-white
so what we saw is the same sort of
change happening but again the key thing
is due to random chance the other thing
that we need to notice is that genetic
drift is really going to only occur in
small populations so again when you
chose out 15 you don't really get a
change to happen when we only chose out
five that's where change started to
happen think about it with the
probability of flipping a heads or a
tails it's a 50/50 shot so if I ask you
to flip a coin a hundred times do you
think you could ever get 100 heads in a
row
probably not you probably end up with
about a 50-50 between the two but if I
said okay flip a coin only two or three
times could you get all heads or all
else you probably would say yeah it's
more likely that's because when it comes
to probability and random occurrences it
is more likely to get a sway in smaller
sample sizes or in this case smaller
populations so how can this sort of
change happen in the real world what
things are going to cause this random
change in allele frequency one way is
going to be through something known as a
bottleneck effect so if you think about
the neck of a bottle it's very narrow so
this is going to be some sort of event
that's going to cause my population to
narrow or reduce in size so imagine I
have a bottle here with different
colored beads that are going to simulate
or represent different genes inside my
population well let's say that I let
only a few through the neck of that
bottle in this case a couple of blue
make it out one orange the red ones
don't so everyone that's left inside
that bottle dies there vanish so now
when this population repopulate sand
reproduces eventually my new population
looks nothing like the original I've had
a change in allele frequency so
evolutionary change has occurred
remember before we had equal amounts of
red blue and orange now we have mostly
blue and it's not because blue is better
adapted they just got lucky and passed
through that event so a bottleneck
effect is gonna be any event that
reduces the size of a population small
enough okay remember it's got to be
small and therefore it's gonna reduce
the variation of the gene pool
I now have less alleles in my population
but again this is not good because it's
not like with natural selection the blue
ones are adapted maybe the blue ones are
less adapted we don't know but either
way evolution areas changes occurred so
a real-world example of this would be
love with the Florida panther what we've
seen is that the gene pool of the
Florida panther has been reduced because
of a bottleneck effect the bottleneck
effect in this case was humans we've
come in we've encroached on their land
and through hunting and traffic and
through just competition between the
males killing each other there's been a
reduction in their gene pool so
evolution has been occurring in that
sense but not in a good way it's not
because they're more adapted it's just
because of a bottleneck effect another
way we can get genetic drift to happen
is through something known as the
founder effect so if we think about what
the term founder means who's the founder
of Facebook Mark Zuckerberg well he's
the person that started the company so
the founder effect is one let's say we
have our population of greens and blues
here let's say a small amount of them
are the founders of a new population
for whatever reason maybe they got
separated maybe they decided to leave
and start a new colony but either way so
now my new population here when they
reproduce looks nothing like the
original now I have more blues than
greens this was not because blues were
more adapted it's just because the new
population started off different from
the original because again it was a
small sample size so the founder effect
is gonna be when a new population is
started or founded by a small number of
individuals again it's got to be small
resulting again in a loss of genetic
variation
so another real-world example of this is
in humans they're actually a group of
humans that settled German people who
settled in Pennsylvania here in the
United States and they started the Amish
populations in Pennsylvania and what we
noticed is that there is a higher than
normal percentage of people with this
condition here known as polydactyl ISM
having six fingers or six toes now it's
not because for them it's beneficial to
have that it's just because the founders
of that population when only a few
hundred people started it some of them
had that condition so that when they
reproduced and started that colony we
see that sort of trait more abundant in
that population so one thing to realize
about genetic drift is it's not as good
as natural selection because it's not
necessarily selecting for the best trait
it's just through random lucky chance
another way we can get this happened so
another way we can get a change in
allele frequency is through what we know
as gene flows this is not the same as
genetic drift thing about this word flow
to flow means to be able to move so in
gene flow what's gonna happen is we can
get alleles moving into and out of my
population so here's my starting
population what if some red ones decide
they're gonna move in so now I've got a
change in my allele frequency that's
evolution maybe some of my blue ones
move out that also can change my allele
frequency so you notice my starting
population looks nothing like my ending
population so again evolution has
occurred so with gene flow gene flow is
going to be immigration and emigration
moving in and moving out of individuals
it's going to result again in a change
in that allele frequency so to stop and
think for a second does gene flow
increase variation or does it decrease
variation like what happens with natural
selection and with genetic drift and
really could happen either way you saw
here how we could increase variation by
introducing new alleles in
or if some of them leave then we can
decrease variation either way we call
this evolutionary change because we're
getting a change in the number of
alleles inside that population one last
way we can get evolution to occurs
through something known as sexual
selection or sometimes referred to as
non-random mating so let's again go to
my beetle population or insect
population here and say we have equal
amounts of green and equal amounts of
blue what might happen in this
population is that for some reason
individuals start choosing let's say
blue ones as mates they only want to
reproduce with the blue one so over the
course of time we have a change in
allele frequency there are more blues
than greens but it's not because the
Blues are more adapted it's just for
some reason that trait is being selected
for in reproduction so with sexual
selection we're gonna get those changes
to happen due to non-random mating
they're not randomly choosing mates
they're looking for a specific trait an
example of this my favorite example is
in the case of peacocks if you've ever
seen a male peacock before you see it's
beautiful elaborate tail and you might
think to yourself how did this occur and
even Darwin might look at this and say
there's no way that natural selection
would allow this to happen
they're slower it's a lot easier to
catch them if you're their predator so
how is this being selected for how did
this evolved and it was actually through
sexual selection what it turns out is
peahens the females prefer males with
more beautiful elaborate tails because
it signifies to them health so they want
those genes to be passed on to their
offspring so over the course of time we
can get that evolutionary change to
happen so if we revisit our lesson
essential question how can this
evolutionary change occur outside of
natural selection you should be able to
by now think about how genetic drift or
these random changes in allele frequency
can occur gene flow can just be the
movement of these alleles into and out
of a population or just through sexual
selection non-random mating how certain
individuals will choose for certain
traits to reproduce with how all these
things can cause evolutionary change if
you can do that I think you're in great
shape you got it if not feel free to go
back and watch certain parts but I thank
you guys so much for watching