welcome back to bogobiology this video
will discuss natural selection as a
mechanism for evolution
we'll be discussing what natural
selection is specific examples of
directional stabilizing and disruptive
selection
and a few common misconceptions okay
here we go
you're a very lucky human if you think
about it you're only here because every
single one of your ancestors survived
long enough to reproduce
we use the term fitness all the time
however biological fitness
is defined as the ability to survive and
pass on one's genes in a given
environment
the more offspring an organism
ultimately has the more fit it's
considered to be
since you're alive and watching this
your ancestors were all at least
moderately fit
still when it comes to achieving
biological fitness we humans have a fair
amount of help from grocery stores full
of high calorie food
modern medicine and dating apps
organisms in nature generally do not
have these advantages
nature is a very dangerous place
competition for resources like food
water and mates is fierce
diseases and pathogens are rampant and
predators are everywhere
certain traits can mean the difference
between surviving long enough to
reproduce and becoming someone's
mid-afternoon snack
when creatures with certain heritable
characteristics tend to reproduce more
successfully we call it natural
selection
the idea of natural selection became
famous when charles darwin published his
work on the origin of species in 1859 in
it he proposed that each slight
variation
if useful is preserved while some of his
ideas are definitely outdated his theory
of natural selection has stood the test
of time
natural selection is one mechanism that
drives a population's evolution over
time
evolution is a change in a species
characteristics over several generations
and is driven by natural selection
there's a few important things to keep
in mind about how natural selection
works
over time it can improve the match
between an organism and its environment
the better suited an organism is to its
environment the more likely it will
survive long enough to reproduce
if the environment changes an organism
may not be as fit anymore
for instance climate change is causing
temperatures to rise rapidly over much
of the globe putting tremendous stress
on many species that are ill-equipped to
handle hotter conditions
in many parts of the united states we're
starting to lose our oak trees
because they're not as well suited to a
hot environment if a heritable variation
makes an organism more competitive in
its environment
that variation can become more common in
a population over time
we call this accumulation of
advantageous changes over the course of
many generations
adaptation this brings us to
misconception number one individuals
don't evolve
populations evolve individual organisms
retain the same genes throughout their
lives generally speaking
they can't simply swap out their genes
for a more favorable set whenever they
feel like it
a population however can change its
composition over time
if a gene gives certain organisms an
advantage those organisms will
eventually increase in frequency
this begs the question of how and why
these variations and traits arise to
begin with if they can't be swapped out
at will
on the whole random mutation is
responsible for the largest share of
genetic variability
there are three categories of mutation
positive neutral and negative
positive mutations increase in organisms
fitness in some way shape or form they
make it more likely that the creature
will survive long enough to reproduce
negative mutations decrease in organisms
fitness neutral mutations have no effect
on fitness positive or negative
now we reach misconception number two
variation is not goal directed
variation is random organisms can't
choose which genetic variations they
would like to have
before darwin a scientist named
jean-baptiste lamarck suggested that
organisms purposely acquired traits over
time
which they then passed on following this
line of thought a giraffe would spend
years and years
stretching its neck to reach higher food
would somehow magically develop a longer
neck and then would have offspring with
longer necks too
we now know that this is not correct
this brings us to a slightly more
nuanced misconception number three
survival of the fittest isn't quite the
whole story
it's more like survival of the fit
enough these creatures who survive to
pass on their genes aren't necessarily
always the biggest the strongest or the
fastest
often more than one variety of a trait
is good enough
the runner-up giraffes can also survive
to reproduce too
now to the types of natural selection
there are three categories of natural
selection
directional selection where one extreme
is favored stabilizing selection where
an intermediate is favored and
diversifying selection where both
extremes are favored simultaneously
these can be a bit difficult to envision
so we're now going to go over an example
and a graph for each one
you might see graphs like this on
various standardized tests
they're very popular on exams like the
ap the ib
the a level et cetera they can sometimes
have different colors or orientations
but the most important thing is to
remember that one line usually shows
what a population used to look like
and the other line shows what the
population looked like later on
in this video the black line shows the
starting population and the green line
shows what it comes to look like
after being put under pressure from
certain conditions in directional
selection
one extreme phenotype is favored rather
than an intermediate
directional selection can be a subtle
shift or it can occur more dramatically
as in the famous case of the peppered
moths
prior to the industrial revolution a
population of light-colored moths
inhabited the city of manchester in the
uk
these moths blended in beautifully
against the neutral colored tree bark
and buildings
darker colored moths were virtually
unknown as they were so easy for
predators to see
however by the year 1848 the moth
population had almost entirely changed
color
dark colored moths were now the norm and
light-colored mods were more rare
why the city of manchester rapidly
became industrialized right around that
time
the new coal-burning factories covered
the city with a dark greasy soot
with the new darker habitat predators
could easily spot the light-colored
moths
and any moths born with the dark color
mutation were much more likely to
survive long enough to reproduce
over time the population shifted from
nearly all light-colored moths to nearly
all dark moths
other examples of directional selection
include darwin's famous population of
finches who were more likely to survive
if their beaks were better suited to the
food that was most readily available
in stabilizing selection an intermediate
phenotype is favored rather than one or
both extremes
a great example is a peacock tail male
peacocks are famous for their enormous
colorful tails which they use to attract
female peacocks usually females prefer
the males with the largest and most
highly colorful tales because they
signal that the male is healthy and
probably has good genes
based on this we might assume that the
goal is to have as large of a tail as
possible
however in addition to attracting all
the ladies enormous tales have a couple
of significant drawbacks
males with exceptionally large tails are
much more likely to be eaten before they
have a chance to pass on their jeans
the best of both worlds is to have a
tail that is large enough to attract a
mate but not
so large as to prevent you from flying
male peacocks actually can
fold up their tails and fly but not very
far other examples of stabilizing
selection include birth weight in humans
and stem height
in plants in both cases the intermediate
is favored
in diversifying selection both extremes
are selected for simultaneously while
the intermediate is selected against
diversifying selection is much more rare
relative to directional and stabilizing
selection
if you think about it it's pretty rare
to have two extreme phenotypes that are
equally useful at the same time but it
does happen
rock pocket mice are an excellent
example of diversifying selection
much of the southwestern u.s is made up
of hot dry desert climate with
light-colored sand
the mice tend to have light-colored
coats and it allows them to hide from
predators
however in an area called the valley of
fires in new mexico the light sand is
suddenly punctuated by a large area of
dark rock
this area is known as the el mel piece
lava flow
scientists believe that it is from a
volcanic eruption that occurred roughly
five thousand years ago
in the area surrounding the lava flow
dark colored rock pocket mice can also
be found
prior to the lava flow any rock pocket
mouse born in the desert with a dark
coat as a result of random mutation
likely would not survive long enough to
reproduce
it would stand out against the
light-colored sand and would be an easy
target for predators
when the lava flow suddenly added areas
of darker habitats
dark-colored mice suddenly were better
able to survive
today across the valley of the fires
region we can now find both light
and dark colored mice but what about
intermediately colored mice
a mouse with an intermediate coat would
not be well suited to either the light
sandy environment or the dark rocky
environment
because it would struggle to hide from
predators it would likely not survive
long enough to reproduce
because both the light and dark extremes
are favored our final population graph
would look like this
as a wrap up let's quickly explore the
concept of darwin awards
in theory natural selection will allow
creatures with advantageous
characteristics to proliferate and those
with disadvantageous characteristics to
be weeded out over time
darwin awards are not real awards for
achievement they're meant as a joke
these prizes are given out annually to
honor humans who do something
so incredibly stupid that they remove
themselves from the gene pool
and supposedly improve the human species
in the process
these are my favorites from the last two
years number one
after a snowstorm a man tried to drive
around barricades and warning signs onto
live wires and died via electrocution
number two a person tried to take a
selfie with an injured and very angry
bear
and finally number three a man
accidentally shot off his own
reproductive organs while carrying an
unholstered loaded gun in his underwear
while shopping at walmart
even though he didn't die in the process
and it's not a true darwin award
he removed his ability to reproduce
therefore he removed himself from the
gene pool
if like me you are a terrible human
being and found these funny
you can read more of them at wwe
that wraps up our discussion of natural
selection evolution and adaptation and i
hope it was useful
stay safe out there and please remember
to like comment and subscribe