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.
OK, 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ideas are definitely outdated, his theory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
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 want.
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 in and out at will.
On the whole, random mutation is
responsible for the largest share of genetic variability.
There are three categories of mutations;
Positive, Neutral and Negative.
Positive mutations increase an organism's
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 organism's
fitness, and  neutral mutations have no effect on fitness either 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. By 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 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.
These runner-up giraffes can also survive
to reproduce too
Now to the types of natural selection.
There are three categories:
Directional selection where one extreme is favored,
stabilizing selection where
an intermediate is favored,
and diversifying/disruptive 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, etc.
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.
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.
Other examples of directional selection
include Darwin's famous population of
finches.
They 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 male peacock's tail.
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.
This signals 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 genes.
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 or "disruptive" 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 there 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 Malpais lava flow.
Scientists believe that it is from a
volcanic eruption that occurred roughly
5000 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, the dark-colored mice suddenly were better able to survive.
Today across the Valley of 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
some live electrical 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 thus 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 www.darwinawards.com.
That wraps up our discussion of natural
selection, evolution and adaptation, and i
hope it was useful.
Stay safe out there, and please remember
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