Hi, I'm Hank. And I'm a human, but let's pretend for a moment
that I'm a moth. And not just any moth, a peppered moth.
Now let's pretend that I'm living in London in the early 1800s,
right as the industrial revolution is starting. Life is swell.
My light-colored body lets me blend in with the light-colored
lichens and tree bark, which means birds have a hard time seeing me,
which means that I get to live.
But it's starting to get noticeably darker around here with all these
coal-powered factories spewing soot into the air, and suddenly
all the trees have gone from looking like this to looking
So thanks to the soot-covered everything, I've got problems.
But you know who doesn't have problems? My brother.
He looks like this Yeah, he has a different form
of the gene that affects pigmentation.
Moths like him represent about 2 percent of all the peppered
moths at the start of the industrial revolution.
But by 1895 it'll be 95 percent!
Why? Well, you're probably already guessing, as the environment
gets dirtier, darker moths will be eaten less often, and therefore
have more opportunities to make baby moths.
The white ones will get eaten more, so over time,
the black-colored trait will become more common.
As for me? [Eaten.]
This, my friends, is a wonderful example of
natural selection. The process by which certain inherited traits
make it easier for some individuals to thrive and multiply,
changing the genetic makeup of populations over time.
For this revelation, which remains one of the most important
revelations in biology, we have to thank Charles Darwin, who first
identified this process in his revolutionary 1859 book,
On the Origin of Species by Natural Selection.
Now lots of factors play a role in how species change over time
including mutation, migration and random changes in how frequently
some alleles show up, a process known as genetic drift.
But natural selection is the most powerful and most important cause
of evolutionary change, which is why today we're going to talk
about the principles behind it, and the different ways
in which it works.
Darwin came to understand the process of selection because he
spent his adult life, even most of his childhood, obsessed with
He studied barnacles, earthworms, birds, rocks, tortoises, fossils,
fish, insects and to some extent, even his own family.
I'll get back to that in a bit.
But it was during Darwin's famous voyage on the H.M.S. Beagle
in the 1830s, a surveying expedition around the world,
that he began to formulate this theory. Darwin was able to
study all kinds of organisms, and he kept amazing journals.
Looking back on his notes, he hit upon a couple of
particularly important factors in species' survival.
One of them was the many examples of adaptations he noticed on
his journey. The ways in which organisms seemed to be nearly
ideally shaped to enhance their survival and reproduction in
Maybe the most famous example of these were the variations of beaks
Darwin observed among the finches in the remote Galapagos Islands
off the coast of South America. He observed more than a dozen
closely-related finch species, all of which were quite similar
to mainland finch species, but each island species had
different shaped and sized beaks that were adapted to the food
available specifically on each island.
If there were hard seeds, the beaks were thick.
If there were insects, the beaks were skinny and pointed.
If there were cactus fruit, the beaks were sharp
to puncture the fruit's skin.
These superior inherited traits led Darwin to another idea,
the finches' increased fitness for their environment, that is, their
relative ability to survive and create offspring.
Explaining the effects of adaptation and relative fitness
would become central to Darwin's idea of natural selection.
And today we often define natural selection, and describe how it
drives evolutionary change, by four basic principles,
based on Darwin's observations.
The first principle is that different members a population
have all kinds of individual variations.
These characteristics, whether their body size,
hair color, blood type, facial markings, metabolisms
or reflexes, are called phenotypes.
The second is that many variations are heritable and can be passed
on to offspring. If a trait happens to be favorable,
it does future generations no good if it can't be passed on.
Third: this one tends to get glossed over a lot, even though
it's probably the most interesting, is Darwin's observation that
populations can often have way more offspring than resources,
like food and water, can support.
This leads to what Darwin called "the struggle for existence."
He was inspired here by the work of economist Thomas Malthus,
who wrote that when human populations get too big,
we get things like plague and famine and wars,
and then only some of us survive and continue to reproduce.
If you missed the SciShow Infusion that we did on human overpopulation
today and Malthus's predictions, you should check it out now.
This finally leads to the last principle of natural selection,
which is that, given all of this competition for resources,
heritable traits that affect individuals' fitness can lead to
variations in their survival and reproductive rates.
This is just another way of saying that those with favorable traits
are more likely to come out on top and will be more successful
with their baby-making.
So to wrap all these principles together, in order for natural
selection to take place, a population has to have
variations, some of which are heritable, and when a variation
makes an organism more competitive, that variation will
tend to be selected.
Like with the peppered moth. It survived because there was
variation within the species, the dark coloration,
which was heritable, and in turn allowed every moth
that inherited that trait to
better survive the hungry birds of London.
But notice how this works. A single variation in a single
organism is only the very beginning of the process.
The key is that individuals don't evolve.
Instead, natural selection produces evolutionary change
because it changes the genetic composition of entire populations,
and that occurs through interactions between individuals
and their environment.
Let's get back to Darwin for a minute.
In 1870, Darwin wrote to his neighbor and parliamentarian
John Lubbock requesting that a question be added to England's
census regarding the frequency of cousins marrying and the
health of their offspring.
His request was denied, but the question was something
that weighed heavily on Darwin's mind,
because he was married to Emma Wedgwood, who happened to be
his first cousin.
Her grandfather was Josiah Wedgwood,
founder for the company that remains famous for its
pottery and china.
Oh, and he was also Darwin's grandfather.
In fact, much of Darwin's family tree was...complicated.
His marriage to Emma was far from the first Wedgewood-Darwin pairing.
Darwin's maternal grandparents and mother were also Wedgwoods,
and there were several other marriages between cousins
in the family, though not always between those two families.
So Darwin, and to a greater extent his children, carried more genetic
material of Wedgwood origin than Darwininan. And this caused
some problems, the likes of which Darwin was all too aware of,
thanks to his own scientific research.
Darwin of course spent time studying the effects of
crossbreeding and inbreeding in plants and animals,
noting that consanguineous pairs often resulted in weaker
and sickly descendants. And the same was true of his family.
Emma and Charles had 10 children, three of whom died in childhood
from infectious disease, which is more likely to be
contracted by those with high levels of inbreeding.
And while none of Darwin's seven other children had any deformities,
he noted that they were "not very robust"
and three of them were unable to have children of their own,
likely another effect of inbreeding.
Now, so far we've been talking about natural selection in terms
of physical characteristics, like beak shape or coloration.
But it's important to understand that it's not just organism's
physical form, or its phenotype, that's changing but its
essential genetic form, or genotype.
The heritable variations we've been talking about are a function
of the alleles that organisms are carrying around. And as organisms
become more successful, evolutionarily speaking,
by surviving in larger numbers for longer and having more kids,
that means that the alleles that mark their variation
become more frequent.
But these changes can come about in different ways.
To understand how, let's walk through the different
modes of selection.
The mode we've been talking about for much of this episode is an
example of directional selection, which is when a favored trait is
at one extreme end of the range of traits, like from short to tall,
or white to black, or blind to having super-night-goggle vision.
Over time this leads to distinct changes in the frequency of that
expressed trait in a population, when a single phenotype is favored.
So our peppered moth is an example of a population's trait
distribution shifting toward one extreme, almost all whitish moths,
to the other extreme, almost all blackish.
Another awesome example is giraffe necks. They've gotten
really long over time because there was selection pressure
against short necks, which couldn't reach all of those
But there's also stabilizing selection, which selects against
extreme phenotypes and instead favors the majority that are well
adapted to an environment. An example that's often used is a
human's birth weight: Very small babies have a harder time defending
themselves from infections and staying warm, but very large
babies are too large to deliver naturally. Because of this, the
survival rate for babies has historically been higher for those
in the middle weight range, which helped stabilize
average birth weight. At least, until Cesarian sections
became as common as bad tattoos.
So what happens when the environment favors extreme traits
at both ends of the spectrum, while selecting against
the common traits? That's disruptive selection.
Now examples of this are rare, but scientists think they found
an instance of it in 2008, in a lake full of tiny crustaceans
called Daphnia. The population was hit with an
epidemic of yeast parasite, and after about a half-dozen
generations, a variance had emerged in how the Daphnia
responded to the parasite. Some became less susceptible to
the yeast, but were smaller and had fewer offspring. The others
actually became more susceptible to the parasite, but were bigger
and able to reproduce more, at least while they were
still alive. So there were two traits that were
being selected for, both in extremes and both to the exclusion
of each other: susceptibility and fecundity.
If you got one, you didn't get the other.
An interesting example, of selection being
driven by a parasite.
Now while these are the main ways that selective pressures can
affect populations, those pressures can also come from
factors other than environmental ones like food supply or predators
or parasites. There's also sexual selection,
another concept introduced by
Darwin and described in The Origin of Species as depending
"not on a struggle for existence, but a struggle between individuals
of the same sex, generally the males, for the possession
of the other sex."
Basically, for individuals to maximize their fitness,
they not only need to survive but they also need to reproduce more,
and they can do that one or two ways:
One, they can make themselves attractive to the opposite sex.
Or two, they can go for the upper hand by intimidating, deterring
or defeating the same-sex rivals.
The first of these strategies is how we ended up with this:
I mean, the peacock tail isn't exactly camouflage. But the more
impressive the tail, the better chances a male will find a mate
and pass its genes to the next generation.
Sad-looking peacock tails will diminish over generations,
making it a good example of directional sexual selection.
The other strategy involves fighting, or at least looking like=
you want to fight, for the privilege of mating,
which tends to select for bigger or stronger
or meaner-looking mates.
And finally, thanks to us humans there are also un-natural forms
of selection, and we call that artificial selection.
People have been artificially selecting plants and animals
for thousands of years, and Darwin spent a lot of time in Origin of
Species talking about the breeding of pigeons and cattle and plants
to demonstrate the principles of selection.
We encourage the selection of some traits and discourage others.
It's how we got grains that produce all those nutrients.
Which is how we managed to turn the gray wolf into domesticated
dogs that can look like this
or like that, two of my favorite examples of artificial selection.
Now these are different breeds of dogs-
Oh, where you goin'? No. No.
But they're both still dogs. They're the same species.
Technically, a corgi and a greyhound could get together and
have a baby dog, though it would be a weird looking dog.
But, what happens when selection makes populations so different
that they can't even be the same species any more?
Well, that's what we're going to talk about next episode on Crash Course Biology:
How one species can turn into another species.
In the meantime, feel free to review what we've gone over today,
ask us questions down in the comments below, or on Facebook or Twitter, We'll see you next time.