- 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
likens in tree bark, which means that birds
have a hard time seeing me and 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 like this. 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% of all the peppered moths at the start of the Industrial Revolution, but by 1895, it'll be 95%. Why? Well, you're probably already guessing, as the environment gets dirtier, darker moths will be eaten less often and therefore will have more opportunities to make baby moths, while the white ones will get eaten more so overtime the black color
trait will become more common. As for me. (upbeat music) My friends, it's 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, random changes in how
frequently some allele 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 natural selection because he spent his adult life, even most of his childhood, obsessed with observing nature. He studied barnacles,
earth worms, birds, rocks, tortoises, fossils, fish, insects, and to some extent even his own family. And I'll get back to that in a bit. But it was during Darwin's
famous voyage on the HMS Beagle in the 1830s 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 adaptions he noticed on his journey. The ways in which organisms
seem to be ideally shaped to enhance their survival and reproduction in specific environments. 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 shape and size 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 to 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. So, first principle is that different members of a population have all kinds of individual variations. These characteristics, whether they're body size,
hair color, blood type, facial markings, metabolisms, reflexes, they're called phenotypes. The second is that many of
these 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, and 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 Thomas Malthus, an economist 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. It's just another way of saying that those of favorable traits are more likely to come out on top and will be more successful
with their baby making. So, in order 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 a
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 devolve. 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. (happy piano music) 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 weighted heavily on Darwin's mind 'cause he was married to Emma Wedgwood, who happened to be his first cousin. Her grandfather was Josiah Wedgwood, founder of the company that remains famous for
its pottery and china, 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
Wedgwood-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 Darwinian. And this cost some problems, the likes of which
Darwin was all too aware 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 descendance, 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 that 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 an
organism's physical form, or its phenotype, that's changing, but its essential genetic
form, or its 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, and 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 whose trait distribution is shifting toward one extreme,
almost all whitish moths, to the other extreme, almost all blackish. Another awesome example
is giraffes' necks. They've gotten really long over time because there was selection
pressure against short necks which couldn't reach all
of those delicious leaves. 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 middle-weight range, which helps stabilize
the average birth weight. At least until cesarean sections became as common as bad tattoos. So, what happens when the
environment favors extreme traits at both ends of the spectrum whilst selecting against common traits? That's disruptive selection. Now, examples of this are rare but scientists think that
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 a yeast parasite and after about a half
a dozen generations, a variance had emergence in how the Daphnia
responded to the parasite. Some became less susceptible to the yeast but were smaller and had fewer organism, the others actually
became more susceptible 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. Also 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 of 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 the chances
a male will find a mate and will 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 unnatural
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 the Origin of Species talking about breeding of pigeons and of cattle and of plants to demonstrate the
principles of selection. We encourage the selection of some traits and discourage others, that'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 are you going? No, no. But they're still both 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 anymore? Well, that's what we're
gonna talk about next episode on Crash Course Biology. How one species can turn
into another species.