Evolution: It’s a Thing – Crash Course Biology #20


Congratulations! This is our last
episode of our section on Evolution and Genetics,
which puts us at the halfway mark of CrashCourse Biology. So far we’ve learned about DNA,
genetics, natural selection, how cells multiply, populations,
speciation, replication, respiration, and
photosynthesitation. I’m so proud of you. But I couldn’t let this section
end without discussing the iscussion that everybody can’t
help but discuss these days: Evolution. It’s a thing. It’s not a debate. Evolution is what
makes life possible. It allows organisms to adapt to
the environment as it changes. It’s responsible for the enormous
diversity and complexity of life on Earth, which not only provides
organisms with sources of food and some healthy competition. It also gives us some truly
awesome stuff to marvel at. And even though evolution makes
living things different from one another, it also shows
us how we’re all the same. All of life, every single thing
that’s alive on the Earth today, can claim the same shared heritage,
having descended from the very first microorganism when life
originated on this planet 3.8 billion years ago. There are people who will say
that this is all random- It’s not. And that this clumsy process could
not be possible for the majestic beauty of our world. To them, I say, well at least we
agree that our world is beautiful but, well you’re probably
not going to enjoy the rest of this video. To me, there are two sorts
of people in the world, those who are excited about
the power and beauty and simplicity of the
process of evolution, and those who don’t understand it. And somehow, I live in a country
where only 40% of the population believes that evolution is a thing. The only possible reason for that
that I can accept is that they just don’t understand it. It’s time to get real, people. First, let’s understand what we
mean when we talk about the theory of evolution. Evolution is just the idea that
gene distribution changes over time, which is an indisputable
fact which we observe all the time in the natural world. But the THEORY of evolution is a
large set of ideas that integrates and explains a huge mass of
observations from different disciplines including embryology,
paleontology, botany, biochemistry, anatomy and geophysics. In every day language, the
word “theory” means “hunch” or even “hypothesis.” But in science, a theory is an
idea that explains several phenomena at once. Thus, The theory of evolution is
a bunch of ideas that explain many things that we, as humans,
have observed for thousands of years. It’s the theory that meticulously
and precisely explains the facts, and the facts are indisputable. So let’s spend some time going
through the facts, and how evolution explains them
all so well. First, fossils: The fossil record shows that
organisms that lived long ago were different from
those that we see today. Sounds obvious, but two
hundred years ago it seemed a little bit crazy. When scientists first
started studying dinosaur fossils in the 1820s,
they thought that all dinosaurs were basically giant iguanas. That’s why the first fossil
dinosaur was named Iguanodon. It wasn’t until the fossils of
two-legged dinosaurs started showing up in the 1850s that
scientists had to grapple with the idea that organisms of the past
were somewhat similar to ones today like, dinosaurs were reptiles, but
many of them took on a diversity that’s barely recognizable to us. And of all those ancient
not-really-iguanas were all extinct, either dying out
completely or evolving into organisms that survive
today, like birds. Fossils make it clear that only
evolution can explain the origin of these new kinds of organisms. For instance, fossils taught us
that whales used to walk. Whales are cetaceans, a group of
mammals that includes porpoises and dolphins, and biologists long
suspected that whales descended from land mammals. Partly because some modern whales
still have the vestigial remnants of a pelvis and hind-limb bones. But it wasn’t until recently,
the 1990s and 2000s, that the pieces really came together. First, paleontologists discovered
fossils of DOR-oo-dons, cetaceans that had different
skulls from modern whales but still had the same
vestigial leg bones. Then they found even older fossil
remains of another cetacean that actually had hind
legs and a pelvis. The pelvis wasn’t fused to
the backbone like ours is, so it did swim like a whale,
but more importantly, it still had ankle bones And they were ankle bones that
are unique to the order that includes bison, pigs,
hippos and deer. So by following these clues left
behind in fossilized bones, paleontologists were able to track
the origin of whales back to the same origin as bison and pigs. This leads us to another series of
facts that evolution explains: Not how animals were different, but
how they are incredibly similar. Last week we talked about
Carl Linnaeus and how he classified organisms by their
structural similarities. Well he didn’t know anything
about evolution or genetics, but when he began grouping
things in this way, he hit upon one of evolution’s
most prominent clues: homologous structures. The fact that so many organisms
share so many finely detailed structures shows us
that we’re related. Let’s go back to the whale. Like my dog, Lemon, and me,
the whale has two limbs at the front of its body,
its front flippers. And so does this bat, its wings. Inside our limbs we all have the
very same structure: one longish bone on top, connected to two thin
bones at the joint, followed by a cluster of small bones called
the carpals, and then our fingers, or digits. We each use our forelimbs for
totally different purposes: the bat flies, the whale swims,
Lemon walks and I… you know, jazz hands! Building limbs like this isn’t the
most efficient way to swim or fly or walk. Our limbs have the same structure
because we descended from the same animal, something like
this more-gan-uh-cah-don here, which, yeah, has the
same forelimb structure. In the first stage of
our existence, every vertebrate looks almost
exactly the same. Why? Because we’re all descended from
the same initial vertebrates. So our structures are the
same as other mammals and other vertebrates, sure, but it
also turns out that our molecules are the same as, like, everything. In fact, if we were ever to
find life on Mars or something, the sure fire way of knowing
whether it’s really extra-terrestrial is to check and
see if it has RNA in it. All living things on our planet use
DNA and/or RNA to encode the information that makes
them what they are. The fact that we all use the same
molecule itself suggests that we are all related,
even if very distantly. But what’s more, by sequencing
the DNA of any given creature, we can see precisely
how alike we are. The more closely related
species are, the more of the same DNA sequences they have. So the human genome is
98.6% identical to that of the chimpanzee, our closest
evolutionary relative, and fellow primate. But it’s also 85%
the same as a mouse. And I wonder how you’re going to
feel about this, about half of our genes are the same as in fruit
flies, which are animals, at least. So, just as your DNA proves that
you descended from your parents, your DNA also shows that you
descended from other organisms and ultimately, from that one
prokaryotic microorganism 3.8 billion years ago that
is the grandparent of us all. Now when it comes to species
that are very similar, like say, marsupials, their
distribution around the world or their biogeography, is also
explained extraordinarily well by the theory of evolution. Animals that are the most similar,
and are the most closely related, tend to be found in the same
regions, because evolutionary change is driven in part
by geographical change. As we talked about in
our speciation episode, when organisms become isolated
by physical barriers, like oceans or mountains, they take their
own evolutionary courses. But in the time scales
we’re talking about, the geographical
barriers are much older, and are often even the
result of continental drift. So, marsupials. You know about marsupials. They can be found in many places,
but they aren’t evenly distributed around the world. By far the highest concentration
of them is in Australia. Even the majority of mammal fossils
in Australia are marsupials. So why is Australia rife with
kangaroos, koalas and wombats while North America
just has, opossums? Fossils show us that one of
marsupials’ earliest ancestors found its way to Australia before
continental drift turned it into an island 30 million years ago. More importantly, after Australia
broke away, placental mammals like us evolved on the main landmass and
quickly outcompeted most of the marsupials left behind,
in what would become North and South America. So, very few marsupials
remain in the Americas, while Australia has been drifting
around like some kind of marsupial Love Boat. Darwin’s finches are another
example of biogeographical evidence As he wrote in The Origin of
Species, Darwin observed that different species of finches on
separate Galapagos islands were not only similar to each other but
were also similar to a species on the South American mainland. He hypothesized that the island
finches were all descendants of the mainland finch and changed
over time to be more fit for their environments,
a hypothesis that genetic testing has since confirmed. Now, you’ll remember, I hope,
a few weeks ago, when I told you about Peter and Rosemary Grant, the
evolutionary biologists/lovebirds who have studied Galapagos
finches since the 1970s. One of their greatest contributions
came in 2009 when studying finches on the island of Daphne Major. They discovered that the offspring
of an immigrant finch from another island and a Daphne Major finch
had become a new species in less than 30 years. This is just the latest example
of our fourth body of evolutionary evidence:
direct observation of evolution. The fact is, we have seen evolution
take place in our own lifetimes. One of the fastest and most common
changes we observe is the growing resistance to drugs
and other chemicals. In 1959, a study of mosquitos in a
village in India found that DDT killed 95% of the mosquitos
on the first application. Those that survived reproduced and
passed on their genetic resistance to the insecticide. Within a year, DDT was killing
only 49% of the mosquitos, and it continued to drop. The genetic makeup of the mosquito
population changed because of the selective pressures caused
by the use of DDT. But it’s not just tiny
changes in tiny animals, we’ve also observed larger
animals undergoing some pretty striking changes. In 1971, for instance, biologists
transplanted ten Italian wall lizards from one island off
the coast of Croatia to another. Thirty years later, the immigrant
lizards’ descendants had undergone some amazing, fundamental changes
like, even though the original lizards were mainly insect eaters,
their digestive systems had changed to help them exploit the island’s
most abundant food source: plants. They actually developed muscles
between their large and small intestine that effectively created
fermenting chambers, which allowed them to digest vegetation. Plus, their heads became wider and
longer to allow them to better bite and chew the grasses and leaves. These are all great examples of
microevolution, allele frequency changes that happens rather
quickly and in small populations. Macroevolution is just that
microevolution on a much longer time scale. The sort of thing that turns
hippos into whales is a lot harder to observe for a species that,
200 years ago, thought dinosaurs were big iguanas, but part of the
power of the human mind is being able to see far beyond itself and
the time scales that our own individual lives are limited to. And I for one, am
pretty proud of that. Let’s all at least agree
that the world is a beautiful and wonderful place. And life is worth studying
and knowing more about, and that’s what Biology is. If you want to go back and watch
parts of this video again please click on the annotations in the
little table of contents over there. If you have questions for us,
please leave them on Facebook or Twitter or in the
YouTube comments below. Thanks to everybody who
helped put this together. And we’ll see you next time.