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Lindsay Turnbull:
Hello, my name's Lindsay Turnbull.

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I teach biology at the University of Oxford.

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In this video, I want to show you how evolution
by natural selection has shaped every organism

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on our planet.

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It's the most important theory in biology
and it forms the core of chapter two of my

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book Biology: The Whole Story.

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In 1819, an unusual moth emerged from its
cocoon in a woodland

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 somewhere in northern England.

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It belonged to a species called the peppered
moth but it didn't look quite like all the

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other peppered moths that were emerging around
it.

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Normal peppered moths have speckled pale wings,
but this moth didn't have wings like that.

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Instead, it had beautiful, sooty black ones.

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The moth was, in fact, a mutant.

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There had been a change to some of the letters
in its genome, so its cells were producing

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more black pigment than normal.

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Now, we've already seen that most mutations
are harmful, so we might prophecy doom for

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this new arrival.

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But instead, naturalists would watch in awe
as this moth and its descendants

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became wildly successful.

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How?

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During the 19th century in northern England,
the Industrial Revolution was well underway.

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Victorian engineers had worked out how to
burn coal to heat water, to produce steam,

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to drive newly invented machines so they could
produce things cheaper and faster 

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than ever before.

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But coal is a dirty fuel.

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Burning coal produces clouds of black soot
and these settle out onto houses, schools,

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lungs, and even trees.

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Worse, coal contains impurities, like sulphur,
so when you burn coal, you also burn sulphur,

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and that produces sulphur dioxide, which is
an acid gas, and that's very toxic to the

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crusty grey-green lichens that 
 normally plaster trees.

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So all this soot and this sulphur dioxide
transformed the tree trunks of northern England.

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Where they'd formally been speckled grey-green,
they were now columns of purest black.

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If you were a pale, peppered moth sitting
on a tree trunk, then this transformation

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was disastrous.

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For birds, moths make a good meal, and if
they can see them, they will eat them.

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These pale speckled wings of the original
form of the peppered moth used to provide

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very good camouflage against the tree trunk
in unpolluted woodlands.

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But now, against the newly blackened tree
trunks, these moths were sticking out like

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sore thumbs.

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In fact, you could say that the peppered moth
was on the verge of extinction.

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Salvation lay in the mutant moth.

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Its sooty black wings provided perfect camouflage
against the newly blackened tree trunks,

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and so the birds ignored it.

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When it had offspring, they too inherited
the black wings, and so over a few decades,

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the black moth became more and more common,
while the original pale speckled form was

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almost impossible to find.

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The story of the peppered moth reveals that
species are not set in stone.

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They can evolve as the typical genome 
 changes over time.

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This change begins with a mutation in a single
lucky individual, but it spreads through a

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population because individuals that carry
the mutation are more successful

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than those that don't.

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This almost ludicrously simple 
process forms the core

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of the Theory of Evolution by Natural Selection.

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It was independently proposed by Charles Darwin
and Alfred Russell Wallace in 1858.

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And although it has been attacked by various
religious figures over time, its position

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at the heart of biology has never been in
serious doubt.

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So let's take a look at this theory.

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Now, I have to tell you, it's a pretty unusual
kind of theory, unique in biology.

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The way it's laid out is we look at a set
of assumptions, there are three crucial ones,

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and we have to decide whether or not we believe
that those assumptions are true.

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Because if they are, then there is a logical
conclusion, and that conclusion is that species

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will evolve to become better adapted to their
environments.

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So let's take a look at the assumptions.

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The first one is that there is heritable variation
among individuals.

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Well, that sounds a bit of a mouthful, but
what does it mean?

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Well, heritable variation is genetic variation,

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 variation that can be passed on 
from a parent to offspring.

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And if we look at the peppered moth, we see
that we do have that heritable variation

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because we have the two forms, 
the pale speckled form and the black form.

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And we now know that the black form was caused
by a mutation in the genome, so it is heritable.

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The second point, the second assumption, is
that there is going to be competition among

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the individuals in a population so that not
all of them are going to survive and thrive.

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And that happens because generally more individuals
are born than the environment can support.

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The pressure in this environment is predation
by birds.

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Some of them are going to be eaten by birds,
but which ones?

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And that leads us to point three.

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And point three says that not all 
individuals are equal.

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This heritable variation from point one 
actually matters.

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So some individuals are more likely to survive
and have more offspring perhaps than others.

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In the case of the peppered moth, in the polluted
woodlands, it's the black moths, they are

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going to survive better and therefore have
more offspring than the pale ones.

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And so because those three assumptions are
true, we are going to see evolution in action.

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The peppered moth is going to evolve to be
better adapted to its environment.

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The population, in fact, is going to change
from one that looked like this

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into one that looked like this.

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And that in a nutshell is evolution in action.

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So since Darwin's day, the theory of evolution
has been tested extensively, and there's a

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mountain of evidence to support it.

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But in his day, Darwin himself was concerned
with one or two issues, and the biggest thing

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he was concerned about was time.

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So he was absolutely convinced that all life
on Earth was descended from a single common

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ancestor, something that we believe 
today very strongly.

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But if that's true, then it must have taken
a really long time for natural selection to

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have produced that extraordinary 
array of creatures.

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So one of the best pieces of evidence we have
that there was other life on Earth in the past 

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is of course fossils.

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Now, Mary Anning is one of the great figures
of palaeontology.

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She lived in Dorset, around the same time
as Darwin and Wallace, and she looked for

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fossils in the cliffs near her home.

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She found the first ichthyosaurs and a wonderful
skeleton of a long neck plesiosaur, and those

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were huge marine reptiles that lived at the
same time as the dinosaurs.

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So what these fossil finds showed is that
the Earth had looked very, very different

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in the past.

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And that was a brand new idea in the 19th
century, the idea that the Earth had been

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populated by fabulous beasts.

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So today we are confident that the Earth is
about four and a half billion years old.

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Now, that is just a span of time that any
puny human is going to find difficult to comprehend.

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And of course, we appear very, very late in
that time.

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So there's various comparisons that are made.

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If we just squeeze the history of life into
a single day, then we are going to appear

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on the last stroke of midnight.

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If we give it a year, then we are going to
appear at 11:35 PM on the last day.

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But any way you look at it, humans are not
a major part of the history of life.

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Now, amazingly, we actually think that life
itself evolved pretty quickly.

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The early Earth was molten, but by about 4
billion years ago, we think the first cells

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were getting up and running.

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But then until about half a billion years
ago, all life on Earth was single celled.

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So half a billion years is about 540 million
years ago.

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The first animals appear, and the good news
about them is they have hard parts.

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So some of them have shells and some of them
have bones and some of them have teeth.

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And then we start to get a 
much better fossil record.

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So that period, from 540 million years ago
to the present day,

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is known as the Eon of Visible Life.

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Now that aeon can be split 
into three great eras.

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There is the Paleozoic, 
which means ancient life,

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the Mesozoic, which is middle life,

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and the Cenozoic, which is recent life. 

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And those eras are dominated by different
creatures.

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So the Paleozoic is the longest era 
and that oversaw therefore

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the biggest changes to the planet.

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For example, it was during the Paleozoic that
the plants colonised the land and the continents

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became green and then 
some animals followed them.

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The Mesozoic was dominated by 
dinosaurs on land,

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but also the ichthyosaurs and 
plesiosaurs in the ocean

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and winged pterosaurs 
in the skies. 

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The Cenozoic, the recent era, has been dominated
by mammals and birds.

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So that's the one that we are 
most familiar with.

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Now, the great eras are punctuated by two 
mass extinction events.

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The first one that lies between the Paleozoic
and the Mesozoic

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is often called the Great Dying.

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And that's just to emphasise how bad it was.

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It was caused by massive sustained volcanic
eruptions that pumped out carbon dioxide and

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hiked up global temperatures very rapidly.

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And that led to the loss of about 90% of species
on Earth, which is a pretty catastrophic event.

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The mass extinction between the Mesozoic and
the Cenozoic was different.

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That had an extraterrestrial cause.

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So a huge asteroid hit the Earth in what's
now the Gulf of Mexico

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and that caused wildfires to spread 
around the globe and would've looked,

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I don't know, almost like Armageddon. 

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So that gives you the span of time and the
major time points.

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And of course, what you can see is that the
diversity of life on Earth has really fluctuated,

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building up and then crashing back down again
during a mass extinction.

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But what's interesting is today there are
probably more species on our planet

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than at any time in the past.

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Well, I do hope you enjoyed that episode,
and if you did, then please do share it with

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friends and colleagues.

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If you'd like to have your own copy of the
book, then there is a link below.

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There's a lot more in chapter three about
evolution.

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In particular, there's a lot of stuff about
how multicellular beings evolved.

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That's actually quite a puzzle for evolutionary
biologists.

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Otherwise, don't forget to join us next time
for episode three, which is all about

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sexual reproduction.