Lesson video

Hello, and welcome to this lesson from the Oak National Academy.

Today's lesson is taken from the unit fossil evidence, selective breeding and explaining evolution.

And the title of today's lesson is Darwin, Wallace and the theory of evolution by natural selection.

So we're gonna be talking a lot about the history of the theory of evolution by natural selection and some of the really important scientists who helped contribute to it, including Charles Darwin and Alfred Russel Wallace.

Hiya, I'm Mrs. Wheate and I'm gonna be your teacher for today's lesson.

By the end of today's lesson, you'll be able to describe the evidence and scientific discoveries that led to the development of the theory of evolution by natural selection.

Let's have a look at some of our keywords.

So we have five keywords for today's lesson.

We have theory, a general explanation that applies to many situations.

Evolution, the process in which the characteristics of species change over many generations, sometimes becoming new species.

Natural selection, organisms that are better adapted to their environment are more likely to survive, reproduce, and pass on their genes to their offspring.

Artificial selection, the process in which humans choose organisms with desirable traits and breed them in order to make offspring with these desirable traits.

This is also known as selective breeding.

And finally, we have DNA, the chemical molecule made of nucleotides, which contains the genetic code for making proteins in all living organisms. So that was a lot.

Don't worry if you've not got all of that memorised right now.

I wouldn't expect you to have, and we're gonna go through each of these in lots of detail as they come up in the lesson.

But if you wanna read through them again, I'm gonna be quiet for five seconds, or if you want a bit longer, maybe you wanna write them down, pause the video and click play when you're ready to continue.

Today's lesson is in three parts.

First of all, we'll talk about the theory of evolution by natural selection, and we'll spend some time understanding what that is.

Then we're gonna go on and we're gonna talk about Charles Darwin and Alfred Russel Wallace and their contribution to developing the theory of evolution by natural selection.

And then we're gonna talk about the role that the discovery of DNA had in understanding evolution.

But first of all, the theory of evolution by natural selection.

Unsurprisingly, I love science.

I think it's one of the most interesting things you could learn about.

And part of the reason for that is that scientists aim to understand how the world around us works.

And they do this by, first of all, asking a question about the natural or physical world.

So let's take, for example, how do plants grow? That's the question someone's trying to figure out an answer to.

So in order to try and answer that question, scientists make observations and collect evidence.

After that, if they've collected enough evidence and made enough observations, they might be ready to suggest an explanation for how do plants grow.

Something really important in science is that you consider other scientists' work as well and share your ideas with them.

Maybe someone halfway across the world is trying to figure out how plants grow as well.

And by collaborating with them, you can get to a better explanation.

And something else that I really love about science is that it isn't finished, it isn't done.

If I discovered a reason why plants grow, I wouldn't then be like, okay, guys, nobody else work on this.

I have the definitive answer.

No one needs to research this anymore.

I've solved it.

That's not how science works, that's not how scientists work.

It's a continually improving field and discipline.

So it's really important in science that explanations for things are improved as new evidence and new understanding becomes available.

And so if you've kind of gone through all of those steps, sometimes an explanation has enough evidence and is widely accepted enough by enough scientists that it becomes a theory.

And the theory we're gonna talk about today is the theory of evolution by natural selection.

And we're gonna talk about how it, first of all, kind of went through all these steps from asking a question, how do animals change over time? How do organisms evolve to then becoming a widely accepted theory that it is today.

The characteristics of species change over many generations.

This is known as evolution.

So natural selection is a theory that scientists use to explain how and why evolution occurs.

The theory of evolution by natural selection, like all theories, has lots of supporting evidence, has been well tested by scientists, and it is widely accepted by most scientists.

It applies to a large number of situations as well.

So let's look at this whole phrase, the theory of evolution by natural selection, and let's break it down.

So we have the first part, evolution.

So evolution means the characteristics of species change over many generations.

There is a lot of fossil evidence for this, and we can see it happening in examples like bacteria and some animals.

Maybe you've heard about peppered moths before.

It's an example of evolution that occurred during the Victorian era.

So evolution is actually a fact.

The characteristics of species changing over many generations, that's an observable fact.

The theory part of this statement is the natural selection part.

Natural selection is the mechanism or the way that evolution occurs.

So natural selection means organisms that are better adapted to their environment are more likely to survive, reproduce, and pass on their genes to their offspring.

That's the theory part.

There's an idea in people's heads about how the word fact and theory work that's a bit wrong.

So a lot of people think that the word fact and theory have very similar meanings, and the only difference between them is that a fact is more true than a theory.

That's not how those words work in science.

A fact is an observation that has been shown to be true over and over and over again.

A theory is a general explanation that applies to a huge number of situation and links and incorporates many different facts.

So a theory explains why we see certain things, kind of explains the facts.

So put really simply, a fact is a type of observation.

A theory is a type of explanation.

So natural selection is a theory that the vast majority of scientists think that evolution works by, the mechanism which kind of drives evolution.

It's not quite the same as a fact.

It is very trustworthy.

It has lots and lots of evidence.

It's not like how we would use the word theory in everyday language.

In everyday language, you might say something like, "Oh, I've got a theory that such and such a football team are gonna win the Euros." That's not really a theory in terms of how we use it in science.

So often people dismiss theories in science as, "Oh, well, it's just a theory, it can't be true." It's really important to remember that some key terms we use in science, like the word theory, have specific and different meanings to how we use them in everyday language.

Okay, let's see if you understood that.

Evolution by natural selection is just a theory.

That means it's probably not true.

Is that true or is that false? Take five seconds or pause the video if you want some more thinking time.

Click play when you're ready to see the answer.

Okay, let's see the answer.

That is false, but why is that false? Let's justify that answer.

Is it false because A, the word theory and science means the same as fact? It is definitely true.

Or is it false because theories are well-evidenced and accepted by most scientists, so are very likely to be true.

Again, take five seconds or you can pause the video if you want more thinking time.

It is B, theories are well-evidenced and accepted by most scientists, so are very likely to be true.

Well done if you got that right.

So here's a quick recap of how natural selection works.

So first of all, there's variation.

Due to random chance, some individuals of a species have different traits or characteristics.

So some people have got blue eyes, some people have got brown eyes, animals have different fur colour, plants are different sizes.

So all of that is an example of variation.

So then we have advantage.

Some traits can give an individual an advantage.

So having fur and living in a really cold place, if you're an animal, that's an advantage.

Having small leaves and existing in a really, really dry habitat, that's an advantage for a plant.

We then have competition.

So the advantage that the organism has will allow it to compete successfully.

Organisms are always in competition with each other for finite resources.

So for animals, that's things like mates and territory.

For plants, that's a thing like space, water, minerals, and sunlight.

So the resources around us are finite, so that means there's only a certain amount of it and it will run out.

And so these advantages that organisms have help them compete for these really finite resources.

Okay, next, we have survival.

So having an advantage, being really well adapted to your habitat, so the environment that you live in, makes you more likely to survive.

If you're more likely to survive, you're more likely to survive to an age where you can have offspring.

And then we have inheritance.

So it becomes more likely if you survived and reproduce that you are then able to pass on these adaptive traits or these traits that were advantageous to your offspring.

And over generations, these advantageous, these helpful traits become more and more common.

So that's the process of natural selection.

And let's see if you understood that.

Complete the sentences about natural selection.

Use the words from the list, traits, survive, similarities, compete, differences, and evolve.

There are blank in the traits of individuals of a species.

This is called variation.

Some of these give individuals an advantage over each other, which makes them more likely to blank successfully for resources.

They are then more likely to blank reproduce and pass on their blank to the offspring.

So use the words from the list above.

You can use one more than once, or you can use some of the words not at all.

And see if you can fill in these blanks.

You have five seconds, or if you want some more time, you can pause the video and click play when you're ready to see the answer.

Okay, let's go through the answers.

There are differences in the traits of individuals of a species.

This is called variation.

Some of these give individuals an advantage over each other, which makes them more likely to compete successfully for resources.

They are then more likely to survive, reproduce and pass on their traits to their offspring.

Well done if you got that right.

Let's have a go at our first practise task of today's lesson.

So number one, what are the features of a scientific theory that make it trustworthy? Number two, describe in steps how scientific theories are developed.

So you'll need to pause the video now to give yourself enough time to think about these questions, to answer them.

Click play when you're ready to see the answers.

Good luck, everyone.

Okay, let's have a look at our answers.

What are the features of a scientific theory that make it trustworthy? Has lots of supporting evidence, has been well tested by scientists, is widely accepted by most scientists and applies to a huge number of situations.

Number two, describe in steps how scientific theories are developed.

So first of all, asking a question about how the natural physical world works.

Then making observations of collecting evidence, suggesting an explanation, considering other scientists' work and sharing ideas and explanations.

Improve the explanation as new evidence becomes available.

And if that explanation has enough evidence and is widely accepted by scientists, it becomes a theory.

Well done if you got that right.

So far in today's lesson, we've talked about the theory of evolution by natural selection.

We've discussed what a theory means, and we've specifically talked about natural selection and what that means.

Now we're gonna talk about Charles Darwin and Alfred Russel Wallace and their contribution to the development of the theory of evolution by natural selection.

So there were actually many scientists who contributed to the theory of evolution by natural selection.

But my guess is that you've probably only heard of one.

In this section of the lesson, we're gonna talk about two of the most, well-known and prominent: Charles Darwin, who's the one I suspect you've heard most about, but we're also gonna talk about Alfred Russel Wallace, who I suspect you've not heard very much about at all.

So they were both British scientists who lived during the 19th century, the 1800s.

And during this time, the majority of people in the Western world believed that species remained the same and they didn't change over time.

That was the thinking at the time.

But this idea was beginning to be challenged.

And part of the reason it was becoming challenged is because of emerging fossil evidence.

So fossil evidence had led some scientists to the conclusion that the characteristics of species change over many generations.

So here's an example of a fossil that was discovered in the 19th century, a fossil of an Archaeopteryx, and that was this animal, which didn't exist anymore.

It had some reptilian features and some bird-like features.

And that was really confusing for people at the time.

They were like, "Is this a bird? Is this a dinosaur? What's going on?" So discoveries like that led people to think, "Okay, well, this animal doesn't exist anymore.

Maybe it changed.

Maybe it evolved into another animal that does exist at the time." So there were many different ideas for how this change over time might have occurred.

Both Darwin and Wallace were trying to find an answer to the same question.

What causes the characteristics of species to change over many generations? So this is the question that they started with.

Let's check to see if we're following that.

True or false? Darwin is the only scientist who developed the theory of evolution by natural selection.

Is that true or is that false? Take five seconds or pause the video if you want more thinking time.

Click play when you're ready to see the answer.

That is false.

Okay, why is that false? Let's justify the answer.

So is it false because A, many other scientists, including Alfred Russel Wallace, also contributed a theory? Or is it false because Alfred Russel Wallace was the only other scientist who contributed to the theory? Again, take five seconds or click pause if you want a bit more thinking time.

Click play when you're ready to the answer.

A is the correct answer.

Many other scientists, including Alfred Russel Wallace, also contributed to the theory.

Great job if you got that right.

So they had their question and then they both travelled around the world, collecting observations as evidence to try and understand better.

Darwin spent most of his time in South America, and Wallace made two trips, firstly, Amazon Basin and then to Southeast Asia.

So despite visiting really different places all across the world, very, very different geography and biology, they made really similar observations.

One of the observations that Darwin made on his travels was he noticed the similarities and differences between the features of fossils and modern species.

So from fossil evidence, he discovered many new species, including an extinct species of giant ground sloth as seen in the picture here.

That is absolutely enormous.

There are no sloths anywhere near that size anymore.

Modern sloths are small creatures that live in trees, kind of like a small monkey sized.

He identified them as belonging to the same family as modern sloths.

And so that was really confusing for him.

These are these giant sloth-like creatures that existed a very long time ago.

Where are they now? Did they just die out? Maybe something else happened to them.

And observations like these helped Darwin to understand that species were replaced by different versions of each other over time.

An observation that Wallace made was after studying thousands of plant and animal species, he noticed how different species were separated by different geographical boundaries, like rivers or mountain ranges.

One example of this is how the different species of howler monkey are found on different sides of the Amazon River.

This led him to think that species are adapted differently to their unique environments.

Darwin also noticed the important role that the environment played in the distribution of species.

So Darwin studied the animals on this group of islands off the coast of South America called the Galapagos.

He noticed differences in the physical characteristics between the animals that lived on different islands or in different places on the islands.

And this led him to think that species are adapted to their environment and that the best adapted species are more likely to survive.

And so a really famous example of this are Darwin's finches.

So you can see from the diagram here that their beaks are really highly specialised.

And the reason for that is because they're really adapted to the specific food that they eat.

So we've got the species number one marked here, has an enormous beak, and then all the way down to species number four, which has got a really tiny beak.

So they're all really closely related species, but with these really different beaks.

So the finches that live in areas with hard seeds and nuts have larger and harder beaks.

And that makes a lot of sense, right? If they're trying to crack open seeds with really hard shell casings, they're gonna need a much bigger and a much harder beak to do that.

The finches that live in areas that had fruit and insects available have much smaller and much narrower beaks 'cause that helps them to eat the insects and the fruit.

The finches have adaptations that make them better suited to the environment that they are in.

And making that observation about animals was really helpful in understanding and developing the theory of evolution by natural selection.

Darwin also took inspiration from selective breeding, although he called it artificial selection.

He carried out experiments crossbreeding pigeons, showing that you can change the characteristics of a species.

Through these crossbreeding experiments, Darwin actually showed that all the different types of pigeons and doves that you can get, they all originally descend from the rock dove or sometimes called the feral pigeon.

So this led him to think that a similar process occurs in nature, one in which nature shapes the changes in characteristics of species instead of humans doing it like he did in his experiment with the pigeons.

Okay, let's see if you understood that.

Darwin noticed similarities between fossils and living species.

This led him to think A, a process similar to artificial selection occurs in nature.

B, the traits of an organism acquired during its lifetime are passed onto the offspring.

Or C, species are replaced by different versions over time.

Take five seconds or if you want longer, you can pause the video.

Click play when you're ready to see the answer.

Okay, the answer is C, species are replaced by different versions over time.

Well done if you got that right.

So they went round the world collecting the observation and the evidence, and they both independently came to the same conclusion.

Organisms that are better adapted to their environment are more likely to survive, reproduce, and pass on their traits to their offspring.

This became known as natural selection, a process that they suggested drove evolution.

So when Wallace discovered this, he wrote to Darwin in 1957.

They didn't really know each other, but they were aware of each other as scientists who worked in the same field.

And Wallace shared his idea of natural selection with Darwin.

Darwin was really shocked to read that Wallace had the same explanation for evolution as he did.

He'd been working on this idea for 20 years.

He didn't know that someone else had a really similar idea.

So in 1958, they co-authored a scientific paper to share their findings with the rest of their scientific community.

And then in 1959, Darwin finally published the book he'd been working on for about 20 years, his now famous book, "On the Origin of the Species "By Means of Natural Selection." Okay, let's see if you understood that.

Wallace sent his findings to a fellow scientist, Charles Darwin.

This is because scientists, A, ask questions about the natural world.

B, make observations to collect evidence.

C, suggest explanations for how the natural world works.

Or D, share their findings with other scientists.

Take five seconds or click pause if you need more time.

Click play when you're ready to see the answer.

Okay, here's the answer.

It is D.

So this is because scientists share their findings with other scientists.

Well done if you got that right.

This is the second practise task of today's lesson.

Let's have a go at it.

Imagine that you're a newspaper of journalist in 1828 and Darwin and Wallace's scientific paper has just been published.

Write a short article that summarises their work.

Include the following in your article: the question Darwin and Wallace were trying to answer, one piece of evidence that helped them to develop their theory, and a simple explanation of their theory of evolution by natural selection.

So you'll need to pause the video now to give yourself enough time to answer the question.

Click play when you're ready for the answer.

Good luck.

Let's check your answer.

Did your article include the following information? So it needed to include the question they were trying to answer, and that was Darwin and Wallace wanted to know what caused the characteristics of species to change over time.

Did your article include one piece of evidence that helped them to develop their theory? So you could have put discovering a fossilised giant sloth led Darwin to believe that the species were replaced by different versions over time.

Wallace noticed that different species of howler monkeys lived on different sides of the Amazon River.

Darwin noticed that different finches that lived in areas with different food available had beaks that were highly specialised to their food.

And Darwin also noticed how the characteristics of organisms could be changed over time by selecting and cross-breeding pigeons.

Did your article include a simple explanation of their theory of evolution by natural selection? Organisms that are better adapted to their environment are more likely to survive, reproduce and pass on their heritable features to their offspring.

This is called natural selection.

It causes the features that are passed on to become more common in each generation, and it changes the common characteristics of species over generations.

Well done if you got that right.

So we have talked about what a theory means, and we've talked about evolution by natural selection.

We've also discussed the really important contributions that both Charles Darwin and Alfred Russel Wallace made to the development of the theory of evolution by natural selection.

And now we're gonna talk about the really important discoveries around DNA that helped support the theory of evolution by natural selection.

So really sadly, despite all of Darwin and Wallace's hard work, the theory of natural selection was not widely accepted at their time during their lifetime for a number of reasons.

One of the main reasons was that scientists at the time, including Darwin and Wallace, they couldn't explain how organisms passed on their traits to their offspring.

We now know that the answer is DNA, and it's the molecule that is passed on from parents to offspring, and it stores instructions that control our characteristics.

But it took the hard work of many, many other scientists for people to understand this.

Let's see if you're following so far.

True or false? The theory of evolution by natural selection was instantly accepted by the wider scientific community.

Is that true or false? Take five seconds or if you need some more thinking time, click pause, click, play when you're ready to see the answer.

That was false.

Okay, why was it false? Let's justify that answer.

Is it false because, A, it was rejected entirely due to lack of evidence? Or B, it was rejected in part because the mechanism for inheritance was not yet understood.

So again, take five seconds or if you want more time, click pause, click play when you're ready to see the answer.

The answer was B.

It was rejected in part 'cause the mechanism for inheritance was not yet understood.

Well done if you got that right.

So these are some of the scientists that helped develop our understanding of DNA and in a really similar way to how most people know the name Charles Darwin, but probably don't know the name Alfred Russel Wallace, there are people that have been left out of the story here.

It wasn't really possible or practical to put in the name of every scientist who's ever worked on DNA.

So this is a selection of the most prominent.

One of these scientists was Gregor Mendel, a monk born in Austria in 1822.

So Mendel carried out a series of experiments to discover the basic principles of heredity using pea plants in his monastery garden.

And so he took different types of peas, different colours, different textures, and bred them together and recorded the results.

And doing this over and over and over again helped him understand that the traits that organisms have must be passed on by some factor of inheritance.

And he couldn't explain what this factor was, where it was physically inside the organism.

And so a lot of his ideas weren't really accepted at the time.

So then we have in 1869, the chemical molecule DNA was discovered by the Swiss biochemist Friedrich Miescher.

He extracted DNA from the nucleus of white blood cells and worked out the chemical groups it was made up of, and he termed the newly discovered substance nuclein.

He suspected that this substance may have been involved with heredity, but he wasn't able to prove this.

That took the work of some other scientists.

In 1910, American evolutionary biologist Thomas Morgan began crossbreeding experiments on flies.

He concluded from his experiments that the traits were controlled and passed on from parents' offspring by genes arranged along chromosomes inside the cell's nucleus.

However, he did not know which molecule made up genes, and many people at the time thought it was probably protein.

Then many scientists over the years attempted to identify the molecule that made up genes, that the genetic material we have is made up of.

In 1943, a team of scientists finally proved that it was DNA.

DNA was the genetic material in cells that made up genes and controlled heredity.

Further really important work on DNA included the first X-ray image of DNA in 1952 by a scientist called Rosalind Franklin and the discovery of the structure of DNA.

The 3D structure of DNA in 1953 by James Watson and Francis Crick.

Okay, let's see if you understood that.

Put these events in order to describe how scientific ideas about inheritance changed over time.

A, Thomas Morgan concluded genes control inheritance.

B, a team of scientists in 1943 concluded DNA was genetic material.

C, Gregor Mendel concluded that the traits are passed on by a factor of inheritance.

D, Rosalind Franklin made an image of DNA using X-rays.

E, Friedrich Miescher discovered the DNA molecule.

Take five seconds or if you want some more thinking time, click pause, and click play when you're ready to see the answers.

Okay, let's have a look.

First of all, we have C, Gregor Mendel concluded that traits are passed on by a factor of inheritance.

E, Friedrich Miescher discovered the DNA molecule.

A, Thomas Morgan concluded genes control inheritance.

B, a team of scientists in 1943 concluded DNA was the genetic material.

So finally, D, Rosalind Franklin made an image of DNA using X-rays.

Great work if you got that right.

We're onto the final practise task of today's lesson.

So the first part of the practise task is to match the scientists to their discovery.

So we have the scientists the Gregor Mendel, Friedrich Miescher Rosalind Franklin, and then grouped together are James Watson and Francis Crick.

Their discoveries: X-ray image of DNA, 3D structure of DNA, traits passed on from parents offspring, extracted DNA molecule from the nucleus of a white blood cell.

So pause the video to think about that and match the scientist to their discovery.

Okay, let's take a look at your answers.

So Gregor Mendel discovered that traits are passed on from parent to offspring.

Friedrich Miescher extracted the DNA molecule from the nucleus of a white blood cell.

Rosalind Franklin created an X-ray image of DNA and James Watson and Francis Crick discovered the 3D structure of DNA.

Great work if you got those right.

Alex is learning about evolution in school.

He says, "Darwin played the most important role in the acceptance of the theory of evolution by natural selection." To what extent do you agree or disagree with Alex's statement? Explain your answer.

You'll need to pause the video now to give yourself enough thinking time.

Click play when you're ready to see the answer.

So this is a model answer, which includes some of the things that you might have included in yours.

Darwin played an important role in developing the theory of evolution by natural selection, but he was not the only scientist involved.

Alfred Russel Wallace conducted similar research to Darwin and came to the same conclusions as him.

Many other scientists contributed to the wider acceptance of the theory by helping to explain how traits were passed on from parents to offspring, including Mendel who investigated inheritance and scientists who identified DNA as the genetic material that stores the instructions for inherited traits.

Great work if you got that right.

Amazing work today.

Let's summarise today's lesson to help us remember it.

Scientists develop theories to explain how the natural world works.

Darwin and Wallace were scientists who helped develop the theory of evolution by natural selection.

They did this by travelling the world, making observations and collecting evidence to help them explain how and why species evolve.

The theory of evolution by natural selection was not widely accepted at the time, in part because the mechanism for how species passed on their traits to their offspring was not known at the time.

Many other scientists helped discover and explain the significance of DNA, which led to the wider acceptance of the theory of evolution by natural selection.

Again, really great work today.

I hope you enjoyed the lesson, and hopefully I see you again soon for our next lesson.