Lesson video

Welcome to this lesson from the Oak National Academy.

Today's lesson is on natural selection at the genetic level, and it's taken from the unit Variation and natural selection at the genetic level.

I'm Mrs. Wheat and I'm gonna be your teacher for today's lesson.

By the end of today's lesson, you'll be able to explain how natural selection leads to advantageous genetic variants becoming more common over generations.

Let's have a look at our keywords.

We've got four keywords today.

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

Adaptation.

A feature that organisms have that helps 'em to live in a particular place and survive.

A genetic variant is a region of DNA in which the sequence of nucleotide bases has been changed and phenotype, the observable traits of an organism.

So if you want to read those through again, I'll be quiet and disappear for five seconds, so you can do that.

But if you want even more time, pause the video so you can read through them or copy them down and then click Play when you're ready to move on with the lesson.

Today's lesson is in three parts.

First of all, we'll talk about mutations and natural selection.

Then we'll move on to an example of that in plants.

And finally, we'll look at an example of that in humans.

But first of all, let's talk about what mutations are and how they affect natural selection.

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

Let's look at a classic example of that.

So we have these moths here, they're called peppered moth, and they were an example of natural selection that occurred during the Victorian era that's really well researched and well studied.

So we can see in my picture here, let's look at the tree with the pale bark and the pale lichen.

So on that tree, we have two different types of moths.

They're the same species, but within this species, we've got different types.

We're just looking at the darker-colored moth and the lighter-colored moth within this species.

So on the tree with pale bark, it's harder to see the paler coloured moth.

It's camouflaged, and so those moths have got an advantage.

They're more likely to survive.

So if a predecessor's coming along, a bird is trying to eat the moss, the bird is less likely to eat the pale-colored moth.

And so those ones are the ones that are more likely to survive and reproduce and then pass on their genes to their offspring and have pale-colored offspring as well.

If there's a sudden environmental change, which did actually happen, pollution from the Industrial Revolution caused a lot of trees near cities to turn, their lichen to turn darker.

their lichen to turn darker and the bark of the tree to turn darker.

So if we look in that example, you can see that the darker-colored moth is now got the advantage.

It's camouflaged, much harder to see.

So the same process, natural selection happens.

The darker-colored moth has an advantage.

It's less likely to be eaten by birds, so more likely to survive, reproduce and have darker-colored offspring.

So that's an example of natural selection.

And hopefully you can see from the example that that isn't possible.

So in order for natural selection to occur, there must be variation within a population, within a species.

If all these moths are exactly the same, they would all equally be likely to survive and reproduce, pass on their genes.

It's the fact that there is variation.

Some are darker and some are paler, which means that they are differently advantaged.

They're in competition for a finite resource, or they're in competition from, well, they're trying to hide from these birds and not get eaten by these birds.

And so that variation is what is driving natural selection.

So in this part of the lesson we'll talk about, well, where does variation come from? So the variation we see in species is a result of mutations in DNA, and let's talk specifically about what that means.

Mutations are changes in the nucleotide base sequence of DNA.

If you can't remember or you haven't learned that yet, let's recap what those words, nucleotide base sequence DNA mean.

Here we have one strand of DNA.

DNA is usually two-stranded, and the strands are wrapped around together to make a helix shape.

We are just gonna look at one strand of DNA right now.

DNA is a molecule, it's a polymer, which means it's made up of lots of tiny repeater units.

And these repeater units are called nucleotides.

Part of the nucleotide molecule are bases.

There are four bases: adenine, guanine, cytosine, and thymine, we're gonna talk about, but we abbreviate those at GCSE to A, T, C, and G.

So each nucleotide can have one of four bases, and those bases form a code which tell our body which amino acids to make.

So let's look at our initial statement again.

Mutations are changes in the nucleotide base sequence of DNA.

So if one of those letters, A, T, C, or G changes somehow, that creates a mutation in our DNA.

So these changes create a genetic variant.

A genetic variant is a region of DNA in which the sequence of nucleotide base has been changed.

So if we say that this strand of DNA here is our original sequence in DNA, this is what it was originally like, and now we have a strand of DNA which is almost exactly the same, but this one is the genetic variant.

Can you see the difference between them? Can you see where the mutation is? Take five seconds to see if you can spot where the mutation is, or pause video, if you want more thinking time, click Play when you're ready to see the answer.

Okay, there it is.

<v ->So all the other bases, those letters</v> A, T, C, and G, are the same except this one.

So in the original sequence of DNA, it was an A.

The genetic variant sequence of DNA, it's a C, and that change can have consequences.

Most mutations, so remember that's the changes in the base sequence of DNA, have no effect on an individual's phenotype, but some do.

Now, that word phenotype, that means the observable characteristics of an organism.

So whether you have freckles, whether you have a genetic condition like type one diabetes, your hair colour, your eye colour, whether or not you're left-handed or right-handed, all of those characteristics, that makes up your phenotype.

So some mutations create base triplets that code for different amino acids, which can change the protein that is synthesised and causes differences in phenotype.

That's quite abstract, so let's have a look at some diagrams that will make it a bit more concrete.

So here we have the original base sequence again.

And so each of those three bases, the triplet codes, they will code for a different amino acid.

So here we have J, GAA, that codes for glutamic acid, that's amino acid.

You don't need to know any names of amino acids, by the way.

I'm just showing you as an example.

And then the next three bases, that's TCA, that makes serine.

So if we look at a genetic variable, we've got one or more of those which are different.

So in this example, the second base, which was an A in the first one is now a T.

So that makes a different amino acid that makes an amino acid called valine.

And the next three makes serine again.

So that change can have consequences for the phenotype.

It might not, but it might.

So if a mutation does happen to change phenotype, the change can be harmful.

For example, some elephants inherent a mutation that causes females to be born without tusks and males to die before birth.

They can be advantageous.

A mutation that leads to birds having longer beaks mean that they're more able to get food from bird feeders or they could have no effect at all.

So human beings' earlobes can be either attached or detached.

That doesn't really confer any kind of evolutionary advantage.

It just kind of is.

So you might hear the word mutation and be like, oh, this is some really harmful thing, possibly caused by radiation, but a mutation doesn't necessarily do anything to your phenotype.

Okay, let's see if you understood that.

True or false? Mutations in DNA are always harmful.

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

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

It's false.

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

Is it false because, mutations can have no effect, be harmful or be useful, or because mutations are always useful to the individual? Again, take five seconds or pause the video if you want more time.

It is false because mutations can have no effect.

They can be harmful or they can be useful.

Well done if you got that right.

Genetic variants that code for advantageous traits become more common over generations because of natural selection.

And this is because organisms that have advantageous traits are more likely to be better adapted to their environment, more likely to survive, more likely to reproduce, and more likely to be the ones that pass on their genetic variance to the next generation.

So here we've got another classic example of natural selection.

So these are Darwin's finches or the Galapagos finches, and they have beaks that are really highly specialised, highly adapted to the type of food that they eat.

And so we have a finch that's got a really huge beak and a finch that's got a really small beak.

And these differences in beak size are caused by genetic variants.

And that leads to some of the birds being better adapted to different areas.

And so then they will thrive while other birds who are not as well adapted to that to eat the food in that area are more likely to die.

Okay, let's see if you understood that.

Which statement is true? Advantageous genetic variants become less common over generation.

Advantageous genetic variants become more common over generations.

Advantageous genetic variants do not become any more or any less common over generations.

Take five seconds or if you want more time, click Pause, click Play when you're ready to see the answer.

The correct answer is B, advantageous genetic variants become more common over generations.

Well done if you got that right.

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

Explain what a genetic variant is and what it is caused by.

What is the name of the process that causes advantageous genetic variants to become more common over generations? And in your own words, explain why advantageous genetic variants become more common over generations.

Okay, you'll need to pause the video now to give yourself enough time to have a go at those questions.

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

Let's have a look at the answers.

Explain what a genetic variant is and what it is caused by.

A genetic variant is a region of DNA in which the sequence of nucleotide bases has been changed.

It's caused by a mutation.

Number two, what is the name of the process that causes advantageous genetic variants to become more common over generations? That's natural selection.

Number three.

In your own words, explain why advantageous genetic variants become more common over generations.

So advantageous genetic variants cause advantageous traits in the phenotype of organisms. Individuals that have these advantageous traits are better adapted to their environment.

Due to natural selection, they are more likely to survive and reproduce.

Therefore, they are more likely to be the ones who pass on their genetic variants to the next generation.

Well done if you got those right.

We finished the first part of today's lesson, and now, we're gonna move on and talk about an example of mutations in natural selection in plants.

I'm gonna start by introducing the plant we're gonna talk about.

So we're talking about white clover.

And white clover is a species of plant that grows across Europe, Asia, and North America.

It grows in grassy, rural areas and has become common in grassy areas in cities as well.

The white clover plant produces hydrogen cyanide, a poisonous compound that deters herbivores from eating it.

However, the white clover plant in urban areas produces much less hydrogen cyanide.

And scientists take the natural selection as the reason for this change.

Now we're gonna talk about the steps of natural selection that led to this change in the white clover.

So natural selection starts with variation.

A random mutation of DNA of the white clover plant created a genetic variant.

Individual clover plants with this genetic variant produced much less hydrogen cyanide.

This is an example of variation and to visualise this on the DNA level, so here we have represented the original sequence in DNA of the white clover, okay? And watch what happens.

This represents the genetic variant, the one that produces much less hydrogen cyanide.

So we can see that there's a base that's different.

It's not always one base.

It could be far more than that.

And so that change in the nucleotide base sequence that's causing a mutation, which has led to this genetic variant, the clovers that don't produce hydrogen cyanide.

So after variation, there's advantage.

So in cities, the white clover plant that produce less hydrogen cyanide had an advantage.

And this is because the white clover plants in cities don't need to deter herbivores.

There aren't really like sheep loose grazing in cities generally.

So producing a lot of hydrogen cyanide was a waste of energy and nutrients which are finite.

Not producing as much hydrogen cyanide was an advantageous adaptation because these individuals, the white clover that were in cities, they could put more of their energy and more of their nutrients into growth as opposed to producing as hydrogen cyanide that wasn't really doing anything.

So advantage leads to competition.

White clover plants, like all plants, are in competition with each other for limited resources such as light, seed dispersers, pollinators, space, water, and minerals.

The white clover plants that produced less hydrogen cyanide were better adapted to compete because they could grow larger.

They were more likely to compete successfully for resources as they are devoting less of their energy to producing this hydrogen cyanide.

After competition, we have survival, reproduction, and inheritance.

So due to natural selection, white clover plants that produce less hydrogen cyanide were more likely to survive and reproduce.

Many of the offspring inherited this advantageous genetic variant.

So it became more common in each generation.

After many generations, the majority of urban white clover plants produced far less hydrogen cyanide.

Okay, let's check to see if you understood that.

Why do white clover plants in cities not produce hydrogen cyanide? Is it, A, because the white clover plants cleverly chose to adapt to their surroundings as there were no herbivores in the cities, or is it B, random mutations in every white clover plant in cities created a genetic variants that cause 'em to not produce hydrogen cyanide.

C, hydrogen cyanide is poisonous to the white clovers.

D, those that didn't produce hydrogen cyanide were better adapted, so were more likely to survive and reproduce.

Take five seconds or if you want more thinking time, click Pause, click Play when you're ready to see the answer.

The answer is D.

Those that didn't produce hydrogen cyanide were better adapted so were more likely to survive and reproduce.

Let's look at why the other answers are wrong.

So A, the white clover plants clearly chose to adapt.

Organisms can't choose to be better adapted to the surroundings.

I can't decide to be taller 'cause it's helpful.

I just am the height that I am based on, 80% is genetics, 20% is environment, but my choice isn't really a part of that at all.

We happen to be better adapted or worse adapted to our environment.

B, random mutations in every white clover plant.

So it wasn't in every white clover plant, it was in a few white clover plants.

And then eventually, that became more common over time.

C, hydrogen cyanide is poisonous to white clover plants.

That was a misunderstanding of the example there.

Hydrogen cyanide is poisonous to the herbivores, which eat white clover plants.

Okay, hopefully that made sense and well done if you got that right.

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

Cacti have evolved to have spines instead of leaves.

The spines are an adaptation that enable cacti to live in desert habitats, which are very dry.

Mutations in the ancestors of cacti created genetic variants that cause spines to develop instead of leaves.

Explain why these genetic variants became more common as cacti evolved over many generations.

You'll need to pause a video now to give yourself enough time to think about your answer and to write it down.

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

Let's check your answer.

Plants lose water through their leaves.

The genetic variants gave cacti the advantageous trait of having spines instead of leaves, which reduced water loss.

The spines also projected the water-filled stem of cacti from being eaten by animals.

Early cacti that had this advantageous trait of spines were better adapted to survive in the dry desert environment.

Due to natural selection, they were more likely to survive and reproduce, so more likely to pass on the genetic variants to the next generation.

This repeated over generations, so the genetic variants became more common in each generation.

Well, I dunno if you got that right.

Okay, so we looked at what a mutation is and how that affects natural selection.

We've talked about an example of that in plants, and now we're gonna talk about an example in humans.

Humans and most mammals drink milk when they are young.

Some lose the ability to digest milk as they grow older.

The enzyme lactase is needed to break down lactose sugar present in the dairy products such as milk.

In most mammals, once infants start eating solid food, they stop producing this enzyme lactase so they can no longer digest the sugar lactose.

So where do humans fit into this? Around 10,000 years ago, mutations occurred that created genetic variants in humans that made them lactose tolerant.

People with these genetic variants continue to produce the lactase enzyme into adulthood, which enabled them to digest lactose.

So that's what lactose tolerance is, the ability to continue to digest lactose out of infancy and into adulthood.

Roughly half of humans now have these genetic variants, although they're distributed really unevenly across the world.

So in some cultures, you have very, very low lactose tolerance or lactose intolerance.

And in other cultures and populations, you have extremely high lactose tolerance.

Lactose tolerance is incredibly common among Bedouin people, and they live in North Africa and in the Middle East.

The Maasai in East Africa have incredibly high levels of lactose tolerance and also Irish people and Scandinavian people, really, really high levels of lactose tolerance.

So scientists think that natural selection led to the prevalence of these genetic variants in some areas.

Let's see if you understood that.

What has led to lactose tolerance becoming very common in some populations of human beings? A, natural selection, B, random chance, or C, selective breeding? Take five seconds to think about it or click Pause.

If you want more time, click Play when you're ready to see the answer.

It is A, natural selection.

Great job if you got that right.

So let's go through the steps of natural selection again, but with this example, this time.

So first of all, variation.

A random mutation created a genetic variant.

Individuals with this genetic variant continue to produce the lactase enzyme into adulthood.

This is a example of variation.

So to visualise that, again, we've got our original sequence of DNA, made up of nucleotides repeating over and over and over again with different bases.

And here we have the genetic variant.

And on the penultimate nucleotide, we have a different base.

So the top one, we have a green one, the bottom one, we have a blue one.

And those are representing the basis A, T, C, or G.

Right, next.

We've got advantage and competition.

In populations which animals are farmed for milk, being able to digest lactose is an advantageous adaptation.

I just wanna pause here before anyone starts picking on the lactose intolerant person in the room saying like, I'm more evolved than you.

It's not true.

That's not how evolution works.

You're not more evolved and more complex than someone because you can drink milk, good for you.

It's just about, in specific populations, where there's limited food or the only food would be drinking milk from farm animals, that's an advantage.

Now, drinking milk's not really an advantage at all.

We have huge supermarkets.

We can get so many different alternatives for food and for dairy alternatives.

So this was a historic advantage.

It's not an advantage anymore.

So I'm sorry, you are not more evolved, right? So let's start again.

In populations in which animals are farmed for milk, being able to digest lactose is an advantageous adaptation.

So again, we're talking 10,000 years ago.

People with the genetic variants had more people with, so this genetic variant had more food sources available to them, and this gave them an advantage.

All organisms compete for finite resources such as mates, territory and food.

And individuals with the ability to digest lactose into adulthood competed more successfully for food.

So next, we have survival, reproduction, inheritance.

Due to natural selection, individuals with these genetic variants were more likely to survive as they had access to more food sources.

This meant they were more likely to reproduce.

Many of their offspring inherited these advantageous genetic variants, which became more common in each generation and eventually, the majority of individuals in certain parts of the world, like I mentioned before, if you are Bedouin, if you're Maasai, if you are Irish, or if you are Scandinavian ancestry.

So in those areas, it was an advantage and it was an adaptation.

So yeah, like I said, for example, more than 90% of people with Irish descent are able to digest lactose into adulthood.

So for whatever reason, at the time that that mutation came into the population of Irish people, it was incredibly advantageous to be able to digest milk due to their specific food environment.

And that's led to incredibly high tolerance amongst Irish people.

Okay, let's check to see if we're following that.

Put these statements in order to describe the process of natural selection.

A, random mutations can lead to variation in a population.

B, this makes them more likely to survive and reproduce.

C, some mutations create genetic variants that cause adaptations, which give individuals an advantage.

D, many of the offspring produced have the genetic variants and over generations, they become more common.

E, these adaptations allow 'em to compete successfully.

Take five seconds or if you want more time, pause the video, click Play when you're ready to see the answer.

Right, let's put these in order.

So the first one is, A, random mutations can lead to variation in a population.

Next, we have C.

Some mutations create genetic variants that cause adaptations, which give individuals an advantage.

Then we have E, these adaptations allow them to compete successfully.

Next is B.

This makes them more likely to survive and reproduce.

And finally D.

Many of the offspring produced have these genetic variants and over generations, they become more common.

Great job if you got that right.

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

Andeep and Lucas are learning about evolution in school.

They learn fossil show that humans with thicker and stronger face bones became more common, as the human species evolved.

They say, so Andeep says, "Natural selection caused stronger face bones to appear in humans for the first time." Lucas says, "Natural selection caused stronger face bones to become more common in population over generations." Who is correct and who is incorrect? Explain your answer.

So you'll need to pause a video now to give yourself enough time to think about your answer and to write it down, click Play when you're ready to see the answer.

Good luck.

Okay, let's have a look at your answer.

So who's understanding is correct? Explain your answer.

Andeep is incorrect.

Random mutations would have caused stronger face bones in humans to appear for the first time.

He said natural selection caused it to appear for the first time.

Lucas is correct.

Natural selection causes advantageous traits, and the genetic variants that cause them, to become more common over generations.

Great job if you got that right.

Okay, let's try this next bit.

Individuals with stronger face bones were more likely to survive fights and can then reproduce, passing on their features to the next generation.

So number two, describe in steps how natural selection led to stronger face bones becoming more common as the human species evolved.

Include the following ideas in your answer.

Variation, mutation, genetic variant, advantage, competition, survival, reproduction, inheritance.

Again, pause the video now to give yourself enough time to think about your answer and to write it down, click Play when you're ready to see the answer.

Okay, let's look at your answer.

Describe in steps how natural selection led to stronger face bones becoming more common after human species evolved.

There was variation among humans.

Random mutations created genetic variants that gave some individuals stronger face bones.

Individuals with these genetic variants were better adapted to their environment and had an advantage over those that had weaker face bones.

Individuals with stronger face bones were more likely survive fights and reproduce.

Many of their offspring inherited the advantageous genetic variants that caused stronger face bones, and eventually, over many generations, all humans have stronger face bones.

Great job if you got that right.

We have nearly finished today's lesson.

Let's summarise what we learned to help it stay in our memories.

The process of natural selection involves following steps.

Number one, variation.

Due to genetic variants caused by random mutations, there is variation among populations.

Next, we have advantage.

Some genetic variants cause adaptations in the phenotype, observable characteristics of an organism, which give individuals an advantage.

Three, competition.

Having an advantage allows individuals to compete more successfully.

Four, survival.

They're more likely to survive.

Five, reproduction.

They're more likely to have offspring.

And finally, six, inheritance.

Many of the offspring inherit the genetic variants and over generations, have become more common in the population.

Really great work today.

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