Welcome to this lesson from the Oak National Academy.

Today's lesson is about sexual and asexual reproduction.

And because it's taken from the unit, Variation and natural selection at the genetic level, we'll be talking about this in the context of evolution and natural selection.

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 explain how sexual and asexual reproduction affect variation and evolution within populations of organisms. Let's have a look at our keywords.

We have five keywords for today's lesson, which are evolution, the process in which the characteristics of species change over many generations, sometimes becoming new species; genetic variation, differences between individuals that are caused by the genetic material that they inherit; sexual reproduction, the process of creating offspring through the fusing of gametes; asexual reproduction, reproduction involving one parent, giving genetically identical offspring; and finally, genome, all of the genetic material of an organism.

So if you want some more time to think about that and process that, I'll be quiet for five seconds so you can read those through.

But if you want longer than five seconds, then I suggest you pause the video to give yourself enough time to read through them thoroughly or maybe copy them down, and then click play when you're ready to continue with the lesson.

Today's lesson is in two parts.

First, we'll talk about sexual reproduction and then we'll talk about asexual reproduction.

And, again, for both types of reproduction, we will talk about them in the context of variation, natural selection, and evolution.

So let's get started by talking about sexual reproduction.

There must be genetic variation within a population of organisms for evolution to occur.

When there is variation, some individuals may be better adapted to their environment than others, and this leads to natural selection where the better adapted individuals survive and reproduce.

So, if we look at giraffes as an example, so giraffes with longer necks may compete more successfully for food.

You may have wondered why giraffes have such long necks, and the reason behind that is natural selection.

So we can see from the picture here that some of the giraffes have shorter necks, some of them have longer necks.

Over time, the giraffes with longer neck were favoured by natural selection.

So, giraffes with longer necks found it easier to reach up and get leaves higher up on trees, which is their main food source, and so they were more likely, they had this advantage of having a longer neck, so they were more likely to survive and reproduce and pass on their genes to their offspring.

Over time, over many generations, this pattern continued until we have giraffes today that all have much longer necks than their ancestral species.

So without variation, so again, if all these giraffes had the same neck length, there's no variation, without this variation, there's no advantage when competing for resources, such as leaves from a tree, so individuals are equally likely to pass on their genes to the next generation.

So variation is essential for natural selection to occur, and natural selection is one of the main mechanisms by which evolution, this change in species over many generations, how that occurs.

So genetic variation is super important, and it can arise as a result of the way in which organisms reproduce.

Now, all living organisms reproduce, it is a common process of all living things.

So I've got lots of pictures here, lots of different types of organisms. We've got a bacterium, we've got a tuna fish, we've got a worm, a sea urchin, some mushrooms, which represent the kingdom Fungi.

We've got trees, owls, flowers, and insects.

All of those incredibly different living things need to reproduce, need to create more of themselves, need to make offspring.

So, there are two forms of reproduction: sexual reproduction and asexual reproduction.

So, let's take a break for a second and let's see if we understood that.

True or false? Sex reproduction is a life process that all organisms carry out.

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.

Okay, let's check out the answer.

It is false.

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

Is it false because some organisms reproduce asexually? Or is it false because not all organisms reproduce? Again, take five seconds, or if you want some more time to think about it, click pause, click play when you're ready to see the answer.

So the correct answer is A, some organisms reproduce asexually.

So B is incorrect because all living organisms reproduce.

It's one of the key life processes for living organisms. But not all organisms sexually reproduce.

Some reproduce asexually.

Well done if you got that right.

Most animals and plants have the ability to reproduce sexually.

So they do this by making cells called gametes.

And gametes contain half of the genome of the parent or the organism that's trying to do the reproducing.

Remember that the word genome means all the genetic information in an organism.

So gametes contain half of the genome, or the genetic information in an organism, and we'll come back to why that is in a few slides time.

So in animals, male gametes are called sperm cells and female gametes are called egg cells or ova.

Here we've got a sperm cell.

Here we've got an egg cell.

So egg cells are much, much bigger than sperm cells.

This isn't to scale.

This is just to give you a rough idea of the fact that egg cells are bigger and some of the main structures.

So here we have the nucleus, and inside the nucleus we have the chromosomes, which store the genetic information, the DNA.

So here we're gonna look at where those gametes are produced.

So first in animals, so testes produce male gametes.

This is a diagram of the male reproductive system but on the inside.

This is what it would look like if you were performing surgery on the male reproductive system.

So we've got these two structures here, either side of the penis, and those are the testes, and their job is to make sperm, the male gametes.

Here we have a diagram of the female reproductive system.

Again, this is what it would look like internally if you were performing surgery on the female reproductive system.

And we have these two structures, either sides of the uterus, called ovaries, and they produce female gametes, the egg cells, or you could also call them ova.

Now we're gonna look at where these are produced in plants.

So we're looking specifically at flowering plants, and flowering plants produce pollen grains, which contain male gametes.

The ovary contains the female gametes.

Now we know what a gamete is and where those gametes are made, we can talk about the process of sexual reproduction.

So in sexual reproduction, a male gamete, the sperm, and a female gamete, the egg cells, fuse in a process called fertilisation.

So in my diagram here, we've got the female gamete and we've got the male gamete, and those join together, or fuse, to create this new cell called a zygote.

So fertilisation creates a new cell called a zygote, and this zygote has a complete genome, all of the genetic information of an organism, and eventually develops into the adult form of an organism.

So if you remember, I said previously that the gametes contain half of a genome, and this is because a gamete's job is to fuse with another gamete.

And so they both have half the amount of DNA they need that when they join together, the new organism that's created, the zygote, has a full set of DNA.

And this zygote then eventually grows to become born humans.

It becomes an embryo, a foetus, it will be born, it becomes a baby, a child, an adult.

In a plant, it's a seed, turns into a seedling, turns into eventually the full, like, adult form of the organism, the plant itself.

A key feature of sexual reproduction is that it produces offspring that are genetically different from one another and from their parents.

I'm sure you know from observing the families around you that even though people who are biology related look similar, they don't look identical unless they're identical twins.

And the reason for this is because of the genes we inherit.

So we have two different versions for every single gene we have.

And a gene is a unit of DNA that controls a characteristic by making a protein.

So these different versions of genes are called alleles.

So here we have here a diagram of a very, very simplified cell.

I'm just showing the chromosomes.

I'm not showing the nucleus, I'm not showing ribosomes, 'cause I just wanna focus on what's happening with the chromosomes.

Now, it's important to mention that these kind of X-shaped chromosomes, this is what chromosomes look like after they've been replicated so that they can then split and reproduce into new cells.

So, we've got an organism here which has got two chromosomes.

These are chromosomes.

I'm showing red as this chromosome this organism inherited from its mother, and the blue is representing that this chromosome is the chromosome that was inherited from this organism's father.

And the point here in yellow that I've labelled, these are the organism's alleles, those versions of a gene.

So for example, I've got brown eyes, my mom has brown eyes, my dad has blue eyes.

So in my DNA, I will have the allele that controls brown eyes and the allele that controls blue eyes, even though that my phenotype, what you can see when you look at me is I've got brown eyes, I carry that allele from my father on some of my chromosomes that causes blue eyes.

So one set of alleles comes from our biological mother, as I've said, and the other comes from our biological father.

And during meiosis, which is this process which creates gametes, each gamete receives a randomly selected allele.

So, first part of meiosis here, we've got our cell, it's divided into two, and the the male chromosome, and the chromosome that came from the mother and the chromosome that came from the father are randomly distributed into one of those two new cells.

This happens again, so we start with one cell and we end up with four cells, which have been randomly allocated alleles from the mother and alleles from the father.

And it's this random selection of alleles which makes all the gametes genetically different from one another, which is why you will look different from your siblings unless you're an identical twin because you were made from a different egg cell and a different sperm cell from your biological mothers and fathers, which had alleles, different alleles that are randomly distributed amongst them.

So these cells that I've created here don't seem very different, and that's because I've just created an organism here which has got two chromosomes.

We have 46 chromosomes.

So you're gonna have to imagine this process happening but 46 times over.

So there's 23 chromosomes from your mother, your biological mother, and there's 23 chromosomes from your biological father.

This process is happening for all of them.

So each of those gametes is a really random selection of alleles from your mother and alleles from your father due to this random assortment of alleles.

So this random assortment of alleles and the gametes means that sexual reproduction leads to genetic variation amongst the offspring.

Genetic variation helps species survive environmental changes because some individuals will have beneficial traits, giving them a survival advantage through natural selection.

So if we think about these piglings.

Piglings? Piglets.

These are all siblings.

So, they are all siblings.

They've all got the same biological mother, same biological father, but they all look different.

And so being different is an advantage, 'cause if we have a sudden environmental change, for example, a change in climate, perhaps temperature drops five degrees where they're living.

So if we have one of these piglets, 'cause they're all different, perhaps one is hairier, and that would then mean it is better insulated.

So it will radiate less heat away from its body and then it's more likely to survive colder temperatures.

So, the fact that there are differences among the piglets mean that it's more likely that one of them will have an advantage which helps 'em to survive a sudden change.

If they are all exactly the same, so they're all exactly the same and they're all exactly equally suited to their current environment, again, they're not able then to cope with any environmental change.

So that's a really big advantage of sexual reproduction.

It creates this differences, this variation, which makes organisms less susceptible to sudden environmental change.

A disadvantage, however, of sexual reproduction is that it produces offspring at a much slower rate than asexual reproduction.

We'll talk about that in more detail in the second half of the lesson, but just briefly.

Organisms have to go out and choose and find a mate, that takes time.

Organisms, mammals have to become pregnant, that takes time.

Obviously, gestation in human beings, pregnancy in human beings, takes nine months, that takes time.

Organisms have to reach sexual maturity.

So you are not born with the ability to sexually reproduce, that develops over time.

So, all of these things, we'll talk, again, we'll talk about more in detail in the second half of the lesson.

But all of those things contribute to the fact that sexual reproduction is a much more time-consuming process than asexual reproduction.

Okay, let's see if you understood that.

Which of the following statements are true? Sexual reproduction produces genetically identical offspring.

During meiosis, alleles are randomly distributed among gametes.

Plants can reproduce sexually.

Each gamete contains an organism's entire genome.

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

Let's have a look at the answers.

So, the correct answers are B, during meiosis, alleles are randomly distributed among gametes, and C, plants can reproduce sexually.

So if we look at why A is wrong, sexual reproduction produces genetically identical offspring, that should say genetically different offspring.

And D, each gamete contains an organism's entire genome.

It should say that each gamete contains half of an organism's genome.

Well done if you got that right.

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

So answer the questions and complete the task below.

So number one, the photo shows two parents and their children.

Explain why there is genetic variation among the siblings even though they have the same biological parents.

And number two, describe how sexual reproduction leads to opportunities for evolution to take place.

So you'll need to 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 the answers.

So number one, explain why there is genetic variation among the siblings even though they have the same biological parents.

Even though the siblings have the same parents, they were all created using different gametes.

Each gamete an organism produces is unique due to the maternal and paternal alleles being randomly distributed among gametes when they are created.

Great job if you got that right.

Okay, number two, describe how sexual reproduction leads to opportunities for evolution to take place.

Sexual reproduction leads to genetic variation among the offspring produced.

Variation is essential for evolution to take place.

In evolution, by natural selection, the best adapted organisms are more likely to survive and reproduce.

If there is genetic variation among organisms, then there is more opportunity for individuals to be better adapted to their environment.

Really great job if you got that right.

We've done the first half of today's lesson, sexual reproduction, now we're gonna talk about asexual reproduction in the context of variation, natural selection, and evolution.

Not all organisms reproduce sexually.

Most unicellular organisms produce asexually.

So in asexual reproduction, the genome of the cell is copied and the cell divides into two genetically identical cells.

So in bacteria, in prokaryotes such as bacteria, they reproduce asexually by a process called binary fission, which is similar to mitosis, but bacteria don't have nuclei and other structures, so we call this process binary fission.

So, many unicellular eukaryotes asexually reproduce by mitosis.

But it's not just unicellular organisms that reproduce asexually, many multicellular organisms can also reproduce asexually.

So, plants such as spider plants and strawberry plants produce flowers for sexual reproduction.

Remember in flowering plants, they produce pollen, and they have an ovary which contain the gametes in order for sexual reproduction.

So these flowering plants, some of them also, such as spider plants and strawberry plants, can produce genetically identical offspring through asexual reproduction.

So here's a picture of a spider plant.

We have a horizontal stem going off the side, which is called a runner.

At the end of this horizontal stem, you can see these tiny little baby spider plants.

And those are clones, which means exactly the same DNA, genetically identical, they are clones of the parent spider plant.

So, this spider plant can produce tiny white flowers for sexual reproduction, but it can also produce these clones through asexual reproduction as well.

And some animals are also able to asexually reproduce.

So in the absence of males, female hammerhead sharks are able to use asexual reproduction to produce genetically identical offspring.

They kind of become pregnant with a clone of themselves, which is so interesting.

There's a few fish and there's a few reptiles which is able to do this and I think it's really, really fascinating.

Okay, let's see if you understood that.

True or false? Only microorganisms can reproduce asexually.

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

Okay, that was false.

Let's justify the answer.

Why was that false? Was it false because plants and animals can also reproduce asexually? Or was it false because living organisms all use sexual reproduction to reproduce? Again, take five seconds or pause the video if you want more time.

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

Okay, it was false because, A, plants and animals can also reproduce asexually.

Well done if you got that right.

The main advantage of asexual reproduction is that it generally occurs at a faster rate than sexual reproduction.

For example, in optimal conditions, some bacteria can divide every 20 minutes.

For sexual reproduction, however, an organism must reach sexual maturity before it can reproduce.

And this can take weeks, organisms like mice can start reproducing at around four weeks, and this can take months or even years depending on the organism.

Giant horses take 20 years to reach sexual maturity.

So that's a long time for an organism to be waiting to reproduce.

Also, asexual reproduction only requires one organism to reproduce.

Sexual reproduction requires two organisms, or at least two different gametes to reproduce.

In asexual reproduction, you have one cell which is dividing into two, so only one organism is needed.

You don't need to then go out and find a mate in order to reproduce.

A disadvantage of asexual reproduction is that the offspring produced are genetically identical to the parent.

The only opportunities for genetic variation to occur during asexual reproduction are if: an error is made when the genome is copied before the cell divides or the organism encounters a mutagenic substance such as UV radiation.

So here we have some DNA sequences.

Each of these molecules is a nucleotide.

And on these nucleotides, we have bases, which make up the genetic code.

So you can see in my genetic variant line of, in my genetic variant line of nucleotides, the last nucleotide is different.

It's changed.

There was a mutation.

So these kinds of mistakes, these mutations, can be made by copying errors when the cell is dividing or if a substance like UV radiation, which is mutagenic, comes into contact with the cell that is reproducing.

So this means that there is less variation among organisms that only asexually reproduce.

And this lack of variation means there are fewer opportunities to evolve and it makes them vulnerable to sudden changes in the environment.

Let's look at an example of that.

So some of the food crops we eat are clones.

For example, the bananas we buy from supermarket are all clones.

And clones, this means they are genetically identical as they are produced through asexual reproduction.

So all the bananas we buy in the supermarket are genetically identical to each other and to their parent organism.

So because of their lack of genetic variation, they are more susceptible to sudden changes to the environment, for example, a new disease.

So, this did happen.

So for example, in the 1950s, the main type of cultivated banana, called the Gros Michel, was wiped out by a fungal disease called Panama disease.

So there used to be a type of banana which was better than the current cultivated banana that we eat.

Apparently it was sweeter, had a more bananary taste, and the skin was tougher, so it made them more easy to transport around the world, but they were all wiped out by Panama disease.

And so the bananas were susceptible to this disease as they're all clones from each other and none of them had any resistance.

So that's an example of how genetic variation is useful in organisms. Okay, let's see if we understood that.

Let's check to see if that made sense.

Which of the following statements are true? A, the rate of asexual reproduction is faster than sexual reproduction.

B, there is less opportunity for organisms to evolve in sexual reproduction.

C, cells made by asexual reproduction are genetically identical.

Or D, bacteria cannot evolve because they reproduce asexually.

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

Let's look at the answers.

So the correct answers are A, the rate of asexual reproduction is faster than sexual reproduction, and C, cells made by asexual reproduction are genetically identical.

Let's look at the incorrect answers.

There's less opportunity for organisms to evolve in sexual reproduction.

That's false because there's actually more opportunity.

And D, bacteria cannot evolve 'cause they reproduce asexually.

That's false because bacteria can still evolve, but they have less genetic variation.

Having less genetic variation means that there are fewer opportunities for natural selection to occur, but there is still some genetic variation due to copying errors and due to mutagenic materials.

So evolution can occur in bacteria.

Well done if you got that right.

Okay, it's time for some maths.

You might remember that I said that in optimal conditions, some bacteria can reproduce every single 20 minutes.

So you are expected in some exams to be able to calculate how many bacteria there are after a certain length of time.

So, the following equation predicts how many bacteria there will be after a certain amount of time.

So it's the bacteria at the end of this time period is equal to how many bacteria you had at the beginning times 2 to the power of the number of times they've divided, okay? And we're gonna look at some examples of how to use that equation.

This is an example of a question that you could be asked in an exam.

A single bacterium develops antibiotic resistance due to a mutation.

It takes 20 minutes to reproduce.

After 4 hours, how many antibiotic-resistant bacteria will there be in the colony, this group of bacteria? So how we would do this is the first step is we need to figure out how many divisions there have been.

We have divisions per hour is equal to 60, so that's because there's 60 minutes in an hour, divided by 20.

It told you in this question that it takes 20 minutes for this bacteria to reproduce.

So 60, number of minutes in an hour, divided by 20, the time it takes for this bacteria to reproduce, equals 3.

So there are 3 divisions per hour.

The next thing we need to do is figure out the total number of divisions.

So for that, we need to know the time period specified in this question.

So it says in the question that after 4 hours, how many bacteria are there? So we have 4, the number of hours, times the number of divisions per hour, which we calculated in the previous line as 3.

So 4 times 3 equals 12.

Now we're ready to use the equation.

So the number of bacteria equals 1, it said at the start of this question there is a single bacterium, that's 1, times 2, that 2 stays the same always when we're using this equation, to the power of the number of divisions, which we calculated in the previous line.

So we have 1 times 2 to the power of 12, which equals 4,096.

Okay, now your turn.

A single bacterium develops antibiotic resistance due to a mutation.

It takes 30 minutes to reproduce.

After 7 hours, how many antibiotic-resistant bacteria will there be left in the colony? So, have a look at the example we just did together and see if you can apply that now to this question.

You'll need to pause the video to give yourself enough time to think about your answer.

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

Good luck.

Okay, let's see how you did.

So, first, we need to figure out the number of divisions per hour.

So that was 60, the number of minutes in an hour, divided by 30, because in this question it tells you that the bacteria takes 30 minutes to reproduce.

So 60 divided by 30, that's 2.

So now the number of divisions in this time period, told you in the question that they were asking about a 7-hour time period.

So 7 times 2, the divisions per hour calculated in the previous line, equals 14.

Number of bacteria.

Again, in this question, it said a single bacterium, so that's 1, 1 times 2, which is always the same, and then to the power of 14, the number of divisions, so our answer is 16,384.

That's a lot of bacteria.

Really great job if you got that right.

This is the final practise task for today's lesson.

Answer the questions and complete the task below.

So 1a, there are 8 antibiotic-resistant bacteria in a colony.

This type of bacteria takes 30 minutes to reproduce.

Calculate how many antibiotic-resistant bacteria there will be after 3 hours.

And b, how many more resistant bacteria will there be after another, so an additional, 3 hours.

Right, and question two, compare the process of sexual reproduction with asexual reproduction.

So you'll need to pause now to read through those questions again, really make sure you know what it's asking you to do, to think about your answer, and to write down your answer.

Click play when you're ready to see how you did.

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

1a, there are 8 antibiotic-resistant bacteria in a colony.

This type of bacteria takes 30 minutes to reproduce.

Calculate how many antibiotic-resistant bacteria there'll be after 3 hours.

So, divisions per hour, you need to do 60, the number of minutes in an hour, divided by 30, which it told you in this question is how long this bacteria takes to divide, and that equals 2.

Now the number of total divisions in this time period, so 3, because they were asking about a 3-hour time period, times 2, the number of divisions per hour you calculated in the previous line, that equals 6.

The number of bacteria.

So, it told you that there were 8 antibiotic-resistant bacteria, so that's where the number 8 comes from, times 2, that part is always the same because every time a bacteria divides, you get two of them, to the power of 6, which is the number of divisions you calculated in the previous line, that is 512.

1b, calculate how many antibiotic-resistant bacteria there will be after another 3 hours.

So, divisions per hour, 60, minutes in an hour, divided by 30, the time it takes for this bacteria to divide, equals 2.

Number of divisions equals 3, told you another 3 hours, times 2, number of divisions per hour calculated in the previous line, that equals 6.

Number of bacteria.

So, from 1a, the answer was 512, so that's where we get that from, times 2, that part's always the same, to the power of 6, because that's the number of divisions there were, equals 32,768.

Really great job if you've got those numbers both right.

Right, and number two, compare the process of sexual reproduction with asexual reproduction.

So, sexual reproduction and asexual reproduction are both processes which create new organisms or offspring.

So sexual reproduction requires two gametes to produce offspring.

Asexual reproduction requires one parent and no gametes.

In sexual reproduction, the offspring have genetic variation, whereas in asexual reproduction, the offspring are genetically identical.

And the rate of reproduction in sexual reproduction is much slower than in asexual reproduction.

This is because organisms need to reach sexual maturity and find mates in order to sexually reproduce.

Really great job if you got that right.

Amazing work today.

Let's summarise what we've learned.

In sexual reproduction, two gametes fuse to create genetically different offspring.

An advantage is that this creates genetic variation, which is necessary for evolution to take place.

A disadvantage is that the reproduction rate is slow.

And in asexual reproduction, the genome of a cell is copied and the cell divides into two genetically identical cells.

An advantage is that the rate of reproduction is very fast.

A disadvantage is that there are fewer opportunities for genetic variation.

Again, really, really well done, and I hope to see you soon for our next lesson.