In this video, we will learn how to describe the defining features of asexual and sexual reproduction and some examples of these two types of reproduction. We will also learn what the term genetic variation means and how asexual and sexual reproduction differ in the genetic variation they introduce to their offspring.
There are several characteristics that all living organisms have in common. From the smallest microscopic organisms, like bacteria, to the tallest trees, to humans, like you and I, all of us move, though some of us move faster than others. We must also release energy, which we do through a process called respiration that occurs in almost all of our cells. We’re all sensitive to our surroundings, and we all grow. Though not every individual will reproduce, as a species, some individuals must reproduce for that species to continue. We must all excrete waste products from our cells. And in order to carry out these energy-demanding processes, we must all obtain food or nutrition. By looking at the first letter of each of these essential life processes, we find an easy way to remember them: MRS GREN.
In this video, we’re focusing on reproduction. But what is reproduction, and why is it so important? Reproduction is a biological process by which living organisms are able to produce new individuals, which are otherwise known as offspring. The new individuals that are produced will have some of the genetic information passed on from their parents and so, therefore, will have some of the same characteristics as their parents. For example, this individual might have had the genetic information that codes for black hair passed on by her parents and so also has the black hair characteristic. Many individuals in every generation of a species become parents to a set of offspring. And some individuals will not become parents.
The offspring that are produced form the next generation. This new generation will grow and mature, and they eventually reproduce and have children of their own. And so the process continues. If these stick people did not reproduce, they will eventually die without producing the next generation. If this happened on a larger scale, the species of stick people might become extinct. Without reproduction, the species of living creatures that existed hundreds of years ago wouldn’t still exist today. In fact, many species that became so rare that they have failed to reproduce in the past have become extinct.
While the stick people are not a real species, one real species that was declared extinct in 2020, among many others, was the splendid poison frog, pictured here. The ultimate goal of every species is to ensure that this species continues to survive throughout generations without going extinct. This is exactly why at least some organisms in every generation of every species must reproduce, to ensure that its species survives.
Let’s take a look at the different types of reproduction that we’ll be exploring in this video. There are several different ways in which organisms can reproduce. But these methods can be broadly grouped into two main categories: asexual reproduction and sexual reproduction.
Let’s explore asexual reproduction first. In asexual reproduction, there is only one parent involved. And this single organism is capable of producing offspring that are identical to the parent. It involves two identical daughter cells being produced from one parent cell. Usually, the type of organisms that undergo asexual reproduction exclusively are single-celled organisms, like bacteria or the fungus yeast, although there are a few interesting examples of plants and animals that undergo asexual reproduction, too. As we’ve mentioned, there are several different ways in which organisms can reproduce asexually.
Let’s learn more about this process through an example in a bacterium. Bacteria, or a singular bacterium, are single-celled organisms that can sometimes cause diseases in humans. However, many bacteria are very useful to us, too. For example, those that live in our digestive system can help us to digest our food. Many bacteria reproduce asexually through a process called binary fission. The word binary comes from the Latin word meaning two together, while the word fission refers to things splitting or dividing. This is because in binary fission, the original bacterial cell divides into two new cells, which are called daughter cells.
Binary fission produces two identical cells. This means that every time a bacterium reproduces, the two new bacterial cells produced are completely identical to each other and to the original parent cell. Bacteria are prokaryotic organisms, which means they don’t have a nucleus in their cells. Eukaryotic organisms, for example, the single-celled Amoeba proteus, do have a well-defined nucleus in each of their cells, however. Although it mainly occurs in prokaryotic cells, some eukaryotic cells, like those of the amoeba, can carry out binary fission, too.
Asexual reproduction is a very rapid process, and it doesn’t require a lot of energy. In fact, binary fission in some bacteria can happen in a shorter time as 20 minutes. In contrast to binary fission, which occurs predominantly in prokaryotes, eukaryotes can asexually reproduce using a different type of cell division called mitosis, which also produces two genetically identical daughter cells from one parental cell. Because of this, we say that asexual reproduction in both prokaryotes and in eukaryotes always produces clones of the original parent cell. However, some differences may arise between the two daughter cells produced due to their environmental conditions.
Fungi are a large group of organisms, some of which reproduce asexually. There are some fungi which are unicellular, such as yeast. One yeast you might’ve heard of, which is used to help bread rise, is called baker’s yeast. Yeast cells reproduce asexually through a process called budding, whereby a new organism buds off the original cell. And both of the cells produced will be genetically identical to each other.
Other fungi, such as those which cause bread mold, can use special structures called spores to reproduce asexually. In fact, when you see mold on a piece of bread like this, you’re actually seeing colonies of spores and the fungi trying to reproduce as much as possible to spread all over the bread.
Some plants can also reproduce asexually through a process called vegetative propagation, in which new plants can grow from the vegetative parts of the plant, which include the leaves, stems, and roots. Let’s take a look at this process occurring in this strawberry plant. As you can see, the strawberry plant produced extensions of a stem-like structure called a runner. Runners can grow either above or below the ground surface till they find a spot which is suitable to set down roots in, establishing a new plant nearby to the parent plant.
Let’s take a look at sexual reproduction next. Sexual reproduction always involves two parents of the same species: one male and one female. Since sexual reproduction requires two parents, an individual organism must find a mate of the opposite sex in order to reproduce. Each parent produces many cells called gametes, otherwise known as sex cells or their reproductive cells. These gametes contain half the genetic material of a normal body cell. So let’s find out why.
There are two main stages involved in sexual reproduction: the formation of these gametes and the fusion of the gametes, which occurs through a process called fertilization. The formation of gametes occurs through a type of cell division called meiosis. When cells divide by meiosis, four new cells are formed. And as we mentioned earlier, each of these four gametes will contain half the genetic information of a normal body cell and of this original parent cell.
While this parent cell had four chromosomes, each of these daughter cells only has two. This is because in sexual reproduction, one gamete from each parent fuses together in a process called fertilization to form a zygote with a full set of genetic material. The zygote that’s produced, unlike the clones produced in asexual reproduction, is genetically different from its parents. This zygote grows and changes over time to eventually become a fully developed organism.
So let’s look at how this can happen in different groups of organisms. Sexual reproduction can also happen in several different ways. In animals, the gametes are called the egg cells in females and sperm cells in males. In most mammals, including humans, fertilization and development of the zygote occurs within the mother’s body. And the development process is called a pregnancy. After this period of development, live offspring or babies are born.
In most birds and reptiles, internal fertilization also occurs, but these animals lay eggs. In most fish and amphibians, however, fertilization usually takes place externally to the mother’s body, often in water, and eggs are also produced. After a period of development in these eggs, offspring can hatch out of them.
Sexual reproduction in plants occurs slightly differently. Although many plants can reproduce asexually, several types of plants also undergo sexual reproduction. The reproductive organ in plants is called the flower. The male gametes in plants are found in grains of pollen, which can travel to the female gametes, or the ovule, by moving through certain parts of the flower. Here, the gametes will fuse by fertilization, eventually forming seeds, which can grow into new individual plants. If the pollen grain is produced by the same flower as the ovule that it eventually fertilizes, this is an example of asexual reproduction, as both gametes were produced by the same parent. However, if this pollen comes from a different plant, it is sexual reproduction, as the offspring produced will be genetically different from each parent.
Now that we’ve seen some of the basics of both asexual and sexual reproduction, let’s identify some of the similarities and differences between these two processes using this Venn diagram. Both processes involve producing new offspring and passing on genetic material to these offspring to form a new generation. One clear difference between the two, as you might have noticed, is that asexual reproduction only requires one parent, while sexual reproduction requires two.
As we mentioned earlier, asexual reproduction is a rapid and efficient process that requires very little energy. This also means that organisms that reproduce asexually can give rise to a large number of offspring in a short period of time. Sexual reproduction, on the other hand, is far more time-consuming and demands a great deal of energy. Fewer and larger organisms tend to be produced through sexual reproduction. For example, you might know that a human mother undergoes a nine-month period of pregnancy during which the baby grows and develops. And generally the larger an organism gets, the longer this period is. For example, African elephants carry the young for a period of 22 months before giving birth, which is close to two years.
The offspring produced by asexual reproduction are genetically identical to their parents so can be referred to as clones, while sexual reproduction gives rise to offspring that are genetically different from their parents, as they have a combination of characteristics from both of the two parents.
We can describe these genetic similarities and differences between different organisms through a concept called genetic variation. Genetic variation describes the differences in the genetics of different living organisms and gives these different individuals unique combinations of characteristics. Organisms that reproduce asexually tend to have a very low genetic variation, as their daughter cells are produced through processes like mitosis, which makes them genetically identical to the parent cells.
Organisms that reproduce sexually, however, tend to have a much higher genetic variation due to the production of genetically different gametes through meiosis. Through genetic variation, combinations of genes from each parent give rise to a unique set of characteristics for every new organism. Let’s explore the benefits of having a high genetic variation that’s afforded by sexual reproduction through an example.
The peppered moth, or Biston betularia, is a species of moth with a speckled black-and-white appearance. There are two types of this moth: one with a light-colored body and one with a dark-colored body. In England, just before the Industrial Revolution in the 18th century, dark-bodied peppered moths were very rare and the light-bodied moths were far more common, as they camouflaged well against pale trees, like this birch. While this protected the lighter-colored moths from predators like birds, the darker moths stood out clearly and were gobbled up quickly.
Due to natural genetic variation, however, small numbers of darker peppered moths remained. During the early decades of the Industrial Revolution, however, things started to change for the peppered moths. Dark-bodied moths began to be seen in larger and larger numbers, eventually even bypassing the number of light-bodied moths. How and why do you think this shift happened? Pause the video and have a think.
During the Industrial Revolution, several cities in England began to burn coal to power their factories, which produced lots and lots of smoky black pollution. The air pollution in cities increased to such an extent that the streets, buildings, and trees were blackened with soot. These environmental changes meant that the darker moths now camouflaged better with the darker surfaces, whereas the lighter ones showed up far more clearly.
Let’s imagine a hypothetical situation. If peppered moths reproduced asexually, would they have survived the Industrial Revolution? As asexual reproduction produces clones of the parent organism, all of the peppered moths would be white. Light-bodied peppered moths would in this scenario keep giving rise to more light-bodied moths. This means that all of the moths are now not very well camouflaged and puts the whole species at risk of extinction. Instead, as these moths were reproducing sexually, the gene for a darker body became more prevalent in the Industrial Revolution because it enabled the moths to camouflage against the blackened surfaces and survive predation. While the light-bodied moths were preyed upon, the dark-bodied moths could grow in number, effectively saving the population of peppered moths from going extinct.
This example shows us one of the benefits of a high genetic variation to increase the resilience of a species against environmental changes. It provides a tool that gives more possibilities for a species to adapt to their environment in order to thrive in spite of unfavorable conditions.
Now that we’ve learnt some more about the different types of reproduction, let’s summarize the key points that we’ve covered in this video. We’ve learnt that there are two types of reproduction: asexual reproduction and sexual reproduction. In asexual reproduction, there is only one parent involved, while sexual reproduction requires two parents: one male and one female. Asexual reproduction is rapid, while sexual reproduction is a lengthier process that requires finding a mate. While asexual reproduction produces clones of the parent organism with no genetic variation in their offspring, the offspring produced through sexual reproduction have a combination of characteristics from both parents and are genetically different from those parents. So they tend to have a high genetic variation.