In this explainer, we will learn how to describe the defining features of asexual and sexual reproduction and compare the genetic variation of the offspring produced.
There are several characteristics that all living things, from the smallest microscopic organisms like bacteria to the tallest trees, have in common. Every organism consumes some form of food, they all use this food as fuel for energy, and they all grow. Reproduction is also considered to be one of the most important biological processes for the survival of a species. What is reproduction, and why is it so important?
Reproduction is the process by which living organisms produce offspring of the same species, passing on their genetics and characteristics. Many individuals in every generation of a species become parents to a set of offspring, which form the next generation, as you can see in Figure 1. This new generation grows and matures, eventually reproducing to form another new generation, and the process continues!
Reproduction is the biological process by which living organisms produce new individuals or offspring.
Let’s think about why reproduction is so important. Without reproduction, the species of living creatures that existed hundreds of years ago would not still exist today. The ultimate goal of every organism is to ensure that its species continues to survive through generations, without dying out or going extinct. This is exactly why every organism reproduces—to ensure that its species continues!
There are several different ways by which organisms can reproduce, but these methods can be grouped into two main categories: asexual reproduction and sexual reproduction.
In asexual reproduction, there is only one parent involved—a single organism capable of producing offspring that are identical to the parent. Usually, single-celled organisms like bacteria and yeast undergo asexual reproduction, although there are a few interesting examples of plants and animals that use this method too!
Key Term: Asexual Reproduction
Asexual reproduction is reproduction that only involves one parent.
There are several different ways by which organisms can reproduce asexually. Let’s learn more about the process through the example of a bacterium.
Bacteria are single-celled organisms that can sometimes cause diseases. Some bacteria reproduce asexually, through a method called binary fission.
The word binary means “two”, and the word fission is used to describe the process of splitting or dividing. In binary fission, the original bacterial cell divides into two new cells, each of which is called a daughter cell. You can see a diagram of this process in Figure 2.
Key Term: Binary Fission
Binary fission is a type of asexual reproduction by which a cell divides into two identical daughter cells.
Binary fission produces two identical cells. This means that every time the bacterium reproduces, the two new bacterial cells are completely identical to one another and to the original cell.
Bacteria are prokaryotic organisms, which means that they do not have a nucleus in their cells. Eukaryotic organisms, on the other hand, have a well-organized nucleus in each cell.
In some eukaryotic cells, binary fission can also be used as a form of asexual reproduction from a single parent. For example, in Amoeba proteus, a unicellular eukaryotic organism, two identical daughter cells can be produced from one parental cell.
In eukaryotes, asexual reproduction can also happen through a type of cell division called mitosis, which produces two identical daughter cells from one single parental cell. Because of this, we say that asexual reproduction, in both prokaryotes and eukaryotes, always produces “clones” of the parent organism.
Key Term: Mitosis
Mitosis is a type of cell division where one cell divides to produce two new cells that are genetically identical.
Asexual reproduction is a very rapid process, and it does not require a great deal of energy. In fact, binary fission in some bacteria can happen in as short a time as 20 minutes!
Fungi are a large group of organisms, some of which reproduce asexually. There are some fungi that are unicellular, such as yeast. One type of yeast you might have heard of, which is used to help bread rise, is called baker’s yeast. Yeast cells reproduce asexually through a process called budding.
Other fungi, such as that which causes bread mold, can use special structures called spores to reproduce asexually.
Plants can sometimes reproduce asexually through vegetative propagation, in which new plants can grow from the vegetative parts of the plant, which include their leaves, stem, and roots. For example, Bryophyllum plants have special structures called leaf buds on the margins of their leaves, as you can see in the photo below, which can give rise to a new plant.
Example 1: Identifying the Properties of Asexual Reproduction
A student writes the following summary about asexual reproduction:
Asexual reproduction is used by one organism to produce new offspring. Asexual reproduction creates offspring that are all genetically different, and it does this quickly.
What is incorrect about this statement?
- Asexual reproduction involves two parents, not just one.
- Asexual reproduction produces genetically identical offspring, not genetically different ones.
- Asexual reproduction is a slow, time-consuming process, not a quick one.
In order to figure out which part of the summary is incorrect, let’s break it down and analyze each piece of information.
The summary begins with “Asexual reproduction is used by one organism to produce new offspring.” This is true! The defining feature of asexual reproduction is that there is only one parent involved, which gives rise to new individuals of the same species.
The next part is “Asexual reproduction creates offspring that are all genetically different ….” Asexual reproduction always produces identical cells, either through simple binary fission in prokaryotes or through mitosis in eukaryotes. The offspring created through asexual reproduction are therefore called clones, since they are identical to each other and to the parent organism. This means that this part of the summary is incorrect.
The final piece of information says “... and it does this quickly.” Again, this is true! Asexual reproduction is a very quick process. Some bacteria can reproduce in as short a time as 20 minutes!
The incorrect part of the statement is, therefore, that asexual reproduction produces genetically identical offspring, not genetically different ones.
Sexual reproduction always involves two parents of the same species, one male and one female. One cell from each parent, which is called a gamete or sex cell, fuse together in a process called fertilization to form a zygote. This zygote grows and changes over time to become a fully developed organism.
Gametes are an organism’s reproductive cells, which contain half the genetic material of a normal body cell.
Key Term: Zygote
A zygote is formed by the fusion of male and female gametes in sexual reproduction. The zygote grows and develops to become a new offspring.
Sexual reproduction can also happen in several different ways. In most mammals, including humans, fertilization and development happen within the mother’s body, and live offspring, or babies, are born. Birds and some reptiles, on the other hand, lay eggs after fertilization, out of which offspring hatch after a period of growth and development.
Although some plants do reproduce asexually, several types of plants also undergo sexual reproduction. The reproductive organ in plants is the flower. In plants, the male gametes are found in grains of pollen, which travel to the ovule, or female gamete, found within the flower, as shown in Figure 2. Here, the gametes fuse through fertilization, eventually forming seeds, which grow into new plants.
In Figure 4, you can see diagrams of sexual reproduction in plants and in animals.
Since sexual reproduction requires two parents, an individual organism must find a mate of the opposite sex in order to reproduce.
There are two main processes involved in sexual reproduction: the formation of gametes and the fusion of these gametes, or fertilization. The formation of gametes happens through a type of cell division called meiosis. When cells divide by meiosis, four new cells are formed, each with half the genetic material that the original cell had. Figure 5 shows the process of gamete formation by meiosis.
Key Term: Meiosis
Meiosis is a type of cell division where one cell can produce four genetically different cells with half the number of chromosomes. Meiosis is how gametes are made.
The offspring produced through sexual reproduction have a combination of different characteristics and are therefore not identical to their parents.
Example 2: Recognizing the Specialized Cells of Sexual Reproduction
Sexual reproduction involves specialized cells that combine in a process called fertilization. What are these specialized cells called?
Sexual reproduction is the type of reproduction that involves two parents: one male and one female. A single sex cell, called a gamete, from each of these parents must fuse together in a process called fertilization to form a zygote, which develops into the offspring.
If we look at the options in the question, we can eliminate spores, since these are cells involved in asexual reproduction. Buds are undeveloped parts of plants that grow into plant organs, and seeds are formed when fertilization happens in plants. These two options can also be eliminated.
A zygote is formed by the fusion of two gametes. The gametes, therefore, are the specialized cells that combine in a process called fertilization.
Now that we have been over the basics of both asexual and sexual reproduction, we can begin to point out some differences between these two processes.
One clear difference between the two, as you may 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, requiring 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, larger offspring are produced through sexual reproduction. For example, as you may know, human mothers undergo a 9-month period of pregnancy during which the baby grows and develops. African elephants carry their young for a period of 22 months before giving birth, which is close to 2 years!
Earlier on in this explainer, we talked about how the offspring produced through asexual reproduction are clones of their parents, while sexual reproduction gives rise to offspring with a combination of characteristics from both parents. We can describe these similarities and differences through a concept called genetic variation.
Key Term: Genetic Variation
Genetic variation describes differences between living organisms. It is what gives different individuals unique combinations of characteristics.
Organisms that reproduce asexually have low genetic variation, whereas organisms that reproduce sexually have a high level of genetic variation. Through genetic variation, combinations of genes from each parent give rise to a unique set of characteristics for every new organism.
The Venn diagram in the figure below shows the differences between asexual and sexual reproduction and the features they have in common.
Genetic variation is an extremely important concept in evolutionary biology. Let’s see what this means 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, as you can see in the photos below.
In England, at the time of the Industrial Revolution in the 18th century, peppered moths exhibited a fascinating example of the benefits of genetic variation.
Before this, dark-bodied peppered moths were very rare, and the light-bodied moths were far more common. During the early decades of the Industrial Revolution, however, the numbers changed: dark-bodied moths began to be seen in larger and larger numbers, bypassing the number of light-bodied moths! How and why did this shift happen?
During the Industrial Revolution, several cities in England began to burn coal to power their factories. This increased the air pollution in these cities to such an extent that the streets, buildings, and trees were blackened with soot. As you can imagine, the high levels of pollution caused the people to experience several health issues. The blackened surfaces, however, also posed problems for the peppered moth!
Before they turned black from soot and pollution, tree trunks provided light-bodied peppered moths with the perfect background to camouflage with, protecting them against predators like birds. When these light-bodied moths settled on blackened tree trunks, however, they showed up clearly, making them easy prey!
The photo below shows two light-bodied peppered moths on the left-hand side and two dark-bodied peppered moths on the right-hand side. As you can see, the darker moth camouflages well with the darker surface, whereas the lighter one shows up clearly.
Let’s imagine a hypothetical situation. If peppered moths had reproduced asexually, would they have survived the Industrial Revolution? Asexual reproduction produces clones of the parent organism. Light-bodied peppered moths would, therefore, keep giving rise to more light-bodied moths, putting the entire species at risk of going extinct from predation!
Instead, through sexual reproduction, the gene for the darker body became more prevalent, because it enabled the moths to camouflage against the blackened surfaces and survive predation. While the lighter-bodied peppered moths were preyed upon, the dark-bodied peppered moths grew rapidly in number, effectively saving the population of peppered moths from going extinct!
This example clearly shows the benefits that genetic variation can have in providing a tool that gives more possibilities for a species to adapt to environment changes, in order to thrive in spite of unfavorable conditions.
Example 3: Understanding Genetic Variation
Imagine there are two populations of the same species of plant. Population X has been produced by asexual reproduction and population Y by sexual reproduction. A new disease is introduced to the area. Which population is more at risk? Why?
- Population Y, because this population will have more genetic variation
- Population X, because this population will have less genetic variation
When organisms are produced through asexual reproduction, such as the ones in population X mentioned in the question, they are genetically identical to their parent organism and to one another. We say that these organisms are clones and that they have very little genetic variation.
On the other hand, organisms produced through sexual reproduction, in which two parents are involved, have a high level of genetic variation. In sexual reproduction, gametes, or sex cells, from each parent fuse through fertilization to form a zygote. This means that the offspring, like the ones in population Y in the question, have a combination of characteristics from each parent and are therefore genetically different.
Different individuals of the same generation are also genetically different from one another. This is because each offspring is formed through the fusion of a different pair of gametes, forming different combinations of characteristics.
In the question, a disease is introduced into both populations. Let’s consider population X that is produced by asexual reproduction.
If a plant from population X has a set of genes that cannot tolerate the disease, this means that every single plant in population X will be badly affected, since they are all clones of one another. The low level of genetic variation puts this population at risk of being completely wiped out by this disease.
Now let’s take a look at population Y, produced by sexual reproduction. Each individual plant in population Y has a different combination of genes. Some plants may still be badly affected and might die from this disease. But not all the plants in this population have the same combination of genes!
Some plants might have genes that help them fight off the disease. Some might even have genes that make them completely resistant to the disease! This means that even if some plants in population Y die, the entire group of plants will not be wiped out. The remaining plants will then reproduce further, passing on their genes and ensuring that future generations will also withstand a similar disease. Through genetic variation, therefore, a species can adapt and survive through unfavorable conditions.
The population at greater risk, therefore, is population X, because it will have less genetic variation.
Let’s summarize the key points from this explainer.
- There are two types of reproduction: asexual reproduction and sexual reproduction.
- In asexual reproduction, there is only one parent involved, whereas sexual reproduction requires two parents: one male and one female.
- Asexual reproduction is rapid, whereas sexual reproduction takes long periods of time.
- Asexual reproduction produces clones of the parent organism with no genetic variation.
- Offspring produced through sexual reproduction have a combination of characteristics from both parents and are genetically distinct from their parents.