Lesson Video: Reproduction Biology

Video Transcript

In this video, we will define the process of sexual and asexual reproduction in plants and animals, describe the key differences between sexual and asexual reproduction, and explain the advantages and disadvantages of each. After that, we’ll use what we’ve learned to answer some practice questions then summarize the key points of this lesson at the end.

One of the characteristics that all living things share is the ability to reproduce or to generate offspring of the same species. During sexual reproduction, two parents each contribute half their genetic material to create genetically unique offspring. How does this occur? Normal human cells are referred to as diploid. And diploid cells possess the normal number of chromosomes that you would find in any typical body cell. In humans, there are 23 pairs of chromosomes, or 46 in total.

In order for sexual reproduction to occur, an organism must produce haploid cells that possess half the normal number of chromosomes. In humans, that’s one from each pair, or 23 chromosomes. These haploid cells are called gametes. In humans and most animals, the gametes are the sperm in the egg cell. In plants, they’re referred to as the pollen and the ovule. The gametes each possess a random assortment of each parent’s genetic material, making each gamete genetically unique.

The two gametes combine to create a diploid zygote, returning the cell to the normal number of chromosomes, 46. This process of gametes combining is called fertilization. The zygote inherits half its genetic information from the mother and half from the father. The zygote grows into an embryo and then a fetus and, finally, a genetically unique baby. Because each gamete is genetically unique, every time this couple reproduces, the offspring will also be genetically unique. Sexual reproduction is reproduction that involves two parents, who each contribute half of their genetic material in genetically unique gametes, to produce genetically unique offspring.

Next, we’ll have a look at asexual reproduction. In contrast to sexual reproduction, asexual reproduction only involves one parent. Because there’s no genetic recombination or mixing of different genes, the offspring are identical to each other and also to their parent. These genetically identical offspring are referred to as clones. Asexual reproduction occurs in several different ways. The process illustrated here is called binary fission.

In binary fission, one organism copies its genetic material and simply splits it into two new organisms. Binary fission is common among single-celled organisms, such as amoeba, paramecium, and our next example bacteria. We’ve said that during asexual reproduction, all of the offspring produced are genetically identical. But that’s not always the case. Sometimes, when DNA is being copied, mistakes are made. These mistakes or random changes are called mutations. A mutation that gives rise to traits that give an organism an advantage in survival or reproduction is called an adaptation.

Most mutations are neutral, neither helping nor harming the organism. However, occasionally, a mutation will have harmful or negative effects. Because there’s no genetic recombination in asexual reproduction, if a mutation is harmful but does not actually kill the organism, it can continue to get passed down to future generations.

Organisms like bacteria that reproduce asexually tend to have low genetic variation within populations. Because mutations are relatively rare and there’s no genetic recombination in asexual reproduction, humans have taken advantage of the low genetic variation among bacteria to develop drugs called antibiotics that protect us from bacterial infection. Fortunately for the bacteria, but unfortunately for us, adaptations have arisen that grant some bacteria antibiotic resistance.

Asexual reproduction is reproduction that only involves one parent. The offspring of asexual reproduction are genetically identical to each other and to their parent. They can also be referred to as clones. And because during asexual reproduction there’s no genetic recombination, genetic variation only occurs by mutation and is relatively lower than populations that use sexual reproduction.

Next, we’ll look at examples of sexual and asexual reproduction in animals. The majority of animals reproduce sexually. Take, for example, these two cats. Through the process of genetic recombination we described earlier, they’re able to produce offspring with a variety of traits. Because each of the offspring receives half of their genes from their mother and half their genes from their father and the genes they inherit are random, each of the offspring are genetically unique.

If one of the offspring inherit a trait that makes them more likely to survive or reproduce — a trait that, for example, might make a cat an excellent hunter — we call that an adaptation. Because an adaptation increases the chances of survival or reproduction, they have better chances of passing on their traits to more offspring. The process of adaptations becoming more common within a population is the mechanism of evolution, or how species change over time.

Let’s also take a moment to note here that mutations also occur during sexual reproduction. The fact that both mutations and genetic recombination occur during sexual reproduction means that populations that use sexual reproduction tend to have relatively higher genetic variation than populations that use asexual reproduction.

Next, we’ll look at an example of an animal that uses both asexual and sexual reproduction. One example of such an animal is the Komodo dragon. You may know the Komodo dragon as the heaviest lizards on Earth and vicious predators. Another interesting fact about the Komodo dragon is that they can reproduce either sexually or asexually. This means that when mates are available, the female Komodo dragon can lay eggs that have been fertilized by a male, which will each produce a genetically unique offspring, increasing the genetic variation within the population. Alternatively, if mates become scarce, the female Komodo dragon can lay eggs that have not been fertilized by a male. These eggs will each contain a genetic clone of the mother, decreasing genetic variation but maintaining the population.

This type of asexual reproduction is called parthenogenesis. Having the ability to reproduce sexually or asexually presents an advantage to the species. When conditions are good, they can reproduce sexually, increasing genetic variation and the possibility of adaptation. But if conditions become harsh, they can maintain the population using asexual reproduction, decreasing genetic variation but avoiding extinction.

Next, let’s take a look at sexual reproduction in plants. Most plants can reproduce either sexually or asexually. Plants reproduce sexually using seeds. Seeds are to plants what embryos are to animals. Under the correct conditions, they grow into new individual plants of the same species. Seeds are produced by the fusion of genetically unique haploid gametes. In plants, these gametes are found in the pollen and ovule. The fertilization of the gamete in the ovule by the gamete in the pollen leads to genetic recombination — just like in animals — producing a diploid zygote, which becomes part of a fully developed seed that eventually grows into a plant that is genetically unique.

Just like sexual reproduction in animals, genetic recombination leads to an increase in variation. This also means that if a plant inherits a trait that makes it more likely to survive or reproduce, also called adaptation, this trait is more likely to be passed on to future generation, driving the evolutionary process.

Next, let’s look at an example of a plant reproducing asexually. To investigate asexual reproduction, we’ll use the example of the potato plant. Potato plants can reproduce the way we just described, using seeds. Potatoes also grow a special underground stem, called a tuber. This is the part of the plant humans typically cultivate for food. That tuber stores starch that the potato plant can use for energy. The tuber or potato also has the ability to grow into a new plant on its own. This is useful if the plant itself is physically damaged or killed.

Since this new potato plant was grown from one parent, this is an example of asexual reproduction. If you ever left a potato plant sitting in your kitchen for too long, you’ve witnessed it begin several organs, which would eventually become roots and shoots. Humans have taken advantage of this mode of reproduction in potatoes for generations, sometimes with unexpectedly negative results. In the mid-1800s, a disease called blight wiped out huge portions of potato crops all across Ireland. Because much of the population relied on these potatoes for survival, over a million individuals starved to death. This tragic event is sometimes referred to as the Irish Potato Famine.

But why was blight able to destroy so many plants? By planting potatoes from potatoes and not from seeds, farmers were able to grow more crops more quickly. However, plants grown by asexual reproduction are all genetically identical or clones of each other. Being genetically identical, they were all susceptible to the same disease. Without the genetic variation brought on by the genetic recombination found in sexual reproduction, none of the potato plants had an adaptation that would allow them to survive the blight, and all of the plants withered and died.

Keeping this story in mind, let’s move on to review some of the advantages and disadvantages of sexual and asexual reproduction. We’ll start with sexual reproduction. One of the advantages of sexual reproduction is increased genetic variation within a population. Genetic recombination, the result of the merging of unique gametes, leads to unique offspring. There’s an increased chance of adaptation and of positive traits being passed on to future generations. Another advantage of sexual reproduction is the ability to adapt to a changing environment. This increases a species chance of survival and drives the evolutionary process.

One of the disadvantages of sexual reproduction is that it requires a mate. The process of mating is time- and energy-consuming. Mating can also be risky to the individual organisms. Sexual reproduction also tends to be a much slower process than asexual reproduction.

Next, let’s review what we know about asexual reproduction. When it comes to asexual reproduction, one of the advantages is that it does not require a mate; one organism can generate an entire population on its own. Another advantage of asexual reproduction is that it’s faster than sexual reproduction.

One of the disadvantages of asexual reproduction is that it tends to create populations with relatively low genetic variation. Genetic variation only rises from mutations because of the lack of genetic recombination. And a population low in variation also find itself vulnerable to certain types of risk, as we saw in our potato blight example. Because genetic variation is low, adaptation rate is also slower. This slows down the evolutionary process.

Next, let’s take a look at a practice question.

For the following examples, select the correct type of reproduction, asexual or sexual, that is being described.

For this question, we’re being asked to read a statement about a type of reproduction and to determine if the type of reproduction being described in the example is sexual or asexual reproduction. First, let’s review some key facts about asexual and sexual reproduction.

Asexual reproduction only involves one parent, while sexual reproduction requires two. Asexual reproduction is a generally faster process than sexual reproduction. In asexual reproduction, since there’s only one parent and no genetic recombination, all the offspring are genetically identical to each other and to their parent. This decreases the genetic variation in a population. In contrast, during sexual reproduction, genetic recombination occurs when genetically unique gametes produced by each of the parents merge in a process called fertilization to produce genetically unique offspring. This process increases genetic variation within a population.

Now, let’s return to our example statement. An area of forest has been devastated by forest fires. The remaining trees rapidly produce small saplings from their roots. Is this an example of asexual reproduction or sexual reproduction?

The key clues in this example are the term “rapidly“ and the fact that the trees are producing saplings or offspring from their own roots. We know that asexual reproduction is a much faster process than sexual reproduction. And since the trees are producing saplings from their roots, only one parent is necessary. Based on this information, we can conclude that this is an example of asexual reproduction.

A bee carries pollen grains from one plant to the ovules of another. Is this an example of asexual reproduction or sexual reproduction?

Since this example involves two plants, it involves two parents. Also, pollen and ovules are examples of gametes. Gametes are special reproductive cells used in sexual reproduction. Based on this information, we can conclude that this is an example of sexual reproduction.

A female shark that has been isolated from male sharks produces eggs containing small embryos. Is this an example of asexual reproduction or sexual reproduction?

In this example, we’re told that the female shark has been isolated from male sharks. So, she’s produced eggs with embryos without fertilization. This means that only one parent was required for the production of these offspring. When reproduction occurs with only one parent, we know that we’re dealing with asexual reproduction.

Now that we’ve had a chance to practice what we’ve learned using some sample questions, let’s go ahead and review the key points of this lesson. So, what have we learned about sexual reproduction and asexual reproduction? Well, first, they’re both methods of producing offspring or more members of the same species. Sexual reproduction requires two parents, while asexual reproduction only involves one. Sexual reproduction produces unique offspring through the fusion of genetically unique gametes, a process known as fertilization, while asexual reproduction produces genetically identical offspring, also referred to as clones.

Some advantages of sexual reproduction are that it increases the genetic variation within a population, which increases the possibility of adaptation to environmental changes. These advantages are the direct result of genetic recombination. Some of the disadvantages of sexual reproduction are: since two parents are involved, sexual reproduction requires finding a mate, which is time- and energy-consuming and risky for individual organisms. Also, sexual reproduction tends to be a much slower process than asexual reproduction.

Some of the advantages of asexual reproduction are: asexual reproduction tends to be much faster than sexual reproduction. And since only one parent is involved in asexual reproduction, there’s no need to find a mate. However, since no genetic recombination is involved in asexual reproduction, asexual reproduction leaves the populations that have low genetic variation. This leaves the population vulnerable to certain types of risk. The lower genetic variation also means there are less opportunities for adaptation, meaning that the population is less prepared for drastic environmental change.