Biological Advantages of Sexual Reproduction

Instructor: Amanda Robb
In this lesson, we'll discuss the advantages of sexual reproduction. We'll focus on the advantages of genetic variability in terms of pathogen resistance, genetic diseases, and evolution compared to asexual reproduction.

What Is Sexual Reproduction?

Have you ever wondered how trees reproduce? Your first thought is probably that they produce asexually. Asexual reproduction simply makes a clone of the parent organism without any sperm or eggs. Trees clearly do not have the sexual reproductive organs that humans have, and they definitely do not have physical intercourse in the way humans do, but they actually do reproduce sexually.

They don't have physical sex like animals do, but they still combine eggs and sperm to make a new organism. You might be wondering how this happens. Do you know anyone with spring allergies? Well, it's time to tell your friend that they're allergic to plant sperm. That's right, pollen is plant sperm. The ovaries fertilized by pollen are located within flowers. So, next time you see your car covered in pollen, you can remember it's actually coated in plant sperm!

Trees reproduce sexually with pollen to increase genetic variability
pollen

Genetic Variability

This process of creating sperm and eggs, fusing them, and developing a new individual is costly to the organism. It takes more time and energy than asexual reproduction. So why do it? The main reason is genetic variability, which is a difference in DNA, or genes, of one organism compared to another of the same species. When sperm and eggs are created through the process of meiosis, the DNA of the parents gets shuffled, so no egg or sperm has exactly the same DNA as the parent that made it. Then, to further increase genetic variability, the sperm and egg fuse, creating a new individual that isn't identical to either parent. There are several reasons why this is advantageous. Let's look at each one in detail next.

Fighting Disease

Remember the last time you were sick? A pathogen, either a virus or bacteria, invaded your body, sneaking inside your cells and forcing them to help it replicate itself. This destroyed your cells and caused an immune reaction that made you feel sick. How did this invader get in?

Pathogens attach to tiny markers on the surface of your cells, usually proteins or carbohydrates. The special pattern of these molecules on your cells is determined by your genetics. Each person has slightly different genetics and will have slightly different markers, so pathogens won't affect everyone in the same way. In fact, some people have been discovered that are resistant to human immunodeficiency virus (HIV) because of this very reason. They lack the marker that HIV uses to enter cells, and so the HIV can't get in, and thus can't infect them.

HIV in yellow attaches to an immune cell in green
HIV

Now, imagine if everyone had the exact same markers on their cells because they had the same genetics. If a lethal virus came into contact with this population, everyone would get infected. It's good for everyone to be a little different because there will always be a few people immune to a particular pathogen, leaving room for our species to reproduce and come back from a population crash.

Eliminating Genetic Disease

Pathogens aren't the only risk to our species. Some diseases are genetic, meaning that they arise from broken DNA in our genome. Usually, these diseases are recessive, meaning that a child would have to get one bad copy of the gene from each parent to actually have the disease. So, even if a child has the disease, if they marry and reproduce with a normal partner, their children will be safe.

However, let's imagine that child does have a genetic disease, but they reproduce asexually. Then, all their children and their children's children will also have the disease. It will be impossible to get rid of the trait from the gene pool as long as the diseased individuals keep reproducing.

This problem manifests in inbreeding, both in people who have children with close relatives, or selective breeding of animals for traits. One example is the inbreeding of dogs to bring out desired traits, like Dalmatians. Dalmatians have long been bred for their characteristic coats, but the inbreeding necessary to bring out this trait leaves Dalmatians prone to deafness, kidney stones, and hives because their genetic variability has been decreased.

Inbreeding has resulted in increased susceptibility to genetic disorders
Dalmatian

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