Sexual Reproduction: Definition & Overview

Instructor: John Williams
Biological systems use different forms of reproduction to accomplish different tasks. This article addresses sexual reproduction and its role in genetic variability.

Introduction

While many of us choose not to think about this fact, we as humans are all products of sexual reproduction. This means that our parents likely engaged in the act of sex in order to procreate, or produce a child. Though this activity in humans is often understood in terms of the basics, it is often difficult to understand the complexity of sexual reproduction and the key characteristics that make this process so important. Let's discuss sexual reproduction, and identify some key aspects of this process.

Sexual Reproduction: A Necessary Life Process

There are several characteristics that organisms must possess in order to be considered living. Of these characteristics, reproduction is probably one of the least understood. Reproduction is the process by which an organism produces offspring for the perpetuation of a species. There are two basic categories of reproduction: asexual and sexual.

The goal of asexual reproduction is to simply increase the population of a particular organism. This is often seen in bacteria, fungi, and even some lower animals, such as insects. In this process, the offspring are typically identical clones of the parent or very similar to the parent genetically. Sexual reproduction, however, is different in that the primary goal is to increase genetic variability. This means that sexual reproduction is necessary so that the offspring will have different combinations of genetic traits than the parents.

So here's the obvious question: why do we need genetic diversity? That answer is still being researched to this day, but most scientists can agree that genetic diversity is important for protecting animal species and for the process of evolution. Let's imagine for a second that all humans are identical in terms of genetic makeup. If a disease were to affect one individual in the group, then the entire group would be susceptible to that illness, and the entire population could be wiped out instantly. Instead, genetic variability allows for some individuals to have a different set of traits, and those different combinations may actually protect some of the individuals from mass extinction.

In short, through sexual reproduction, genetic variability improves the chances of species survival.

Different Forms of Sexual Reproduction

Many organisms engage in sexual reproduction, though the methods by which they achieve this process vary. However, the goal for every method is to increase genetic diversity as a means of perpetuating the species.

Bacteria

Have you ever heard of bacteria becoming medically resistant? What about bacteria that are incurable, or difficult to treat? Well, bacteria are single-celled organisms that use sexual reproduction to exchange genes prior to asexual reproduction. Through the exchange of plasmids (circular pieces of DNA) bacteria can give each other new genes and new traits, such as antibiotic resistance. When those bacteria undergo asexual reproduction, the newly formed generations now have copies of that new DNA. While asexual reproduction is how bacteria increase their population size, their ability to develop resistance and survive comes from the sexual reproduction portion of their life cycles.

Bacterial Sexual Reproduction
Bacterial conjugation

Invertebrates

Some invertebrates also use a combination of asexual and sexual reproduction in order to perpetuate the species. For example, Aurelia jellies (Moon jellyfish), undergo sexual reproduction as adults to produce planulae (first-stage offspring). These offspring are genetically a combination of genes from the male and female adult jellies. However, these planulae will eventually form polyps and, in this stage, asexually reproduce to create multiple copies of the DNA from the planulae. While each of the offspring from the polyp will be identical to the others, the sexual reproduction portion ensures that the offspring will be different from the adults.

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