The Evolution of Sexual Reproduction

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Sexual reproduction is one of two ways that organisms reproduce. In this lesson, we'll explore why sexual reproduction is so common as well as how it contributes to diverse populations.

Sexual Reproduction

Living organisms all have one thing in common - they need to reproduce. Every living thing will eventually die so it needs to create a new generation to carry on its genetic lineage. Organisms do this through either sexual or asexual reproduction. Asexual reproduction is pretty straightforward. 'A' means 'without,' so this refers to organisms that reproduce without sex, or fertilization. Instead, they might divide into two or break off a limb, which then develops into a new individual. With asexual reproduction, the offspring is an exact copy of the parent. In other words, a clone.

Many living organisms reproduce through asexual means.
coral budding

But with sexual reproduction, we get something quite different. In sexual reproduction, fertilization occurs, which is when two different individuals create a new offspring called a zygote. Because two individuals put genetic information into the same 'pool,' the offspring is also a unique individual but still retains one half of each parent's genetic information to carry on.

Evolution of Sexual Reproduction

But what we know today as sexual reproduction wasn't always the way it was. Obviously, sexual reproduction didn't just occur out of nowhere, like everything else, it has evolved and changed over time. The exact mechanisms for how sexual reproduction came about are still not fully understood because this is not something that is easily tested in experiments. So instead, scientists try to understand how sexual reproduction works now, how it evolves from this point forward, and why sexual reproduction continues to be a prominent method for many organisms.

We do think that sexual reproduction has influenced some really interesting stuff. For example, sexual dimorphism, which is the difference between sexes within a species, has probably evolved as a result of evolution in sexual reproduction. In many species, males and females look very different and have very different roles as parents, and scientists believe sexual reproduction is at least partly responsible for these differences.

There are also many differences in sexual reproduction among different species. For example, earthworms are hermaphroditic, meaning that they have both male and female reproductive structures. This doesn't mean that they self-fertilize, but it does mean that everyone out there is a potential mate! This significantly reduces the energy it would take to find a suitable partner, increasing your ability to reproduce and create the next generation.

There are also a number of animals (for example some fishes) that will change gender under specific conditions. If the single lead male or female of the population dies, the most dominant individual of the opposite gender will actually switch in order to take his/her place. This provides an opportunity for the population to continue to reproduce, despite losing a prominent member of the group.

Earthworms are hermaphroditic but still need a partner in order to reproduce.
earthworm reproduction

Still, other differences exist, ones that would have evolved from a more primitive form of sexual reproduction. While sexual reproduction involves fertilization, some organisms internally fertilize while others fertilize externally. Some organisms also have very complex sexual reproductive systems while others have fairly simple ones. And again, we may not be able to determine how these differences came about, but we can see how they change from this point forward, as well as see why sexual reproduction would be beneficial in these different forms.

Evolutionary Benefits and Disadvantages

Though we may not understand how sexual reproduction evolved, we do understand that it has significant benefits for populations. The genetic variation that is created in an offspring from two different individuals is considered a driving force for natural selection, which is the mechanism by which evolution occurs. Because new genetic combinations are entered into the population through sexual reproduction, there are lots of opportunities for populations to adapt to their ever-changing surroundings.

Sexual reproduction is common among living organisms.
moth reproduction

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