Key Terms
| Term | Meaning |
|---|---|
| Sexual reproduction | Process of creating new individual using two parent organisms |
| Asexual reproduction | Process of creating new individual using one parent organism |
| Offspring | New organism that results from reproduction |
| Gamete | Sex cell (in males: sperm; in females: eggs) |
| Fertilization | The joining of gametes to form a new organism |
| Zygote | Cell formed during the fusion of two gametes |
Types of reproduction
There are two major forms of reproduction: sexual and asexual.
Sexual reproduction
Sexual reproduction requires two parents. Each parent contributes a gamete - a sex cell that has half of the normal DNA of a regular body cell. In males, the gametes are sperm and in females, the gametes are eggs.
When these two gamete combine during fertilization, the result is a zygote, which then continues to develop into an embryo.
Overview of the process of sexual reproduction.
Asexual reproduction
Asexual reproduction requires only one parent.
There are many types of asexual reproduction. Four major types are:
1) Binary fission: Single parent cell doubles its DNA, then divides into two cells. Usually occurs in bacteria.
Prokaryotic cell undergoing the process of binary fission
2) Budding: Small growth on surface of parent breaks off, resulting in the formation of two individuals. Occurs in yeast and some animals (like the hydra below).
Hydra reproduce asexually through budding
3) Fragmentation: Organisms break into two or more fragments that develop into a new individual. Occurs in many plants, as well as some animals (like coral, sponges, and starfish).
Starfish losing an arm. The arm fragment grows into another starfish and the original starfish regrows its lost arm.
4) Parthenogenesis: An embryo develops from an unfertilized cell. Occurs in invertebrates, as well as in some fish, amphibians, and reptiles.
Baby Komodo dragon produced through parthenogenesis.
Comparing sexual vs asexual reproduction
| Sexual | Asexual |
|---|---|
| Requires 2 parents | Requires 1 parent |
| Sperm fertilizes egg | Single organism makes an exact copy of itself |
| Used by animals, flowering plants, some fungi | Bacteria, some plants and fungi, few animals (sponges) |
| Offspring are different from parents | Offspring are identical to parent |
| Provides genetic variation, but time-consuming | Fast and easy, but no genetic variation |
Common mistakes and misconceptions
Sexual reproduction is not necessarily "better" than asexual reproduction, or vice versa. If one type of reproduction was clearly advantageous over the other, we would see all organisms reproducing in that manner. However, both types of reproduction still exist in various organisms, telling us that each type of reproduction carries some kind of evolutionary advantage.
For example, sexual reproduction allows for variation, but gestation usually takes a long time and it requires heavy parental input. Asexual reproduction occurs quickly, but because all of the offspring have the same genetic information, individuals are more susceptible to disease.
Budding and fragmentation are not the same thing. Although they do appear similar, in fragmentation, the parent body breaks into distinct fragments and each fragment develops into a new individual or offspring. In budding, there must be an outgrowth (bud) that develops on the parent.
Some organisms are able to do both sexual and asexual reproduction. This is particularly true for fungi and plants (and rarely, animals - as in parthenogenesis). Often, the type of reproduction that they undergo depends on their environmental conditions or the point in their growth cycles.
Although sexual reproduction requires two parents, they do not always have to be two separate individuals. This may sound confusing, but some organisms are hermaphroditic, meaning they contain both male and female gametes. In this instance, those organisms are able to self-fertilize. Despite the fact that these gametes come from the same individual, we still consider this sexual reproduction, as two gametes are involved.
