Reproduction, Asexual

Asexual reproduction occurs when a new organism is produced from just one parent. It is a form of reproduction that does not involve the union of gametes (male and female sex cells), and therefore results in an offspring with the same genetic blueprint as the parent. As a result, this offspring is actually a clone of the parent.

BINARY FISSION

Reproduction is the process by which new organisms are produced from existing ones. Asexual reproduction means reproduction without sex, or without male and female sex cells uniting. The simplest form of asexual reproduction is when a single-celled organism like a bacterium splits into two. Called binary fission, this process occurs when a one-celled organism duplicates its DNA and divides into two to form two new identical organisms.

BUDDING

Other organisms like yeast, which is a microscopic, single-celled fungus, reproduce by budding or growing new cells that eventually separate from the parent. Some very simple animals, like hydras and corals, also reproduce this way. Like yeast, a new bud grows directly on the body of the parent. It eventually breaks off and establishes itself as a new, separate organism.

REGENERATION

Reproduction by fragmentation or regeneration is related to budding. It occurs when a part of the parent's body breaks off and grows into a complete, new organism. For example, if a starfish loses an arm, it can grow the arm back. Sometimes the severed arm itself can grow into an entire starfish. A flatworm can also be cut into two and each part can grow into a complete flatworm. Many plants also use a form of asexual reproduction called vegetative reproduction to duplicate themselves. This common method occurs when a plant splits in two, and each segment develops into a separate new plant.

RHIZOMES

Other plants like strawberries and certain types of grass form runners, or rhizomes, that spread out from the parent plant to form plantlets at their ends. The plantlets become independent plants when they develop roots, and the connecting runners disintegrate. Spider plants are excellent examples of plants that use vegetative reproduction.

A CLONE NAMED "DOLLY"

Whether it is a plant or an animal, or whether reproduction occurs by binary fission, budding, or vegetative reproduction, all reproduction that is asexual involves one parent passing on a duplicate of all of its genes to its offspring. This means that the offspring produced by asexual reproduction are genetically identical copies, or clones, of the parent. Until recently, only certain animal cells could be regularly cloned or reproduced asexually. However, in 1996 a sheep named "Dolly" was cloned from a cell taken from an adult sheep. The achievement of such a difficult and complex feat (that is, asexually reproducing a mammal from a single cell taken from an adult rather than an embryo) raises the possibilities of asexual human reproduction. However, such possibilities raise many legal, moral, and ethical questions.

THE ADVANTAGES AND DISADVANTAGES OF ASEXUAL REPRODUCTION

Humans aside, asexual reproduction is a fairly common occurrence in nature and it has its advantages and disadvantages. First of all, organisms that reproduce without sex do not have to expend any energy or resources toward the production of gametes (sex cells). Neither do they have to maintain an elaborate reproductive system or spend time and energy finding and fertilizing (or being fertilized by) a mate. Also, under certain conditions, a great number of individuals can be rapidly produced by asexual reproduction. For example, bacteria can divide every twenty seconds. Another great advantage is that even if there remains only a single individual, organisms capable of asexual reproduction can continue their species without a mate. Finally, when an organism is perfectly adapted to its habitat or particular environment, it will never change and always remain perfectly suited.

However, habitats or environments often change. If an organism does not have the capacity to change, what had been an advantage may become a disadvantage. With asexual reproduction, there is no opportunity for genetic variety. Since offspring are clones of the original parent, there is no way for any new (and sometimes advantageous) traits to be introduced into the population. When all the individuals are identical, that means if one cannot adapt then none can, and the entire species may be endangered.

[See alsoBuds and Budding; Cell Division; Mitosis ]