Asexual reproduction is the reproduction of a single organism without any outside genetic material. Instead of two organisms combining half of their genomes into one new organism, as occurs in sexual reproduction, the solitary parent replicates its own genome completely, resulting in a clone of the parent. Because of this, far fewer genetic mutations occur in asexually reproduced organisms than in sexually reproduced organisms. Asexual reproduction is common in all kingdoms except for the animal kingdom. While there are some animals that reproduce asexually, they are rare exceptions.
Asexual Reproduction in Plants
For the plants that reproduce asexually, there are several parts of the plant that can be involved. The stems are the most common reproductive parts, though this can be somewhat misleading. While commonly considered roots, rhizomes, bulbs, corms, and tubers are actually subterranean stems that put off roots. These underground stems branch out and split apart from each other as they send out their own individual root systems and become separate plants. The strawberry is a good example of a plant that reproduces asexually through its above ground stem. Strawberries send out “runners”, which forms a stolon that sends out roots when it reaches soil. Eventually, these roots develop well enough to support the new plant and it begins to grow rapidly above ground as another strawberry plant.
Some plants do reproduce through their roots, of course. Though most trees reproduce sexually and spread from dropped nuts or seeds, many are also capable of reproducing asexually through the process of suckering. This occurs when shoots form and branch upward from already existing roots. Eventually, these become separate trees in their own right. In this way, a single Elm can eventually become a forest of clones.
Though far less common that asexual reproduction through roots or stems, some plants can reproduce asexually through their leaves. In the case of the Kalanchoe, plantlets form on already existing leaves and take root when they fall to the ground.
Asexual manipulation in plants occurs when people use special techniques to propagate plants asexually that would not normally reproduce in such a way. The plants typically propagated this way are commercially viable food and medicine crops or ornamental plants chosen for their beauty. The most commonly known method of asexual manipulation is probably grafting, specifically the grafting of fruit trees. When people want to propagate apple trees, the most frequent and successful method is by grafting a cut branch from the tree that is wanted into the root and stem of another. The resulting apple tree will be fed and watered through the stock's root system, but the fruit produced will be genetically the fruit of the tree from which the cutting was taken.
Asexual Reproduction in Animals
As stated above, asexual reproduction in animals is rare, but it does happen. In animals such as jellyfish and a number of echinoderms, this occurs through a process of budding in which offspring develop affixed to the parent's body. Once mature, the buds generally split away and take up their own lives. This same process sometimes occurs without the buds splitting apart, as is the case with corals, which continue to grow as new offspring develop.
Another type of asexual reproduction in animals is parthenogenesis, or “virgin birth”. A single parent lays an egg, then the egg develops into another organism without having to be fertilized by a male. This is the norm with some types of reptiles, fish, and amphibians. However some other species use this opportunistically. Aphids usually reproduce sexually, but will reproduce through parthenogenesis in the spring when they find themselves with an abundance of available food. This process is faster than sexual reproduction, and allows the species to take advantage of the abundance of food while it lasts.
During asexual reproduction, a single organism reproduces itself. The offspring has no genetic input from a second parent, as with sexual reproduction. Barring any mutations during reproduction, the offspring is an exact clone of the parent. Most plants that reproduce asexually do so through their roots or by special sections of stem, many of which exist underground like roots. Though rare, there are some plants that reproduce asexually through their leaves. Asexual reproduction is common in every kingdom except for the animal kingdom. There are some animals that can reproduce asexually, but most can’t. Some animals, like jellyfish, reproduce asexually so that the offspring forms as a bud on the parent that eventually breaks off and continues developing as an individual. While rarer even than budding, there are some few animals, like the kimono dragon, that reproduce through parthenogenesis. Parthenogenesis, or virgin birth, occurs when a parent animal lays an unfertilized egg that eventually develops into a new individual and hatches without ever being fertilized.