Asexual Reproduction

Asexual Reproduction, the formation of a new individual from cells of the parent, without meiosis, gamete formation, or fertilization. There are several types of asexual reproduction. Fission is the simplest form and involves the division of a single organism into two complete organisms, each identical to the other and to the parent. Fission is common among unicellular organisms such as bacteria, many protists, some algae (such as Spirogyra and Euglena), as well as a few higher organisms such as flatworms and certain species of polychaete worms. A similar form of asexual reproduction is regeneration, in which an entire organism may be generated from a part of its parent. The term regeneration normally refers to regrowth of missing or damaged body parts in higher organisms, but whole body regeneration occurs in hydroids (see Hydra), starfish, and many plants. Spores are another form of asexual reproduction and are common among bacteria, protists, and fungi. Spores are DNA-containing capsules capable of sprouting into new organisms. Unlike most seeds, spores are produced without sexual union of gametes, that is, reproductive cells.

Budding is another method of asexual reproduction in which a group of self-supportive cells sprouts from and then detaches from the parent organism. Unlike eggs or spores, buds are multicellular and usually contain more than one cell layer. Hydroids and sea squirts reproduce by budding. Vegetative reproduction is common among plants and consists of certain parts that grow out from a main parent plant and eventually root and sprout to form new, independent plants. Examples are the runners of strawberries, the tubers of potatoes, and the bulbs of onions. Parthenogenesis is an important means of asexual reproduction in which new individuals are formed from unfertilized eggs. It occurs in some insects, amphibians, reptiles, fishes, and birds, as well as in some plants.

Asexual reproduction may provide a secondary means of multiplying in organisms that ordinarily reproduce sexually. Under certain conditions this may be the only way to reproduce. For example, if there are no other individuals with which to exchange gametes or, in plants, if pollinators are absent. Asexual reproduction demands less time and energy and may be the most efficient way for certain species to reproduce under harsh environmental conditions. Some species switch between asexual and sexual modes of reproduction in an annual cycle so that each takes place at the most favorable time. Examples of this are aphids that reproduce asexually in the summer but sexually in the fall, and water fleas, which have a similar cycle.

Organisms that can reproduce asexually are particularly well suited to colonize new habitats because a single individual can establish an entire new population. This is the case in many invasive and weedy species, including many pests introduced from one region to another. An example is the common dandelion, which can reproduce from unfertilized seeds.

The offspring of organisms that reproduce asexually are genetically identical to their parents and to each other (see Genetics). Without sexual reproduction, the species cannot benefit from the variability introduced by mixing genes (see Natural Selection). Therefore, evolutionary adaptation to changing environmental conditions may proceed slowly.