Sexual reproduction involves the production of new cells by the fusion of sex cells (sperm and ova) to produce a genetically different cell. Asexual reproduction, on the other hand, is the production of new cells by simple division of the parent cell into two daughter cells (called binary fission). Since there is no fusion of two different cells, the daughter cells produced by asexual reproduction are genetically identical to the parent cell.
The adaptive advantage of asexual reproduction is that organisms can reproduce rapidly, and so colonize favorable environments rapidly.
Bacteria, cyanobacteria, algae, most protozoa, yeast, dandelions, and flatworms all reproduce asexually. When asexual reproduction occurs, the new individuals are called clones, because they are exact duplicates of their parent cells. Mosses reproduce by forming runners that grow horizontally, produce new stalks, and then the runner decomposes, leaving a new plant which is a clone of the original.
Duplication of organisms, whether sexually or asexually, involves the partitioning of the genetic material (chromosomes) in the cell nucleus.
During asexual reproduction, the chromosomes divide by mitosis, which results in the exact duplication of the genetic material into the nuclei of the two daughter cells. Sexual reproduction involves the fusion of two gamete cells (the sperm and ova) which each have half the normal number of chromosomes, a result of reduction division known as meiosis.
Bacteria reproducing asexually double their numbers rapidly, approximately every 20 minutes. This reproduction rate is offset by a high death rate that may be the result of the accumulation of alcohol or acids that concentrate from the bacterial colonies.
Yeasts reproduce asexually by budding, as well as reproducing sexually. In the budding process, a bulge forms on the outer edge of the yeast cell as nuclear division takes place. One of these nuclei moves into the bud, which eventually breaks off completely from the parent cell. Budding also occurs in flatworms, which divide into two and then regenerate to form two new flatworms.
Bees, ants, wasps, and other insects can reproduce sexually or asexually. In asexual reproduction, eggs develop without fertilization, a process called parthenogenesis. In some species the eggs may or may not be fertilized; fertilized eggs produce females, while unfertilized eggs produce males.
There are a number of crop plants which are propagated asexually. The advantage of asexual propagation to farmers is that the crops will be more uniform than those produced from seed. Some plants are difficult to cultivate from seed and asexual reproduction in these plants makes it possible to produce crops that would otherwise not be available for commercial marketing.
The process of producing plants asexually is called vegetative propagation and is used for such crops as potatoes, bananas, raspberries, pineapples, and some flowering plants used as ornamentals. Farmers plant the so-called " eyes" of potatoes to produce duplicates of the parent. With banana plants, the suckers that grow from the root of the plant are separated and then planted as new ones. With raspberry bushes, branches are bent and covered with soil. They then grow into a separate plant with their own root system and can eventually be detached from the parent plant.
Leone, Francis. Genetics: The Mystery and the Promise. Blue Ridge Summit, PA: Tab Books, 1992.
Taylor, Martha. Campbell's Biology Student Study Guide. Redwood City, CA: Benjamin/Cummings, 1990.
Allison, Richard. "Genetic Engineering Studied." Cancer Researcher Weekly (21 March 1994): 13.
Nash, J. Madeleine. "Is Sex Really Necessary?" Time (20 January 1992): 47.
Robertson, John A. "The Question of Human Cloning." The Hastings Center Report (March/April 1994): 6.