Asexual Reproduction Definition
Asexual reproduction occurs when an organism makes more of itself without exchanging genetic information with another organism through sex.
In sexually reproducing organisms, the genomes of two parents are combined to create offspring with unique genetic profiles. This is beneficial to the population because genetically diverse populations have a higher chance of withstanding survival challenges such as disease and environmental changes.
Asexually reproducing organisms can suffer a dangerous lack of diversity – but they can also reproduce faster than sexually reproducing organisms, and a single individual can found a new population without the need for a mate.
Some organisms that practice asexual reproduction can exchange genetic information to promote diversity using forms of horizontal gene transfer such as bacteria who use plasmids to pass around small bits of DNA. However this method results in fewer unique genotypes than sexual reproduction.
Asexual reproduction is practiced by most single-celled organisms including bacteria, archaebacteria, and protists. It is also practiced by some plants, animals, and fungi.
Advantages of Asexual Reproduction
Important advantages of asexual reproduction include:
1. Rapid population growth. This is especially useful for species whose survival strategy is to reproduce very fast.
Many species of bacteria, for example, can completely rebuild a population from just a single mutant individual in a matter of days if most members are wiped out by a virus.
2. No mate is needed to found a new population.
This is useful for species whose members may find themselves isolated, such as fungi that grow from wind-blown spores, plants that rely on pollinators for sexual reproduction, and animals inhabiting environments with low population density.
3. Lower resource investment. Asexual reproduction, which can often be accomplished just by having part of the parent organism split off and take on a life of its own, takes fewer resources than nurturing a new baby organism.
Many plants and sea creatures, for example, can simply cut a part of themselves off from the parent organism and have that part survive on its own.
Only offspring that are genetically identical to the parent can be produced in this way: nurturing the creation of a new organism whose tissue is different from the parents’ tissue takes more time, energy, and resources.
This ability to simply split in two is one reason why asexual reproduction is faster than sexual reproduction.
Disadvantages of Asexual Reproduction
The biggest disadvantage of asexual reproduction is lack of diversity. Because members of an asexually reproducing population are genetically identical except for rare mutants, they are all susceptible to the same diseases, nutrition deficits, and other types of environmental hardships.
The Irish Potato Famine was one example of the down side of asexual reproduction: Ireland’s potatoes, which had mainly reproduced through asexual reproduction, were all vulnerable when a potato-killing plague swept the island. As a result, almost all crops failed, and many people starved.
The near-extinction of the Gros-Michel banana is another example – one of two major cultivars of bananas, it became impossible to grow commercially in the 20th century after the emergence of a disease to which it was genetically vulnerable.
On the other hand, many species of bacteria actually take advantage of their high mutation rate to create some genetic diversity while using asexual reproduction to grow their colonies very rapidly. Bacteria have a higher rate of errors in copying genetic sequences, which sometimes leads to the creation of useful new traits even in the absence of sexual reproduction.
Types of Asexual Reproduction
There are many different ways to reproduce asexually. These include:
2. Budding. Some organisms split off a small part of themselves to grow into a new organism. This is practiced by many plants and sea creatures, and some single-celled eukaryotes such as yeast.
3. Vegetative propagation. Much like budding, this process involves a plant growing a new shoot which is capable of becoming a whole new organism. Strawberries are an example of plants that reproduce using “runners,” which grow outward from a parent plant and later become separate, independent plants.
4. Sporogenesis. Sporogenesis is the production of reproductive cells, called spores, which can grow into a new organism.
Spores often use similar strategies to those of seeds. But unlike seeds, spores can be created without fertilization by a sexual partner. Spores are also more likely to spread autonomously, such as via wind, than to rely on other organisms such as animal carriers to spread.
5. Fragmentation. In fragmentation, a “parent” organism is split into multiple parts, each of which grows to become a complete, independent “offspring” organism. This process resembles budding and vegetative propagation, but with some differences.
For one, fragmentation may not be voluntary on the part of the “parent” organism. Earthworms and many plants and sea creatures are capable of regenerating whole organisms from fragments following injuries that split them into multiple pieces.
When fragmentation does occur voluntarily, the same parent organism may split into many roughly equal parts in order to form many offspring. This is different from the processes of budding and vegetative propagation, where an organism grows new parts which are small compared to the parent and which are intended to become offspring organisms.
6. Agamenogenesis. Agamenogenesis is the reproduction of normally sexual organisms without the need for fertilization. There are several ways in which this can happen.
In parthenogenesis, an unfertilized egg begins to develop into a new organism, which by necessity possesses only genes from its mother.
This occurs in a few species of all-female animals, and in females of some animal species when there are no males present to fertilize eggs.
In apomoxis, a normally sexually reproducing plant reproduces asexually, producing offspring that are identical to the parent plant, due to lack of availability of a male plant to fertilize female gametes.
In nucellar embryony, an embryo is formed from a parents’ own tissue without meiosis or the use of reproductive cells. This is primarily known to occur in citrus fruit, which may produce seeds in this way in the absence of male fertilization.
Examples of Asexual Reproduction
All bacteria reproduce through asexual reproduction, by splitting into two “daughter” cells that are genetically identical to their parents.
Some bacteria can undergo horizontal gene transfer – in which genetic material is passed “horizontally” from one organism to another, instead of “vertically” from parent to child. Because they have only one cell, bacteria are able to change their genetic material as mature organisms.
The process of genetic exchange between bacterial cells is sometimes referred to as “sex,” although it is performed to change the genotype of a mature bacterium, not as a means of reproduction.
Bacteria can afford to use this survival strategy because their extremely rapid reproduction makes harmful genetic mutations – such as copying errors or horizontal gene transfer gone wrong – inconsequential to the whole population. As long as a few individuals survive mutation and calamity, those individuals will be able to rebuild the bacterial population quickly.
This strategy of “reproduce fast, mutate often” is a major reason why bacteria are so quick to develop antibiotic resistance. They have also been seen to “invent” whole new biochemistries in the lab, such as one species of bacteria that spontaneously acquired the ability to perform anaerobic respiration.
This strategy would not work well for an organism that invests highly in the survival of individuals, such as multicellular organisms.
Slime molds are a fascinating organism that sometimes behave like a multicellular organism, and sometimes behave like a colony of single-celled organisms.
Unlike animals, plants, and fungi, the cells in a slime mold are not bound together in a fixed shape and dependent on each other for survival. The cells that make up a slime mold are capable of living individually and may spread or separate when food is abundant, much like individuals in a colony of bacteria.
But slime mold cells are eukaryotic, and can display a high degree of cooperation to the point of creating a temporary extracellular matrix and a “body” which may become large and complex. Slime molds whose cells are working cooperatively can be mistaken for fungi, and can perform locomotion.
Slime molds can produce spores much like a fungus, and they can also reproduce through fragmentation. Environmental causes or injury may cause a slime mold to disperse into many parts, and units as small as a single cell may grow into a whole new slime mold colony/organism.
New Mexico Whiptail Lizards
This species of lizard was created by the hybridization of two neighboring species. Genetic incompatibility between the hybrid parents made it impossible for healthy males to be born: however, the female hybrids were capable of parthenogenesis, making them a reproductively independent population.
All New Mexico whiptail lizards are female. New members of the species can be created through hybridization of the parent species, or through parthenogenesis by female New Mexico whiptails.
Possibly as a remnant of their sexually reproducing past, New Mexico whiptail lizards do have a “mating” behavior which they must go through to reproduce. Members of this species are “mated with” by other members, and the lizard playing the female role will go onto lay eggs.
It is thought that the mating behavior stimulates ovulation, which can then result in a parthenogenic pregnancy. The lizard playing the “male” role in the courtship does not lay eggs.