Living organisms need to reproduce to sustain their species. Some species reproduce sexually and combine their DNA to produce a new organism. Sexual reproduction requires both an egg and sperm that combine to create a new organism that possesses a combination of genes from both parents. Organisms can interact with each other to achieve this goal, or the egg and sperm can travel via other organisms or wind or water currents. This offspring, while it contains genetic traits of each of its parents, is genetically unique. This process results in diversity in populations, which improves the odds of survival in a changing environment.
Other organisms reproduce asexually and create offspring completely on their own. With no other organism involved, all offspring are genetically identical to the parent. This method of reproduction is common among single-celled organisms and plants and animals with simple organizations. It tends to occur more rapidly than sexual reproduction, allowing these species to grow at a faster rate. From the outset, offspring are able to live independently, needing nothing from the parent.
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Asexual reproduction results in offspring with identical genes to that of the parent. This can occur through division, parthenogenesis or apomixis.
Some species are capable of either sexual or asexual reproduction. The simplest organisms have no sex organs, so asexual reproduction is a necessity. Other species, such as corals, may reproduce either sexually or asexually, depending on conditions. Though it occurs infrequently, some species surprise scientists by adapting to asexual reproduction, sometimes where the species or even an individual organism had reproduced sexually in the past. This is most common in species in captivity and in those where no males are present to further the species, but is also in evidence in sharks and snakes in the wild where the populations included both male and females of the species.
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Asexual reproduction happens most frequently in lower-level organisms, such as uni- and multicellular organisms that serve as the primary and secondary producers in an ecosystem. This is beneficial as it enables these organisms to reproduce even when there is no suitable mate for them, enabling them to quickly produce a large number of offspring with the same genetic makeup.
Of course, in some cases a large population with the same genetic makeup may be a disadvantage as it limits a species’ ability to adapt to changing conditions. In addition, any mutations will be present in all individuals. If one organism is genetically susceptible to disease, all its offspring will be as well, so an entire population can quickly be eliminated.
An Organism Divides Itself
There are several ways an organism can create offspring by dividing directly from the parent. This can happen when the parent’s cells divide through the process of fission, when offspring form attached to the parent through budding or when a section of the parent is separated from the parent and then grows the missing part or parts to become a whole separate organism.
Fission Is Simple Division
Fission is the method of asexual reproduction seen in the simplest life forms, such as the amoeba, and tends to occur rather rapidly. In some species, cell division can occur as quickly as every 20 minutes. All eukaryotic cells that do not produce gametes (eggs and sperm) reproduce using mitosis. In this process, two identical daughter cells develop and separate into two distinct organisms.
In the process of binary fission, a cell divides in half and separates so that each half becomes a new independent organism. At its simplest form, fission occurs when a chromosome is replicated and the cell expands to accommodate both chromosomes. The cell then elongates and pinches inward at the center as the two chromosomes move apart before separating and producing two identical cells. In effect, the first organism becomes two organisms of the same size with no damage done to the parent cell.
In other organisms, such as algae, and some groups of bacteria, the parent cell divides multiple times and separates into multiple identical offspring. Using multiple fission, they grow and replicate cellular DNA multiple times, rapidly producing dozens or even hundreds of smaller cells called baeocytes before finally tearing open and releasing the new organisms that are then capable of independent life.
Budding also involves a division. Offspring bud and grow while attached to the parent until mature enough to survive on their own. After separation, the parent organism remains unchanged from its original state. While able to survive independent of the parent, these new organisms are smaller in size at first but continue to grow and mature.
A number of plants reproduce in this manner, including those that grown from corms or bulbs, tubers, rhizomes or plants with a stolon (commonly known as a runner) that forms adventitious roots that emerge separate from the primary root and become a new plant. Other plants grow small buds on their leaves that, when separated from the plant (or when they touch soil), are capable of growing independently. This is how some plants, such as daffodils, “naturalize” or spread on their own.
Strawberry plants have runners, stems that root themselves and create a new plant. Garlic has a corm, which resembles tulip or daffodil bulb, which can divide and separate to create new plants. Ginger and some flowers such as irises form rhizomes that serve as the foundation for new plants. In some species, such as certain cacti, the offspring remain attached to the parent but form their own colony.
Budding is less common in the animal kingdom, but it is seen in some organisms such as yeast and fixed sea life such as hydras, which develop polyps that break off to form new organisms. Some sponges and corals also reproduce asexually. After reaching a certain size, some species form polyps and divide to form a new colony. In other cases, they reproduce sexually, by releasing sperm or eggs that fertilize in the water and are carried off to grow in another location.
Splitting off on Their Own
Fragmentation or regeneration occurs when a parent or an organism “loses” a body part and then regrows what is missing and becomes a new whole. This is common among many worms, sea urchins, sponges and starfish. In the plant kingdom, fragmentation occurs in fungi, lichen, and photosynthetic algae and bacteria.
A recent study revealed details about the reproductive process of freshwater planarian, better known as flatworms. Flatworms are shy organisms that only reproduce in the dark and when they are undisturbed, so scientists needed to use continuous video recordings to determine how the process occurs. They discovered that asexual reproduction in flatworms occurs in a predictable manner, approximately once a month. The process has three stages: waist formation, pulsation and rupture. During the first step, waist formation, a weak point is created so that pulses cause the organism to break or rupture at that weak point. Once the worm has separated into two sections, both pieces regrow the missing section, using stem cells that have been distributed between the two portions.
While this process frequently occurs naturally, artificial reproduction in plants is also possible. This is done through grafting, layering or artificially creating roots by placing cuttings in water for a period of time. Alternately, tissue cultures can be taken and manipulated in a laboratory to create new plants.
Changing With Conditions
Some species use more than one method of reproduction. Some tubers, such as the potato, can reproduce through either budding or when part of the plant separates (in this case, the “eyes”) and is replanted, through fragmentation. Fungi also reproduce through both budding and fragmentation, where asexual spores are produced and released from the parent plant. In some cases, genetic mutations or certain environmental conditions can cause a species that typically reproduce sexually to adapt to asexual reproduction.
Offspring From Unfertilized Eggs
In some cases, asexual reproduction can occur in organisms with sexual organs. In these cases, eggs develop without fertilization. Parthenogenesis is the process by which an unfertilized egg develops into a new organism. This offspring would by necessity have the same genes as its mother.
Parthenogenesis, also known as “virgin birth” occurs most often in plants. Though rare in animals, it has been documented in birds, sharks, rays and squamate reptiles such as snakes and lizards. In this process, an egg develops without fertilization. Invertebrates such as water fleas, aphids, stick insects, some ants, wasps and bees reproduce in this manner. It is common in honeybees where unfertilized eggs produce drones that are haploid males; if the egg is fertilized, it produces a female worker or queen. Certain vertebrates also have reproduced via parthenogenesis; this has been seen mostly in zoos in certain species such as Komodo dragons, and in some sharks when females are isolated from males.
There are two types: obligate and facultative parthenogenesis. Obligate parthenogenesis species are incapable of reproducing sexually while facultative parthenogenesis occurs when species that normally reproduce in a sexual manner instead reproduce asexually.
Obligate parthenogenesis rarely occurs in plants. Within the animal kingdom, it is most frequently seen in lizards and generally only among all-female populations. It also has been seen in one species of snake: the Brahminy blind snake. Facultative parthenogenesis was initially discovered in certain chickens and turkeys in the 1950s and was more recently documented in snakes and varanid lizards. It has also been seen in bony fish and some species of sharks and rays. In many cases, this is thought to happen due to a mutation and may be related to environmental factors.
Commonly seen in some phasmids and mayflies, facultative parthenogenesis is rare among mammals and was long considered to occur only in captivity, and only in populations where females had limited access to males. However, a 2012 study of snakes indicated that parthenogenetic reproduction is not limited to disproportionate sex ratios where there was a shortage of males. In fact, the number of males and females in this study was at or close to even numbers. The data, which showed that the genetic makeup of the offspring was identical to that of the mother, provided evidence that these “virgin births” also happened among snake populations where the presence of male snakes was common. The research also indicates that this happens with more frequency than previously assumed, in up to 5 percent of the snake population studied.
Asexual Reproduction: Natural Cloning in Plants
Apomixis, asexual reproduction in plants via seeds, is a natural way of cloning that allows plant embryos to grown from unfertilized eggs. Apomixis occurs naturally in a number of tropical and subtropical grasses, orchids, citrus plants and in wild species of crops such as beets, strawberries and mangoes. Over 300 species and over 35 families of plants reproduce through apomixis.
Scientists have worked to develop apomictic plants in hopes of producing crops that are of a consistent quality and yield as well as being more tolerant of weather conditions, and being more disease- and insect-resistant. This would also allow the production of favorable hybrid species considered too difficult or expensive to grow using traditional methods. Scientists believe that apomixis technology will reduce cost and breeding time of crops and also avoid the complications associated with sexual reproduction and vegetative propagation.