All living things must reproduce in order to pass down genes to the offspring and continue to ensure the survival of the species. Natural selection, the mechanism for evolution, chooses which traits are favorable adaptations for a given environment and which are unfavorable. Those individuals with undesirable traits will, theoretically, eventually be bred out of the population and only the individuals with the "good" traits will live long enough to reproduce and pass down those genes to the next generation.
There are two types of reproduction: sexual reproduction and asexual reproduction. Sexual reproduction requires both a male and a female gamete with different genetics to fuse during fertilization, therefore creating an offspring that is different from the parents. Asexual reproduction only requires a single parent that will pass down all of its genes to the offspring. This means there is no mixing of genes and the offspring is actually a clone of the parent (barring any sort of mutations).
Asexual reproduction is generally used in less complex species and is quite efficient. Not having to find a mate is advantageous and allows a parent to pass down all of its traits to the next generation. However, without diversity, natural selection cannot work and if there are no mutations to make more favorable traits, asexually reproducing species may not be able to survive a changing environment.
JW Schmidt/Wikimedia Commons/CC BY 3.0
Almost all prokaryotes undergo a type of asexual reproduction called binary fission. Binary fission is very similar to the process of mitosis in eukaryotes. However, since there is no nucleus and the DNA in a prokaryote is usually just in a single ring, it is not as complex as mitosis. Binary fission starts with a single cell that copies its DNA and then splits into two identical cells.
This is a very fast and efficient way for bacteria and similar types of cells to create offspring. However, if a DNA mutation were to occur in the process, this could change the genetics of the offspring and they would no longer be identical clones. This is one way that variation can occur even though it is undergoing asexual reproduction. In fact, bacterial resistance to antibiotics is evidence for evolution through asexual reproduction.
Lifetrance/Wikimedia Commons/CC BY-SA 3.0
Another type of asexual reproduction is called budding. Budding is when a new organism, or the offspring, grows off the side of the adult through a part called a bud. The new baby will stay attached to the original adult until it reaches maturity at which point they break off and become its own independent organism. A single adult can have many buds and many offspring at the same time.
Both unicellular organisms, like yeast, and multicellular organisms, like hydra, can undergo budding. Again, the offspring are clones of the parent unless some sort of mutation happens during the copying of the DNA or cell reproduction.
Kevin Walsh/Wikimedia Commons/CC BY 2.0
Some species are designed to have many viable parts that can live independently all found on one individual. These types species can undergo a type of asexual reproduction known as fragmentation. Fragmentation happens when a piece of an individual breaks off and a brand new organism forms around that broken piece. The original organism also regenerates the piece that broke off. The piece may be broken off naturally or could be broken off during an injury or other life threatening situation.
The most well known species that undergoes fragmentation is the starfish, or sea star. Sea stars can have any of their five arms broken off and then regenerated into offspring. This is mostly due to their radial symmetry. They have a central nerve ring in the middle that branches out into five rays, or arms. Each arm has all the parts necessary to create a whole new individual through fragmentation. Sponges, some flatworms, and certain types of fungi can also undergo fragmentation.
Neil/Wikimedia Commons/CC BY-SA 3.0
The more complex the species, the more likely they are to undergo sexual reproduction as opposed to asexual reproduction. However, there are some complex animals and plants that can reproduce via parthenogenesis when necessary. This is not the preferred method of reproduction for most of these species, but it may become the only way to reproduce for some of them for various reasons.
Parthenogenesis is when an offspring comes from an unfertilized egg. Lack of available partners, an immediate threat on the female's life, or other such trauma may result in parthenogenesis being necessary to continue the species. This is not ideal, of course, because it will only produce female offspring since the baby will be a clone of the mother. That will not fix the issue of lack of mates or carrying on the species for an indefinite period of time.
Some animals that can undergo parthenogenesis include insects like bees and grasshoppers, lizards such as the komodo dragon, and very rarely in birds.
USDA Forest Service Pacific Southwest Research Station/Wikimedia Commons/CC BY 2.5
Many plants and fungi use spores as a means of asexual reproduction. These types of organisms undergo a life cycle called alternation of generations where they have different parts of their lives in which they are mostly diploid or mostly haploid cells. During the diploid phase, they are called sporophytes and produce diploid spores they use for asexual reproduction. Species that form spores do not need a mate or fertilization to occur in order to produce offspring. Just like all other types of asexual reproduction, the offspring of organisms that reproduce using spores are clones of the parent.
Examples of organisms that produce spores include mushrooms and ferns.