Biological evolution, or organic evolution, is the development over time of different taxa of biological organisms due to the selective survival and propagation of individuals of successive generations. The main driving force behind biological evolution is natural selection, the process by which organisms with traits that tend to make them slightly more likely to survive and reproduce therefore tend to pass on more of their genes, resulting in these traits predominating in future generations. In some cases, some change may also occur through artificial selection and other processes.
Biological evolution is often referred to as just "evolution", though strictly speaking this word can refer more broadly to other kinds of developmental processes. Still, where the context minimizes ambiguity, the use of the word "evolution" to refer specifically to biological evolution is common and unobjectionable.
Development through biological evolution is the principal defining characteristic of the universe of Biota. However, this is not to say that all Biotan species have necessarily developed through this process. Life forms created through magic or technological manipulation which bear clear chemical and anatomical similarities to naturally evolved organisms are frequently classified as organisms by analogy. There may even be some magical worlds on which gods or similar beings created all life (save of course, necessarily, for the gods themselves), but where that life, again, sufficiently resembles evolved organisms to be placed in the same taxa. In any case, even where such life forms did not originally arise through typical evolutionary processes, they may be capable of further development and diversification through such processes.
One important prerequisite for the possibility of biological evolution is that traits must be in some way heritable. Organisms must be able to reproduce and to pass on some innate characteristics or tendencies onto their offspring. The mechanisms encoding these heritable traits are encoded are called genes. Genes can take many different forms, depending on the fœdus and the cosmos; among Spirileges of Xi, they take the form of sequences of nucleic acid bases of long molecules of deoxyribonucleic acid, while in Dverelei the genes are held in the arrangement of separate small molecules collectively called the genosystem. In most cases, these included, only inherent traits are passed on through this method; skills and knowledge acquired during the course of an organism's life are not encoded in the genes. However, among other universes the genetic mechanism may allow for the inheritance of acquired characteristics as well.
It is not necessary that every member of a species must be capable of reproduction; among social insects, for instance, only a female "queen" and a few male "drones" actually reproduce and pass on their genes directly, while most of the individuals are sterile workers. Even here, however, the workers share some genes of the queen, and so indirectly play a role in the success of their own genetic propagation.
Another necessary factor for the occurrence of biological evolution is that the passing on of genetic traits to the children have some random variance. A child does not inherit a perfect copy of the parents' genes, or even a perfect combination of genes from multiple parents; rather, occasional transcription errors occur, called mutations. The mutations themselves are not necessarily beneficial to the organism; as a matter of fact, for an already well adapted organsim, a random change is much more likely to be harmful than beneficial, although there is a slight chance that a beneficial mutation may occur that would promote the organism's chances of survival and reproduction. While the mutations themselves are more or less random, however, they provide the raw materials of natural selection which discriminates between them, as organisms with mutations that are harmful to them are more likely to die out with fewer offspring, and therefore beneficial mutations tend to be passed on to future generations. While this nonrandom selection process is responsible for much of the change organisms undergo in evolution, there may also be a random process of genetic drift as genes and traits that provide no particular benefit or hindrance to the organism may change or appear arbitrarily.
The main result of biological evolution over time is that species of organisms tend to become better adapted to their environment. This does not mean that they tend toward some specific pinnacle of adaptation, however; contrary to some nineteenth century misunderstandings (which tend to persist to some degree in common thought even today), there is no unambiguous "evolutionary ladder" that they climb. There are often myriad equally beneficial branches that could be taken, and the course of evolution may be swayed by happenstances such as meteor strikes or weather fluctuations, such that the particular suite of organisms that will exist after a long period of time cannot be predicted. It was by no means inevitable, for example, that evolution on Earth would someday result in anything remotely resembling humans, nor is it likely the case that anything closely resembling humans would ever evolve independently on another planet. The various races inhabiting other planets in many science fiction settings that mostly resemble humans with different skin colors and perhaps slightly altered facial features partake much more of fiction than of science. Nor is it much more sensible to posit an alien planet where an ellogous race evolved from reptiles or insects, since this would imply first the independent evolution there of reptiles or insects, which isn't much more likely than the independent evolution of humans. (This is certainly not to say that such near-human races or alien reptiles or insects may not exist in some worlds, but if they do they must have arisen through a very different process than the usual biological evolution—either biological evolution works very differently in those worlds or some other process is at play.)
While identical species are unlikely to separately arise, however, evolution may hit on similar solutions to similar problems, resulting in unrelated species sharing some similar traits. This process is called parallel evolution. It is due to parallel evolution that fish and whales have independently developed fins and a streamlined shape, for instance, and that so many different kinds of plants have independently evolved tree-like structures. However, parallel evolution works on particular features and adaptations, not on whole organisms, and even there it results in similarities, not in exact duplication; parallel evolution would by no means lead to identical organisms evolving on different planets. It's also worth noting that the environment that organisms are adapting to is a mutable one. It's thus not the case that evolution will ever reach a pinnacle and stop, because by that time the environment may have changed enough to require new adaptations. This is especially true because other organisms are a part of that environment, and are themselves evolving. For two species in competition, this may result in a sort of an evolutionary arms race—for instance, as a prey animal evolves ever better ways of escaping predators, the predators evolve ever better ways of catching the prey anyway.
It is common for evolution to co-opt preexisting structures to new purposes, rather than producing completely new structures ex nihilo. The bones of the human inner ear, for instance, come from the gills of their fish ancestors. Sometimes large changes are made simply by alterations to the rate of an organism's development, or of development of different parts. The phenomenon in which an organism resembles a juvenile version of an ancestor is called neoteny, one good example being that of the human, which in many ways is anatomically and developmentally an overgrown baby.
It could sometimes happen that a previously existing trait or structure becomes no longer useful due to a change in an organism's lifestyle or due to the development of another structure that does the job better; when such a structure persists beyond its utility, it is said to be vestigial. Truly vestigial structures, with no utility at all, do not last long; because it takes energy simply to grow and maintain the structure, and because wasting energy on useless structures takes it away from other uses, such a useless vestige is harmful to the organism, and so will actively be selected against. An organism is likely, therefore, to have few completely vestigial structures, though there may be many structures that initially appear vestigial but which on closer examination turn out to have been turned to a new purpose.
In the competitive venue of natural selection, it is not just individuals that are susceptible to dying out with no descendents. Entire species and higher taxa may eventually find themselves insufficiently adapted to a changing environment, or outcompeted by other species, and may eventually perish without issue. The end of a species or higher taxon is called extinction.
Although biological evolution per se is generally held to be specific to biotic organisms, there is some evidence that some other universes may undergo similar processes. Hathroists often hold that hathra also undergo essentially the same process, one of the factors that leads many of them to place hathra together with biota in the superuniverse of Autoctista. There are even those who believe that manufactured objects (including, but not limited to, life forms of the universe Machillae) evolve in the same sense as biological organisms, that their utility to their creators is analogous to the fitness of a biological organism, as it influences whether or not more similar objects will be made. Some abstract mental constructs such as languages may also undergo sorts of evolutionary processes.
Sometimes mention will be made of "devolution", supposedly a reverse process in which organisms "devolve" into more primitive forms. Strictly speaking, the concept of such devolution is a nonsensical one. While there was once a common saying that ontogeny recapitulates phylogeny, that is that the development of an individual organism reflects the evolutionary development of its species, this is only partly and superficially true. The early stages of a human embryo may bear some resemblance to a fish, but this doesn't mean that a human bears somewhere within it a complete fish genome ready to be reactivated at will. Nor does it make any more sense to suppose that an organism could change into a "more primitive" form that is not necessarily ancestral to it, not only because of the lack of a mechanism of such a change but because of the poor definition of what would make one organism "more primitive" than another. Certainly there's no meaningful way to so rank contemporary organisms; the undeniable fact that a human is more intelligent than, say, a salamander doesn't mean that the human is "more evolved". In short, biological devolution is not a meaningful concept in the context of standard organic evolution, and if there are worlds where it does occur, it must be because evolution works differently there or because there are mechanisms at work other than those supposed.
- Evolution on Wikipedia