Speciation refers to the appearance of a new species of life on earth, particularly as seen in the fossil record. There are many ideas about the process leading to the creation of new species, each typically based on any of the Darwinian theories of biological evolution.
Most of these ideas share the hypothesis that speciation occurs when a parent species (also referred to as a common ancestor) splits into two (or more) reproductively-isolated populations, each of which then accumulates changes from sexual reproduction and/or random mutation (in addition to any other various contributors to genetic change) until the populations are no longer capable of interbreeding (cladogenesis). (This is one definition of what a species is; see species.)
If a single population of a species changes enough over time to be designated a new species while the old species dies out, we have a process called anagenesis. Among simpler forms of life, such as bacteria, single mutations can cause drastic changes (called "saltation") that result in speciation in a very short time. Speciation is also related to a process known as adaptive radiation.
Ernst Mayr proposed a speciation mechanism called allopatry. Allopatry begins when subpopulations of a species become isolated geographically (for example, by habitat fragmentation or migration). The isolated populations are then liable to diverge evolutionarily over many generations as a) they become subjected to dissimilar selective pressures and b) they independently undergo genetic drift; particularly when one of the subpopulations is small (a scenario that leads to the "founder effect"). This kind of speciation is evident in many vertebrates' taxa. See also the issue of Ring species.
Another proposed mechanism of speciation is sympatry, by which new species emerge alongside the old. This might occur if, say, subpopulations become dependent on different plants in the same area; and if variations in mating lead one subpopulation to become reproductively isolated from the other. Many examples of this kind of speciation are found in the invertebrates, especially the insects. Polyploidy is also a very common cause of sympatric speciation. Polyploidy is seen as a mode of speciation in many plants, a well studied example being that of the wheat species. In asexually reproducing organisms, there are other modes that become prominent including horizontal gene transfer by viruses and mutations.
A further mechanism is parapatry, where the zones of two species abut but do not overlap. There is only partial separation afforded by geography, so individuals of each species may come in contact or cross the barrier from time to time.
Which mechanisms of speciation have actually taken place over the course of evolution is a subject of debate, as is the speed with which they occur. Palaeontologists Niles Eldredge and Stephen Jay Gould argued that species usually remain unchanged over long stretches of time, and that speciation occurs only over relatively brief intervals, a view known as punctuated equilibrium. It is entirely possible that speciation has occurred by several mechanisms simultaneously over evolutionary history.