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Speciation refers to the appearance of a new species of life on earth, particular 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.
Speciation mechanisms
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 taxaA taxon (plural taxa is an element of a taxonomy, e. in the scientific classification in biology. Taxa form a hierarchical scheme, each being broken down into subtaxa. In traditional Linnaean taxonomy, taxa are ranked as follows, with some of the less wid. See also the issue of Ring speciesIn biology, ring species present an interesting problem for those who seek to divide the living world into discrete species. The coloured bar to the right shows a number of natural populations, each population represented by a different colour, varying al.
Another proposed mechanism of speciation is sympatrySympatry is one of three theoretical models for the phenomenon of speciation. In complete contrast to allopatry, species undergoing sympatric speciation are not geographically isolated by, for example, a mountain or a river. The speciating populations sha, by which new species emerge alongside the old. This might occur if, say, subpopulations become dependent on different plantGreen algae land plants (embryophytes non-vascular embryophytes Hepatophyta liverworts Anthocerophyta hornworts Bryophyta mosses vascular plants (tracheophytes seedless vascular plants Lycopodiophyta clubmosses Equisetophyta horsetails Pteridophyta "true"s 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 invertebrateInvertebrate is a term coined by Chevalier de Lamarck to describe any animal without a backbone or vertebra, like insects, squids and worms. He divided them into two groups, the Insecta and the Vermes. However, the invertebrates are not a coherent group os, especially the insectSubclass Apterygota Symphypleona globular springtails Subclass Archaeognatha (jumping bristletails) Subclass Dicondylia Monura extinct Thysanura (common bristletails) Subclass Pterygota Palaeodictyoptera extinct Ephemeroptera (mayflies) Odonata ( dragonfls. 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.
Evolutionary biology Ecology
