Speciation
Categories: Speciation | Ecology
Speciation refers to the evolutionary process by which new biological species arise. There are three main ideas concerning the creation of new species (Speciation mechanisms), each based on the degree to which populations undergoing this process are geographically isolated from one another. Speciation mechanisms include allopatric speciation, sympatric speciation, and parapatric speciation.
Speciation mechanisms
Ernst Mayr proposed a speciation mechanism referred to as allopatric speciation. In allopatric speciation, a population splits into two geographically isolated allopatric populations (for example, by habitat fragmentation or emigration). The isolated populations then undergo genotypic and/or phenotypic divergence as they a) become subjected to dissimilar selective pressures and b) they independently undergo genetic drift. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes.
In sympatric speciation, species diverge while remaining sympatric (i.e. the diverging populations are not isolated geographically). Examples of sympatric speciation are found in insects which become dependent on different host plants in the same area. Increased ploidy levels, i.e. Polyploidy, is a mechanism often attributed to causing some speciation events in sympatry. Polyploidy is observed in many plant species, a well studied example being that of wheat. However, it should be noted that not all polyploids are completely reproductively isolated from their parental plants, so an increase in chromosome number may not result in the complete cessation of gene flow between the incipient polyploids and their parental diploids.
In parapatric speciation, the zones of two diverging populations 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.
All forms of speciation have actually taken place over the course of evolution, though it still remains a subject of debate as to the relative importance of each mechanism in driving biodiversity.
There is some debate as to the rate at which speciation events occur over geologic time. While some evolutionary biologists claim that speciation events have remained relatively constant over time, some palaeontologists such as Niles Eldredge and Stephen Jay Gould have 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.
Other related terms: Reinforcement is a process by which prezygotic isolation is selectively favored when postzygotic isolation is sufficiently strong.
Observed speciation
Humans have themselves created many new species or have observed new species appearing in nature. [1][2]
| Basic topics in evolutionary biology |
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| Processes of evolution: evidence - macroevolution - microevolution - speciation |
| Mechanisms: selection - genetic drift - gene flow - mutation |
| Modes: anagenesis - catagenesis - cladogenesis |
| History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis |
| Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics - evo-devo |
| List of evolutionary biology topics | Timeline of evolution | Timeline of human evolution |