Speciation is the evolutionary process by which new biological species arise. Speciation can occur from a variety of different causes and are classified in various forms (e.g. allopatric, sympatric, polyploidization, etc.). Scientists have observed numerous examples of speciation in the laboratory and in nature, however, evolution has produced far more species than an observer would consider necessary. For example, there are well over 350,000 described species of beetles. Great examples of observed speciation come from the observations of island biogeography and the process of adaptive radiation, both explained in an earlier section. The examples shown below provide strong evidence for common descent and are only a small fraction of the instances observed.
The bird species, Sylvia atricapillab, commonly referred to as Blackcaps, lives in Germany and flies southwest to Spain while a smaller group flies northwest to Great Britain during the winter. Gregor Rolshausen from the University of Freiburg found that the genetic separation of the two populations is already in progress. The differences found have arisen in about 30 generations. With DNA sequencing, the individuals can be assigned to a correct group with an 85% accuracy. Stuart Bearhop from the University of Exeter reported that birds wintering in England tend to mate only among themselves, and not usually with those wintering in the Mediterranean. It is still inference to say that the populations will become two different species, but researchers expect it due to the continued genetic and geographic separation
William R. Rice and George W. Salt found experimental evidence of sympatric speciation in the common fruit fly. They collected a population of Drosophila melanogaster from Davis, California and placed the pupae into a habitat maze. Newborn flies had to investigate the maze to find food. The flies had three choices to take in finding food. Light and dark (phototaxis), up and down (geotaxis), and the scent of acetaldehyde and the scent of ethanol (chemotaxis) were the three options. This eventually divided the flies into 42 spatio-temporal habitats.
They then cultured two strains that chose opposite habitats. One of the strains emerged early, immediately flying upward in the dark attracted to the acetaldehyde. The other strain emerged late and immediately flew downward, attracted to light and ethanol. Pupae from the two strains were then placed together in the maze and allowed to mate at the food site. They then were collected. A selective penalty was imposed on the female flies that switched habitats. This entailed that none of their gametes would pass on to the next generation. After 25 generations of this mating test, it showed reproductive isolation between the two strains. They repeated the experiment again without creating the penalty against habitat switching and the result was the same; reproductive isolation was produced
One example of evolution at work is the case of the hawthorn fly, Rhagoletis pomonella, also known as the apple maggot fly, which appears to be undergoing sympatric speciation. Different populations of hawthorn fly feed on different fruits. A distinct population emerged in North America in the 19th century some time after apples, a non-native species, were introduced. This apple-feeding population normally feeds only on apples and not on the historically preferred fruit of hawthorns. The current hawthorn feeding population does not normally feed on apples. Some evidence, such as the fact that six out of thirteen allozyme loci are different, that hawthorn flies mature later in the season and take longer to mature than apple flies; and that there is little evidence of interbreeding (researchers have documented a 4–6% hybridization rate) suggests that speciation is occurring.
London Underground mosquito
The London Underground mosquito is a species of mosquito in the genus Culex found in the London Underground. It evolved from the overground species Culex pipiens.
This mosquito, although first discovered in the London Underground system, has been found in underground systems around the world. It is suggested that it may have adapted to human-made underground systems since the last century from local above-ground Culex pipiens, although more recent evidence suggests that it is a southern mosquito variety related to Culex pipiens that has adapted to the warm underground spaces of northern cities.
The species have very different behaviours, are extremely difficult to mate, and with different allele frequency, consistent with genetic drift during a founder event. More specifically, this mosquito, Culex pipiens molestus, breeds all-year round, is cold intolerant, and bites rats, mice, and humans, in contrast to the above ground species Culex pipiens that is cold tolerant, hibernates in the winter, and bites only birds. When the two varieties were cross-bred the eggs were infertile suggesting reproductive isolation.
The fundamental results still stands: the genetic data indicate that the molestus form in the London Underground mosquito appeared to have a common ancestry, rather than the population at each station being related to the nearest above-ground population (i.e. the pipiens form). Byrne and Nichols' working hypothesis was that adaptation to the underground environment had occurred locally in London once only.
These widely separated populations are distinguished by very minor genetic differences, which suggest that the molestus form developed: a single mtDNA difference shared among the underground populations of ten Russian cities; a single fixed microsatellite difference in populations spanning Europe, Japan, Australia, the middle East and Atlantic islands.
Madeira House Mouse
The Madeira mice are species of mice, descended from the house mouse (Mus musculus), that went through speciation after colonization of the island Madeira in the 1400s. Up to six distinct species are present, driven by Robertsonian translocations (fusions of different numbered chromosomes). This driving force is particularly interesting because the human genome evidences a Robertsonian translocation in its divergence from its ape and ape-like cousins.
The Shortfin Molly (Poecilia mexicana) is a small fish that lives in the Sulfur Caves of Mexico. Years of study on the species have found that two distinct populations of mollies—the dark interior fish and the bright surface water fish—are becoming more genetically divergent. The populations have no obvious barrier separating the two; however, it was found that the mollies are hunted by a large water bug (Belostoma spp). Tobler collected the bug and both types of mollies, placed them in large plastic bottles, and put them back in the cave. After a day, it was found that, in the light, the cave-adapted fish endured the most damage, with four out of every five stab-wounds from the water bugs sharp mouthparts. In the dark, the situation was the opposite. The mollies’ senses can detect a predator’s threat in their own habitats, but not in the other ones. Moving from one habitat to the other significantly increases the risk of dying. Tobler plans on further experiments, but believes that it is a good example of the rise of a new species.
A specific example of large-scale evolution is the polar bear (Ursus maritimus). The polar bear is related to the brown bear (Ursus arctos) but they can still interbreed and produce fertile offspring. However, it has acquired significant physiological differences from the brown bear. These differences allow the polar bear to comfortably survive in conditions that the brown bear could not including the ability to swim sixty miles or more at a time in freezing waters, to blend in with the snow, and to stay warm in the arctic environment. Additionally, the elongation of the neck makes it easier to keep their heads above water while swimming and the oversized webbed feet that act as paddles when swimming. The polar bear has also evolved small papillae and vacuole-like suction cups on the soles to make them less likely to slip on the ice; feet covered with heavy matting to protect the bottoms from intense cold and provide traction; smaller ears to reduce the loss of heat; eyelids that act like sunglasses; accommodations for their all-meat diet; a large stomach capacity to enable opportunistic feeding; and the ability to fast for up to nine months while recycling their urea.
Kirsten Bomblies et al. from the Max Planck Institute for Developmental Biology discovered that two genes passed down by each parent of the thale cress plant, Arabidopsis thaliana. When the genes are passed down, it ignites a reaction in the hybrid plant that turns its own immune system against it. In the parents, the genes were not detrimental, but they evolved separately to react defectively when combined.
To test this, Bomblies crossed 280 genetically different strains of Arabidopsis in 861 distinct ways and found that 2 per cent of the resulting hybrids were necrotic. Along with allocating the same indicators, the 20 plants also shared a comparable collection of genetic activity in a group of 1,080 genes. In almost all of the cases, Bomblies discovered that only two genes were required to cause the autoimmune response. Bomblies looked at one hybrid in detail and found that one of the two genes belonged to the NB-LRR class, a common group of disease resistance genes involved in recognizing new infections. When Bomblies removed the problematic gene, the hybrids developed normally.
Over successive generations, these incompatibilities could create divisions between different plant strains, reducing their chances of successful mating and turning distinct strains into separate species.
Salsifies are one example where hybrid speciation has been observed. In the early 20th century, humans introduced three species of goatsbeard into North America. These species, the western salsify (Tragopogon dubius), the meadow salsify (Tragopogon pratensis), and the oyster plant (Tragopogon porrifolius), are now common weeds in urban wastelands. In the 1950s, botanists found two new species in the regions of Idaho and Washington, where the three already known species overlapped. One new species, Tragopogon miscellus, is a tetraploid hybrid of T. dubius and T. pratensis. The other new species, Tragopogon mirus, is also an allopolyploid, but its ancestors were T. dubius and T. porrifolius. These new species are usually referred to as "the Ownbey hybrids" after the botanist who first described them. The T. mirus population grows mainly by reproduction of its own members, but additional episodes of hybridization continue to add to the T. mirus population.
T. dubius and T. pratensis mated in Europe but were never able to hybridize. A study published in March 2011 found that when these two plants were introduced to North America in the 1920s, they mated and doubled the number of chromosomes in there hybrid Tragopogon miscellus allowing for a "reset" of its genes, which in turn, allows for greater genetic variation. Professor Doug Soltis of the University of Florida said, "We caught evolution in the act…New and diverse patterns of gene expression may allow the new species to rapidly adapt in new environments". This observable event of speciation through hybridization further advances the evidence for the common descent of organisms and the time frame in which the new species arose in its new environment. The hybridizations have been reproduced artificially in laboratories from 2004 to present day.
Welsh groundsel is an allopolyploid, a plant that contains sets of chromosomes originating from two different species. Its ancestor was Senecio × baxteri, an infertile hybrid that can arise spontaneously when the closely related groundsel (Senecio vulgaris) and Oxford ragwort (Senecio squalidus) grow alongside each other. Sometime in the early 20th century, an accidental doubling of the number of chromosomes in an S. × baxteri plant led to the formation of a new fertile species.
The York groundsel (Senecio eboracensis) is a hybrid species of the self-incompatible Senecio squalidus (also known as Oxford ragwort) and the self-compatible Senecio vulgaris (also known as Common groundsel). Like S. vulgaris, S. eboracensis is self-compatible, however, it shows little or no natural crossing with its parent species, and is therefore reproductively isolated, indicating that strong breed barriers exist between this new hybrid and its parents.
It resulted from a backcrossing of the F1 hybrid of its parents to S. vulgaris. S. vulgaris is native to Britain, while S. squalidus was introduced from Sicily in the early 18th century; therefore, S. eboracensis has speciated from those two species within the last 300 years.
Other hybrids descended from the same two parents are known. Some are infertile, such as S. x baxteri. Other fertile hybrids are also known, including S. vulgaris var. hibernicus, now common in Britain, and the allohexaploid S. cambrensis, which according to molecular evidence probably originated independently at least three times in different locations. Morphological and genetic evidence support the status of S. eboracensis as separate from other known hybrids.
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