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Speciation: Three models based on geographyEdit

According to the University of California at Berkeley’s “Understanding Evolution” website, the definition of allopatric speciation is:

Speciation that depends on an external barrier to gene flow (such as geographic isolation) to begin or complete the process of speciation. As they say, it’s “just a fancy name for speciation by geographic isolation.”  Allopatry is believed to be the most common form of speciation (Fischer-Reid, et al, 2013). Rossetto et al (2013) go so far as to label it the “default” method.

Modes

Diagram explaining the four different modes of speciation.

The diagram on the right explains the differences in the four modes of speciation. All four start with an original population, then due to the different separations or alterations of the orginal species, the result is a new distinct species after equillibration of new ranges. 

Note that speciation has occurred when the newly formed population is reproductively isolated from the original population.

The converse scenario, sympatric speciation, entails a non-physical barrier to gene flow, such as  a behavioral difference. This could happen when individuals exploit a new niche within their geographic area, as was seen to occur when some members of the apple maggot fly population began feeding on the introduced domestic apple rather than the native hawthorn (Berlocher & Feder, 2002; Linn et al, 2012). Hawthorn preferring flies mate with other hawthorn preferring flies, while apple flies prefer to mate with other apple flies. There are already some genetic differences. In a similar study, Hebert et al (2013) have also seen reproductive isolation occur in leaf mining flies on their two host species of holly. Once the flies show a preference for one or the other holly, it perpetuates, since flies tend to mate with other flies who prefer the same holly. In an experiment, they mated flies from different holly species together, which resulted in a drastic decline of offspring. They also see genetic differences between the host-preferring flies. In both cases, these scenarios could result in sympatric speciation.

Here ia  link to an article fully explaining the sympatric speciation in the apple maggot fly: Speciation and the Apple Maggot Fly

In a somewhat similar scenario that also does not necessitate geographic isolation, parapatric speciation can occur. In parapatric speciation there is no specific extrinsic barrier to gene flow. The population is continuous, but nonetheless, the population does not mate randomly. Individuals are more likely to mate with their geographic neighbors than with individuals in a different part of the population’s range. In this mode, divergence may happen because of reduced gene flow within the population and varying selection pressures across the population’s range. (UCBerkeley, Understanding Evolution: http://evolution.berkeley.edu/evolibrary/glossary/glossary.php 

Parapatry seems to imply a large range with some variation in the habitat available throughout the area. Fischer-Reed et al (2013) studied a population of eastern redbacked salamanders (Plethodon cenereus) on Long Island which may be in the early stages of speciation in order to determine which mechanism is at work. The salamanders occur sympatrically on the mainland, but spread across the ecological gradient of Long Island, they seem to be occurring parapatrically. There are two definite morps, the redback and the pure lead. (See picture)

Salamanders

redbacked and leadback morphs of the redback salamander

Although they are still one and the same species, the two morphs differ not only ecologically, but genetically and morphologically from one another as well as from P. cenereus found on the mainland. The authors suggest this scenario might eventually result in parapatric speciation. This is due to the habitat preferences that appear to have developed geographically. In the western part of the island, where the habitat is primarily deciduous forest, all of only pure redback morphs are found. In the eastern part of the island, which consists of pine barrens, 100% of the population is composed of the pure-leadback morphs.

Salamander

The Ensatina salamanders of California are a great example of Ring Species and the Parapatric mode of speciation.

The Ensatina salamanders of California are a great example of parapatric speciation. Color differences found in the salamanders are due to intergradation of subspecies.

One original species of salamander spread throughout California and entered new niches, therefore, new subspecies were formed that are reproductively isolated from each other.

The visual to the left maps out the different subspecies of Ensatina salamanders and their relative locations.  


References:          

Berlocher, S.H. and J.L. Feder. (2002). Sympatric speciation in phytophagous insects: moving beyond controversy? Annual Review of Entomology,47. 773-815.

Fisher-Reid, M. C.; Engstrom, T. N.; Kuczynski, C. A.; Stephens, P.R.; Wiens, J. J.(2013).  Parapatric divergence of sympatric morphs in a salamander: incipient speciation on Long Island?  Molecular Ecology, 22(18). 4681-4694.

Hebert, J. B., Scheffer, S. J., & Hawthorne, D. J. (2013). Reproductive Isolation between Host Races of Phytomyza glabricola on Ilex coriacea and I. glabra. Plos ONE, 8(9), 1-6

Linn Jr., C. E., Yee, W. L., Sim, S. B., Cha, D. H., Powell, T. H. Q., Goughnour, R. B. and Feder, J. L. (2012), Behavioral evidence for fruit odor discrimination and sympatric host races of Rhagoletis pomonella flies in the western united states. Evolution, 66: 3632–3641.

Rossetto, B.M.C/,Allen, C.B., Thurlby, K.A., Weston, P.H., Milner, M.L. (2013). Genetic structure and bio-climatic modeling support allopatric over parapatric speciation along a latitudinal gradient. Evolutionary Biology, 12 (1). 1 – 14.

"Understanding Evolution" by University of California, Berkeley: Modes of Speciation

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