Speciation, or the process of forming an entirely new species, can occur in numerous ways. One type of speciation is sympatric- where speciation occurs but in a single geographical area. Although the exact definition of sympatric speciation is constantly argued, there are four main events that must occur for speciation to be classified as sympatric. The original and resulting species must both be from the same geographical area, speciation must occur, the different resulting species should be sister species or monophyletic collections, and their history must show no evidence of an allopatric stage (Fitzpatrick et al. 2008).
Another, more common, type of speciation is allopatric speciation- which involves geographic isolation within a species. Unlike sympatric, this type of speciation involves a species separating into different geographical areas, rather than staying within the same. This usually occurs due to natural barriers such as mountains, rivers, or valleys that divide the population and prevent it from interacting. Due to the lack of communication between the groups, mating between them becomes less frequent, and eventually the original species splits and becomes two distinct species that can no longer successfully reproduce with each other. Similar to sympatric speciation, there were four requirements for speciation to be considered allopatric in the study conducted by Ricklefs and Bermingham- scattering of islands must be low, the groups on each island must eventually reach incompatibility, the isolated groups should move and inhabit other islands, and the new species groups must be unable to reproduce with each other and be ecologically harmonious (Ricklefs and Bermingham 2007).
There are two main types of allopatric speciation- vicariant and peripatric. Vicariant allopatric speciation involves a species being broken up into isolated groups. Peripatric allopatric speciation is when a small piece of the population breaks off and becomes isolated from the rest of the group (Rocha and Bowen 2008). Vicariant speciation is seen in some areas of coral reefs- for example, due to the small strip of land, the Isthmus of Panama, forming between the Caribbean Sea and the Eastern Pacific, numerous new coral and fish species were created because they were isolated by the land. In 2011, a study was performed by Runemark et. al. to determine whether endemic lizards, belonging to the genus Podarcis, from islands in the Greek archipelago had gone through vicariance or dispersal. Seven different small islands off the coast of Skyros were involved in the study and multiple lizards were caught from each. DNA was collected from each lizard’s tail and went through a series of processes to determine the alleles and genotypes of each lizard. Programs were implemented to determine the amount of divergence between each population of lizard. Their results indicated that the allopatric speciation was due to vicariance, but a portion of their data showed that dispersal contributed due to the type of gene flow that occurred (Runemark et al. 2012).
Research on lemon shark, belonging to the genus Negaprion, speciation involved 138 sharks being collected and DNA was extracted from each, specifically mitochondrial DNA. Numerous programs were used to assess molecular diversity, population subdivision, and to define the genetic seascape. A linear regression formula was also used to ensure the information matched an isolation-by-distance model. The results pointed to vicariant speciation- the populations of sharks were isolated due to the African and Eurasia plate converging and due to the rise of the Isthmus of Panama, similar to the case of the coral reef species. Although their evidence points more towards this theory, the dispersal theory cannot be counted out entirely (Schultz et al. 2008).
Most of the allopatric speciation occurs on islands, due to the amount of geographic isolation that is implemented, the land being completely surrounded by water. Continental shelf islands are some of the best areas to study allopatric speciation due to the recent separation from other islands that occurs. In a study conducted by Comes et al., the plant species Nigella was used. The plant group is mainly found in the Southwest Asian and the Mediterranean areas of the world. Six of the species are flowering or fruit bearing, and four of those bloom in the summer months. Millions of years ago, certain climate changes occurred, and the researchers feel the climate events played a huge role in speciation of the Nigella. Some of the first evidence that pointed to Nigella isolation was variations between cpDNA. The study also involved comparing phylogenetic, phylogeographic, and population level approaches to determine the past evolution patterns of the species (Comes et al. 2008). Their results show that allopatric speciation had occurred along with genetic drift. The type of allopatric speciation that showed up more often was vicariant speciation, but it did not occur in all groups. Both of these processes were the driving factors of the speciation that took place throughout the Nigella genus. The groups of Nigella are categorized at an incipient stage in the process of allopatric speciation by the researchers. Unfortunately, the researchers cannot apply this information to other plant species on the islands because there is little data to compare different species with Nigella (Comes et al. 2008).
Another research project was focused in the Lesser Antilles. This study used a small lizard genus, Anolis, that has gone through speciation, creating over 150 species from just two small settled groups. The researchers focused on four different groups from the genus Anolis and their hypothesis involved the four separate groups, which had been extremely isolated from each other, showing no traits relating to reproduction between the other groups. Their procedures involved dividing area the lizard’s inhabited into different transects. They took samples, from the tails, from 48 lizards in each transect. The researchers also observed any traits or adaptations that seemed obscure or advantageous. They incorporated elements of genetic isolation, climate of the lizard groups, adaptations that they have acquired throughout time, and their relationship with lineage to determine if allopatric speciation could have contributed to the creation of multiple species of Anolis. Lineage was tested using mtDNA from the groups, a climate profile was created for each habitat, traits that were observed included but were not limited to color, size, and weight of the lizards, and genetic structure was tested with nDNA and numerous programs to compare and analyze the recorded data. When conclusions were reached, they discovered that the different groups were not performing as species, rather the groups of the same species that lived next to or around each other had more genetic variability than did those who were two separate species. Their data did not support their hypothesis, and it indicated that little to no allopatric speciation had occurred throughout these Anolis species (Thorpe et al. 2010).
Allopatric speciation has proven extremely important in the process of creating new species. It has increased the world’s diversity and allowed for newly adapted species to arise. Both sympatric and allopatric speciation have an extremely large impact on organisms’ everyday lives and although there is numerous amounts of research on the two types of speciation, scientists and researchers are still searching to better understand why it occurs in some places, between some species, and not others. Although some species, like the Anolis species mentioned above, cannot contribute their speciation to allopatric methods, it is still important to understand the other ways in which species form.