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Allopatric speciation

Allopatric speciation

Allopatric speciation
n. Definition: A speciation in which biological populations are physically isolated by an extrinsic barrier and evolve intrinsic (genetic) reproductive isolation

We can define speciation as a process by which the novel genetically independent group of organisms are formed through the process of evolution. In another definition, speciation is explained as the process by which the genetically homogenous population is split into various populations that undergo genetic differentiation and reproduction isolation. Thus, it can be deduced that biogeographic isolation leads to the formation of a new species through speciation.

Geographical isolation means separation either due to geographical changes or migration of some members of the species while others remained at the point of their origin. Geographical isolation examples also include a scenario where a population migrated to an island thus separated from the mainland population. These two populations will evolve separately.


A population that becomes geographically isolated from other populations of the parental species may lead to allopatry. Find out how. Hear more from our Expert: Sympatric vs allopatric speciation. Join us in our Forum.


Allopatric Speciation Definition

Allopatric is a Greek word. Allopatric means “geographical”. Allopatric speciation is also termed as geographical speciation, dumbbell model, and vicariant speciation. So how can we define allopatric speciation? In simple words, it refers to the speciation that occurred between two populations of the same species that had become isolated from each other due to geographical barriers. Speciation is a gradual process by which populations evolve into new species.

Biology definition:
Allopatric speciation is a type of speciation in which biological populations are physically isolated by an extrinsic barrier and evolve intrinsic (genetic) reproductive isolation, such that if the barrier breaks down, individuals of the population can no longer interbreed. Example: Charles Darwin’s Galápagos Finches.

Etymology:
from Greek allos, meaning “other” + Greek patrā, meaning “fatherland”
Synonyms: geographic speciation, dumbbell model; vicariant speciation
Compare: peripatric speciation, parapatric speciation, sympatric speciation

The advantages of speciation include that it teaches the organisms to sustain themselves under severe environmental conditions and the ecological balance of abiotic and biotic components is maintained. On the other side, the disadvantages of speciation include that its data cannot be obtained from fossil fuels. Moreover, the whole process of speciation is absent in the asexual species and it can only be applied to the populations and communities that are in total isolation geographically. (Shapiro et al., 2016)

There are many ways in which the process of speciation occurs. The origin of new species from the old one over time and the multiplication of species wherein a single species is split into several ones are two of the most cited ways of speciation.

Speciation has been observed as a three-stage process:

  1. The rapid isolations of the species 
  2. The divergence in the individualities of the populations that are separated
  3. In the last stage, the species maintain isolation and reproduce themselves

The recent research studies show that the first two steps take place at the same time and thus paving the path for the third stage.


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Causes of Speciation

Many causes are responsible for the process of speciation. Natural selection, genetic drift, migration, chromosomal mutations, natural causes, and reduction of gene flow are some of the notable causes.

In natural selection, various individuals sometimes develop some distinct characteristics that are passed from one generation to another whereas in genetic drift the random change occurs in the allele frequencies in the population.

Although the genetic drift is a major cause of speciation still some scientists argue that such a process is not the cause of speciation rather results in evolution.

There are many cases in which species tend to migrate from one certain location to another in search of food and shelter; thus, they accumulate some diverse features which cause speciation in the next generations.

In chromosomal mutations, natural causes, and reduction of gene flow, speciation occurs due to the progression of chromosome mutations, the occurrence of natural events such as a flood that bisects species into two, and the rapid reduction of the flow of genes respectively.

 

 

Four Types of Speciation

The types and classifications of the speciation are based upon the fact that how much the geographical separation of the population living in that particular area for ages contributes to the process of enhancement in the genes and ultimately giving rise to new species. Hence, based upon the said idea the speciation is divided into four types. They are:

  • Allopatric Speciation
  • Sympatric Speciation
  • Peripatric Speciation
  • Parapatric Speciation

The details of all four types of speciation can be seen in Figure 1. Moreover, the process details with individual steps have also been elaborated in Figure 1.

Types of Speciation
Figure 1: Details and Types/ Classification/ Modes of Speciation. Credit: MicrobeNotes.com.

 

Allopatric Speciation (Biology)

In biology, the type of speciation that occurs when two original populations of the same species and characteristics get separated from each other due to the geographical alterations is called allopatric speciation, and the population that changes are termed as allopatric populations. In such a mode of speciation, the isolation occurs due to the existence of physical barriers, such as rivers, deserts, considerable distances, or mountains. These physical barriers prevent them from regular mating thus a lineage to speciate is caused.

The model for allopatric speciation was studied and then later presented by Ernst Mayr in the early 19th century. The idea behind the working of Mayr’s model was that when the larger populations are divided into smaller groups of populations due to certain geographical barriers, the rise of new species occurs. Thus, the species instead of mating with their original breeds start adapting to their new environments, overcoming the differences and in the end evolving into new species. Hence, allopatric speciation occurs in three phases. In the first phase, the populations become physically separated from each other due to environmental disruptions, the second phase of divergence of the separated species occurs via alterations in the tactics of mating and conditions of the habitat and lastly, they become separated reproductively which means that can’t interbreed and exchange genes with their mother populations rather the whole process of speciation starts in the new community. For the elaborate steps of allopatric speciation, go to: Steps of Allopatric Speciation

The whole progression of allopatric speciation has been subdivided into two main models: vicariance and peripatric. The main difference between both of these models is their population sizes and the mechanism behind the geographical isolation. Allopatry and vicariance are the terminologies that are used to explain the relationships of organisms whose ranges do not interact with each other. The term range for a particular set of species is often referred to as the geographical areas where those organisms are found.

The allopatric speciation of the owl is an example of allopatric speciation, as shown in Figure 2. The Mexican spotted owl and the Northern spotted owl being separated by rivers, and therefore isolated from each other by geographical barrier resulted in allopatric speciation.  They acquired features that differ from each other as they evolved and adapted independently over time as they inhabited different geographical locations with distinct climates and ecosystems. (Wang et al., 2016)

Example of allopatric speciation
Figure 2: Allopatric speciation due to geographical separation: the case of owls geographically separated by a river. Biology LibreTexts.

Sympatric speciation

Sympatric speciation is the type of speciation in which new species are formed for an original population that is neither geographically isolated nor has any barriers in them. The whole concept in which the new species originate from the livings that are present in highly overlapping and undistinguishable areas makes the sympatric speciation different from other types of speciation. Moreover, in bacteria, this type of speciation is very common because they transfer their genes both inside their community and to offspring when they are in the process of reproduction.

The scientists are still exploring the reasons that are responsible for sympatric speciation because it is not very common unlike allopatric, parapatric, and peripatric types of speciation. Still, some of its examples can be found in nature. For instance, it occurs in herbivorous insects when they start feeding and mating on the new plants or when a new geographical plant is introduced in their habitat which suits them pretty well.

Examples of sympatric speciation include the apple maggot flies that laid eggs around 200 years ago and bred only on hawthorns but as of now, their eggs can easily be located on both domestic apples and hawthorns.

Another example is cichlids, Amphilophous sp. (a type of fish) found in Lake Apoyo present in Nicaragua. The researchers closely analyzed two species of cichlids that were very similar but had slight variances in appearance. The scientists concluded that one species of the fish evolved to form the other one recently, but in more particular terms, it is believed that the species of cichlids evolved less than ten thousand years ago. (Richards et al., 2019)

Allopatric vs. sympatric speciation

In allopatric speciation, geographical isolation is the key factor whereas the requirement of geographical isolation is least in sympatric speciation. Similarly, in the former type of speciation, the selection method is the natural selection differentiation method while later, the main selection method is polyploidy. Moreover, the whole process of the creation of new species is very quick in allopatric speciation and it can be found in both plants and animals as compared to the sympatric speciation where the process of generation of new species is low and can only be found in plants.

Peripatric speciation

Peripatric speciation is the type of speciation in which members of the same community, border, or periphery and a larger population separate from them, and eventually in the end they evolve as a separate species with time. It is sometimes referred to as a special type of allopatric speciation when the size of the available isolated subpopulation is relatively small. Hence, in such speciation, the role of genetic drift is enormous as it acts very quickly in smaller populations. Thus, the small number of organisms living in that particular area sometimes carry the rare genes that are transferred to the entire community of new species, and thus under its influence, the set of new species under the peripatric speciation are formed.

London Underground mosquito belonging to the species of Culex pipiens, which was found in London, the United Kingdom in the 19th century, was a variant of the mosquito, Culex pipiens. Petroica multicolor, an Australian bird, is another example of peripatric speciation. (Colvin, A., 2018)

Parapatric speciation

The type of speciation that occurs when a certain population of a certain group of species, mostly separated from each other and have a narrow area from where their ranges may overlap is called parapatric separation. The relative unequal distribution of members divided into the subpopulation and the partial tropological barriers are some of the reasons behind this type of speciation.

Non-random occurrence of mating, the unequal occurrence of gene flow, and the occurrences of populations both in continuous and discontinuous geographical ranges are some of the distinguishing characteristics of parapatric speciation.

The comparative affiliation between the organisms whose ranges don’t overlap significantly but are adjacent to each other is described by the words “parapatric” and “parapatry” in biogeography. Moreover, it has been observed that although the population here is continuous it has been seen that the population is unable to mate randomly.

Anthoxanthum odoratum, a well-known grass species, is a very common example of parapatric speciation. It is seen that some of the species of such grass that are living next to the mines have created the tolerance to heavy metal in their nature while the remaining species that are not found adjacent the miles haven’t got that particular tolerance against the heavy metals, but being close to another there is still a possibility that both of the species can come closer together, mute themselves, fertilize each other and thus they may rise to a new set of species having advanced characteristics. (Yamaguchi & Iwasa, 2017)

Allopatric speciation Sympatric speciation
A physical barrier that separated a continuous population resulting in reproductive isolation

Initial step of speciation: geographical barrier formation

Feature: New species geographically isolated from the original population

Example: Charles Darwin’s Galápagos Finches

A form of speciation occurring inside a continuous population and caused by mechanisms apart from the geographical barrier

Initial step of speciation: genetic polymorphism

Feature: New species coinhabits with the original population in the same geographical region

Example: Parent plants produce offspring that are polyploid. Hence, the offspring live in the same environment as their parents but are reproductively isolated.

Parapatric speciation Peripatric speciation
Speciation triggered by partial isolation wherein the new species budded off from the larger founder species

Initial step of speciation: new ecological niche

Feature: New species acquired a new niche adjacent to the parent’s niche. Because they are only partially separated, the two species may still come in contact from time to time by crossing the barrier until reproductive isolation is reached.

Example: Ensatina salamanders in Central Valley in California

Speciation triggered by partial physical isolation wherein the new species evolved as a sub-population that diverged from the original species

Initial step of speciation: new ecological niche

Feature: New species acquired a new or an isolated niche, e.g. a new habitat within the same geographical area of the original (ancestral) species. Similar to allopatric speciation but entails a relatively smaller unit.

Example: London Underground mosquito, a variant of the mosquito Culex pipiens, which entered in the London Undgerground in 19th century


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Steps of Allopatric Speciation

It is a very well-known fact that before the process of speciation occurred, there was a population of organisms that had the same characteristics and they had all the freedom to mate with each other freely and successfully. Hence, there was always the same set of individuals living in a habitat. As described earlier, the main reason behind the allopatric speciation is the geographical barrier that is created between the population of the same individuality and thus they no longer can be classified as the same species.

allopatric speciation steps
Figure 3: Allopatric speciation – steps. Source: Maria Victoria Gonzaga, BiologyOnline.com.

 

How does speciation occur?

What is the first step in allopatric speciation? In the first step, the geographical change occurs. It results in the separation of the group of organisms from their mother habitat. The earthly changes may occur due to natural and synthetic reasons.

The formation of a new mountain range, the volcanic explosion, the evaluation of new waterways, the rapid growths of new canyons, and the severe outcomes of natural disasters are some of the natural causes of such differences. Another reason for the migration of some populations to another area is human activities such as excessive modernization of the word, air, water, and land populations.

In the next step the developments of genes start. Gene, in biology, is defined as the specific linkages of the nucleotides that have the functionality to control the expressions of single or a couple of traits of the living organisms. The composition of genes includes the availability of DNA and RNA in it. Thus, in the current step of the allopatric speciation, the different characteristics between the populations occur due to the variations present in the genes of such communities.

In the final stage of the allopatric speciation, both populations have reached a point when they become very different from each other and the chances that they may interbreed with one another if placed in the same habitat are dim. The reason behind that is the change in their characteristics, habits, and living preferences.

Drosophila_speciation_experiment
Figure 4: Allopatric speciation in flies from an experiment. (Image source: public domain)

The steps in the allopatric speciation have been shown in Figure 3.  In Figure 4, a schematic diagram shows how allopatric speciation occurs in flies through an experiment. It was conducted in a way that the same set of flies were forced to live in two very different atmospheres. One population of flies was forced to remain alive on a starch medium while the other flies were given a maltose medium. After many generations, although both of the populations of flies belong to the same species, two different types of flies emerged based on the body phenotype. When they were allowed to mingle with each other, the two populations of flies preferred to mate with their own type — an indication of reproductive isolation, and hence, a case of allopatric speciation. (Dodd, 2020).

Examples of Allopatric Speciation

There are many allopatric speciation examples that can be found in the literature. Charles Darwin, an English biologist, and naturalist narrated the phenomena of allopatric speciation in Galapagos finches. It has been observed that there are approximately fifteen various species of finches found in the Galapagos Islands. All of the fifteen species of finches look diverse to each other and have very dedicated beaks to consume insects, flowers, and seeds to meet the nourishment requirements. It is believed that they evolved from a single parent species and later they migrated to various islands, and thus after isolations they evolved as distinct species. Various owls and birds have similarly emerged to possess very diversified features indicating allopatric speciation.

Explain why scientists believe Abert and Kaibab squirrels are examples of speciation

Another very interesting example of such speciation is the Grand Canyon squirrels. About ten thousand years ago, due to the formation of the Grand Canyon, the major population of squirrels was forced to separate from each other and they couldn’t live in the same habitat. Hence, after several thousand years from one species of squirrels, two different squirrels evolved namely Kaibab squirrels and Abert squirrels. The Abert squirrels lived on the south rim canyon over extensive ranges while the Kaibab squirrels were in the north rim of the canyon and had a very small range. Although both of these species had a very similar size, shape, nutrients, diet, and living they evolved to become two different entities because they were no longer in contact with each other over a course of time. Thus, Galapagos finches and the Grand Canyon squirrels are the two most explored and diversified examples of allopatric speciation.

 

Adaptive radiation

The biological process in which organisms tend to diversify their forms, shape, sizes and characteristics due to sudden changes in the environment thus creating new challenges for survival is called adaptive radiation. It should be noted here that various new species originate from a single mother species known as the founder species. Thus, starting from a founder species, various new species are formed that show different morphological and physiological traits. The examples of adaptive radiation have been shown in Figure 5. It shows the various kinds of birds evolved from a single species and how they evolved and differentiated based on various adaptations, such as varying beak phenotypes to become better adapted to the food they eat. (Marques et al., 2019)

Figure 5: Galapagos Island Finches - adaptive radiation.
Figure 5: Galapagos Island Finches – adaptive radiation.

Read this tutorial to learn more about Adaptive Radiation

 

Summary

It can be concluded from the above discussion that the process of speciation plays a very vital role in the origin of various novel species. The process by which the novel genetically independent group of organisms are formed through the process of evolution is called speciation. Natural selection, genetic chromosomal mutations, drift, migration, natural causes, reduction of gene flow are some of the notable causes responsible for the process of speciation. In natural selection, the various individuals sometimes develop some distinct characteristics that are passed from one generation to another whereas in genetic drift the random change occurs in the allele frequencies in the population.

The types and classifications of the speciation are based on the geographical separation of the population living in that particular area for ages contributes to the process of enhancement in the genes and ultimately giving rise to new species. Allopatric, peripatric, parapatric, and sympatric speciation are the four types of speciation.

  • The type of speciation that occurs when two original populations of the same species and characteristics get separated from each other due to geographical alterations is called allopatric speciation. In such a mode of speciation, the isolation occurs due to the existence of physical barriers such as rivers, deserts, considerable distances, or mountains preventing them from regular mating thus a lineage to speciate is caused. The whole progression of allopatric speciation has been subdivided into two main models, i.e. vicariance and peripatric. Galapagos finches and the Grand Canyon finches are the two most explored and diversified examples of allopatric speciation.
  • Peripatric speciation is the type of speciation in which members of the same community, border, or periphery and a larger population separate from them, and eventually in the end they evolve as a separate species with time.
  • Peripatric speciation is sometimes referred to as a special type of allopatric speciation when the size of the available isolated subpopulation is relatively small.
  • Sympatric speciation is the type of speciation in which new species are formed from an original population that is neither geographically isolated nor has any barriers in them.
  • Lastly, the type of speciation that occurs when a certain population of a certain group of species, mostly separated from each other and have a narrow area from where their ranges may overlap are called parapatric separation.

 

 

Try to answer the quiz below to check what you have learned so far about allopatric speciation.

Quiz

Choose the best answer. 

1. Which of this is allopatric speciation?
2. Which of this is the first stage of allopatric speciation?
3. Various individuals develop certain distinct characteristics that are passed from one generation to another.
4. Species migrate in search of food and shelter, and as a result, accumulate some diverse features leading to speciation.
5. Initial cause of speciation

Send Your Results (Optional)

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References

  • Colvin, A. Z. (2018). WikiJournal of Science – Peripatric speciation (Engineering Collection) – Informit. WikiJournal of Science, 1(2), 1. https://search.informit.com.au/documentSummary;dn=251848944342418;res=IELENG
  • ‌Dodd, D. (1989). Reproductive Isolation as a Consequence of Adaptive Divergence in Drosophila pseudoobscura. Evolution43(6), 1308–1311. JSTOR. https://doi.org/10.2307/2409365
  • ‌Marques, D. A., Meier, J. I., & Seehausen, O. (2019). A Combinatorial View on Speciation and Adaptive Radiation. Trends in Ecology & Evolution34(6), 531–544. https://doi.org/10.1016/j.tree.2019.02.008
  • ‌Richards, E. J., Servedio, M. R., & Martin, C. H. (2019). Searching for Sympatric Speciation in the Genomic Era. BioEssays41(7), 1900047. https://doi.org/10.1002/bies.201900047
  • ‌Shapiro, B. J., Leducq, J.-B., & Mallet, J. (2016). What Is Speciation? PLOS Genetics, 12(3), e1005860. https://doi.org/10.1371/journal.pgen.1005860
  • Wang, L., Wan, Z. Y., Lim, H. S., & Yue, G. H. (2016). Genetic variability, local selection and demographic history: genomic evidence of evolving towards allopatric speciation in Asian seabass. Molecular Ecology, 25(15), 3605–3621. https://doi.org/10.1111/mec.13714
  • Yamaguchi, R., & Iwasa, Y. (2017). A tipping point in parapatric speciation. Journal of Theoretical Biology421, 81–92. https://doi.org/10.1016/j.jtbi.2017.03.018

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