Allopatric and Sympatric Speciation: Videos & Practice Problems

Speciation occurs through allopatric and sympatric mechanisms. Allopatric speciation involves populations evolving in different geographic areas, leading to reproductive isolation via dispersal or vicariance. In contrast, sympatric speciation occurs within the same location, often through polyploidy or disruptive selection, requiring mechanisms for reproductive isolation. Polyploidy, common in plants, results from chromosome duplication, while disruptive selection favors divergent phenotypes, necessitating mate choice or niche differentiation for speciation. Understanding these processes is crucial for grasping biodiversity and evolutionary dynamics.

Allopatric and Sympatric Speciation

Reproductive isolation is a crucial factor in the process of speciation, which can occur in two primary ways: allopatric speciation and sympatric speciation. Understanding these concepts is essential for grasping how new species arise and evolve over time.

Allopatric speciation occurs when populations of a species become geographically separated, leading to reproductive isolation. This separation can happen due to various factors, such as physical barriers like mountains or rivers, which prevent individuals from mating. The term "allopatric" derives from the Greek roots "allo," meaning "other" or "different," and "patric," which relates to "country" or "region." Thus, allopatric species are those that inhabit different regions. As these populations evolve independently in their distinct environments, they can accumulate genetic differences, ultimately resulting in the formation of new species.

In contrast, sympatric speciation takes place when populations evolve in the same geographic area, without physical barriers separating them. The prefix "sym" means "same," indicating that these species coexist in overlapping regions. Despite being in close proximity, reproductive isolation can still occur through mechanisms such as behavioral differences, temporal isolation (differences in mating times), or ecological niches that reduce competition. Although sympatric speciation is less common than allopatric speciation, it demonstrates that new species can arise even in the absence of geographic separation.

In summary, both allopatric and sympatric speciation illustrate the diverse pathways through which species can evolve. Allopatric speciation emphasizes the role of geographic isolation, while sympatric speciation highlights the potential for reproductive isolation within shared habitats. Understanding these processes is fundamental to the study of evolutionary biology and the dynamics of biodiversity.

True or False: if false, choose the answer that best corrects the statement.


The difference between allopatric and sympatric speciation is how long it takes for populations to split into separate species.

• A. True.
• B. False; the difference between allopatric and sympatric speciation is if pre- or postzygotic barriers are used to create reproductive isolation.
• C. False; the difference between allopatric and sympatric speciation is how the species respond to recontact in a hybrid zone.
• D. False; the difference between allopatric and sympatric speciation is whether or not it involves a geographical barrier.

Answer: (D)

Allopatric Speciation

Allopatric speciation is a process where populations become geographically isolated, leading to reproductive and genetic isolation. This isolation can occur through two primary mechanisms: dispersal and vicariance.

Dispersal involves the movement of individuals from one location to another, resulting in the colonization of a new habitat. For instance, if a group of butterflies from the mainland is blown to an island, they become isolated due to the surrounding water, which prevents gene flow between the two populations. Over time, without gene flow, these populations can diverge genetically, potentially leading to differences in traits, such as coloration. If this divergence continues long enough, the populations may become incapable of interbreeding if they come into contact again.

Vicariance, on the other hand, refers to a physical splitting of a habitat, often due to geological events. For example, if a mountain range emerges in the middle of a flower field, the flowers on either side become isolated. Similar to dispersal, this separation leads to reproductive and genetic isolation, as there is no gene flow between the two groups. Over time, these populations will also diverge, potentially resulting in distinct characteristics.

While both mechanisms lead to allopatric speciation, they have different implications for the populations involved. Dispersal often results in the founder effect, where a small group loses genetic diversity when establishing a new population. This can lead to rapid adaptation to the new environment. In contrast, vicariance typically involves larger populations that remain in their original locations, which may result in different evolutionary pressures and outcomes.

Understanding these processes is crucial for studying how species evolve and adapt to their environments over time. The key takeaway is that geographic isolation, whether through dispersal or vicariance, plays a significant role in the divergence of populations and the formation of new species.

Allopatric and Sympatric Speciation Example 1

The distribution of the common chimpanzee (Pantroglodytes) and the bonobo (Pan paniscus) is illustrated on a map, where the common chimpanzee's range is marked in orange and the bonobo's in purple, separated by the Congo River. Both species inhabit similar tropical rainforest environments and are believed to have diverged from a common ancestor approximately 1.5 to 2 million years ago.

To determine whether these species are sympatric or allopatric, we recognize that they are allopatric. The term "allopatric" derives from the Greek words "allo," meaning different, and "patric," referring to a region. This indicates that the two species occupy distinct geographical areas, with the Congo River acting as a barrier that prevents overlap in their ranges.

In terms of reproductive isolation, the primary mechanism at play here is habitat isolation. Since both species cannot swim, the Congo River serves as a physical barrier, preventing them from interbreeding. This separation reinforces their distinct evolutionary paths.

If we consider a vicariance event, which refers to a situation where a population is divided by a physical barrier, we can infer that the Congo River is likely around 1.5 to 2 million years old. This age aligns with the timeline of the species' divergence, suggesting that the river's formation split a previously continuous population, leading to the reproductive isolation of the two species.

Conversely, if a dispersal event were responsible for the separation, we would predict that the Congo River is older than 2 million years. In this scenario, it is possible that some individuals of the original population managed to cross the river, leading to the establishment of the two species on opposite banks. However, the exact mechanisms of how they crossed remain uncertain, as both species lack swimming capabilities.

In summary, while the prevailing scientific view supports the idea of a vicariance event leading to the current distribution of these species, there is ongoing debate among scientists regarding the exact processes involved and the age of the Congo River.

Which of the following would be most likely to lead to speciation by vicariance?

• A. Volcanoes in the ocean create new islands.
• B. A change in the environment leads to a grassland becoming a forest.
• C. A small group of organisms are washed to sea during a storm and land on a new island.
• D. A change in climate creates a desert that divides a large grassland.

Answer: (D)

The organisms of the Galapagos islands have become famous for people studying their processes of speciation. Which statement best describes how speciation most likely occurs on islands like the Galapagos?

• A. Speciation likely occurs through vicariance as the organisms that reached the islands split off from mainland populations.
• B. Speciation likely occurs through dispersal as groups of organisms reach new islands and adapt to local conditions.
• C. Speciation likely occurs through sympatric speciation as organisms diverge in different ecological niches.
• D. Speciation likely occurs through hybridization as new organisms interact with resident organisms on islands.

Answer: (B)

Sympatric Speciation

Sympatric speciation occurs when new species arise within the same geographic area, which can be more challenging to understand compared to allopatric speciation, where populations are separated. The key to sympatric speciation lies in achieving reproductive isolation despite the organisms coexisting in the same environment. This process is less common than allopatric speciation and can occur primarily through two mechanisms: polyploidy and disruptive selection.

Polyploidy refers to a condition where an organism has more than two complete sets of chromosomes. In eukaryotes, which typically have two sets (diploid), polyploid organisms can be tetraploid (four sets) or even octaploid (eight sets), as seen in strawberries. When individuals with different chromosome sets attempt to mate, their offspring are often either non-viable or sterile, leading to reproductive isolation. This phenomenon is particularly prevalent in plants, where polyploidy can facilitate speciation.

Disruptive selection, on the other hand, favors divergent phenotypes within a population. For instance, if a population of fish exhibits both green and yellow morphs, disruptive selection may enhance the fitness of these two distinct forms. However, for speciation to occur, a mechanism of reproductive isolation must be established. This can happen through mate choice, where individuals preferentially select partners that resemble themselves, or through microhabitat niches, where organisms occupy different areas within the same environment, limiting their interactions. For example, if green fish only mate with other green fish and yellow fish only with yellow fish, over time, these groups may diverge enough to become separate species.

In summary, while sympatric speciation is less common than allopatric speciation, it can occur through mechanisms like polyploidy and disruptive selection, both of which emphasize the importance of reproductive isolation in the formation of new species.

Strong disruptive selection doesn’t always create new species. What other requirement must be met for speciation to occur?

• A. There must also be reproductive isolation of the divergent phenotypes for speciation to occur.
• B. There must be no stabilizing or directional selection on other traits for speciation to occur.
• C. There must also be chromosomal differences between the groups for speciation to occur.
• D. There must be random mating in the population for speciation to occur.

Answer: (A)

Within the Northeast Pacific, two types of killer whales (Orcinus orca) are found. Resident killer whales feed mostly on salmon, while the transient population feeds mostly on marine mammals. While these populations spend a lot of time in the same waters, they do not interbreed, likely because they use different systems of calls to communicate. Some scientists believe that these two populations of orca are undergoing speciation. What type of speciation is occurring, and what type of reproductive barrier do you suspect is reinforcing the speciation?

• A. Allopatric speciation with behavioral isolation.
• B. Allopatric speciation with habitual isolation.
• C. Sympatric speciation with behavioral isolation.
• D. Sympatric speciation with habitual isolation.

Answer: (C)

Polyploidy

Polyploidy refers to the condition of having extra sets of chromosomes beyond the typical diploid state, which consists of two sets. For instance, a tetraploid cell contains four sets of chromosomes, while strawberries are an example of octaploid organisms, possessing eight complete sets. This increase in chromosome number can lead to speciation, as organisms with differing chromosome counts may produce sterile hybrids when they attempt to reproduce.

There are two primary mechanisms through which polyploidy can occur: autopolyploidy and allopolyploidy. Autopolyploidy arises from errors during cell division within a single organism. For example, a diploid cell (2n) with six chromosomes can undergo a division error, resulting in a tetraploid cell (4n) with twelve chromosomes. When this tetraploid organism undergoes meiosis, it produces diploid gametes (2n = 6). If these gametes self-fertilize, they can form a new tetraploid zygote, effectively creating a new species in just one generation. This self-fertilization is more common in plants, making polyploidy prevalent in this kingdom.

On the other hand, allopolyploidy involves hybridization between two different species followed by a cell division error. For instance, if species one (2n = 4) and species two (2n = 6) produce gametes, their fusion may result in a sterile hybrid due to the mismatch in chromosome numbers. This hybrid, which cannot properly pair its chromosomes, is effectively an unbalanced diploid (n = 5). However, if a cell division error occurs, this hybrid can become a diploid organism with a total of ten chromosomes, termed an allopolyploid. This organism can also self-fertilize, leading to the establishment of a new species.

In summary, autopolyploidy involves chromosome doubling within a single species, while allopolyploidy results from hybridization between different species followed by a doubling of chromosome sets. Both processes illustrate the complexity and significance of polyploidy in the evolution of new species.

Allopatric and Sympatric Speciation Example 2

Plant breeders have successfully developed seedless watermelons through the creation of polyploid plants. Typically, watermelons, scientifically known as Citrullus lanatus, are diploid, with a chromosome count represented as \(2n = 22\). However, breeders can induce tetraploidy, where the chromosome count becomes \(4n = 44\), by applying a mutagen that disrupts the normal segregation of chromosomes during mitosis. This results in cells that possess double the usual number of chromosomes. The tetraploid plants can then self-fertilize to produce a new breed.

When tetraploid plants are crossed with diploid plants, the resulting offspring are sterile and do not produce seeds, which is why seedless watermelons are so popular in the market today. Essentially, if you have enjoyed a seedless watermelon, you have likely consumed a hybrid of tetraploid and diploid watermelon varieties.

In terms of classification, tetraploid watermelons are considered autoploids rather than alloploids. The distinction lies in the definitions: allo- refers to "other" or "different," indicating hybridization between two species, while auto- means "self," indicating a mutation within a single species. Since the creation of tetraploids involves mutagen treatment without hybridization, they are classified as autoploids.

Furthermore, while diploid and tetraploid watermelons are generally regarded as the same species, they can be considered different species under the biological species concept. This concept emphasizes reproductive isolation, which is evident here as the hybrids produced from diploid and tetraploid crossings are sterile. This postzygotic reproductive isolation aligns with the criteria for defining separate species, even though the tetraploid variety was artificially created in a laboratory setting rather than through natural evolution.

Polyploidy creates individuals or gametes with novel numbers of chromosomes. To mate, these individuals must mate with other organisms that are also polyploid. Would this lead to polyploidy being more common in animals or plants, and why?

• A. Animals. Animals tend to have mobile sperm, so the gametes can travel farther to find compatible eggs.
• B. Plants. Many plants can self-fertilize, and gametes from the same plant will have the same number of chromosomes.
• C. Animals. Animal species are more likely to have individuals that are only one sex.
• D. Plants. Plant ovules tend to be larger than animal eggs meaning they can better accommodate the extra DNA.

Answer: (B)

What is the difference between autopolyploids and allopolyploids?

• A. Allopolyploids can only reproduce through self-fertilizing, while autopolyploids are able to cross-fertilize.
• B. Allopolyploids are descended from a single species, autopolyploids begin as hybrids between two species.
• C. Autopolyploids are descended from a single species, allopolyploids begin as hybrids between two species.
• D. Autopolyploids can only reproduce through self-fertilizing, while allopolyploids are able to cross-fertilize.

Answer: (C)

Here’s what students ask on this topic:

What is the difference between allopatric and sympatric speciation?

Allopatric speciation occurs when populations evolve in different geographic areas, leading to reproductive isolation due to physical separation. This can happen through dispersal, where individuals move to a new location, or vicariance, where a physical barrier divides a population. Sympatric speciation, on the other hand, occurs within the same geographic area. It often involves mechanisms like polyploidy, where chromosome duplication creates reproductive barriers, or disruptive selection, where divergent phenotypes are favored, leading to reproductive isolation through mate choice or niche differentiation. Both processes result in the formation of new species but differ in their geographic and reproductive isolation mechanisms.

How does polyploidy contribute to sympatric speciation?

Polyploidy contributes to sympatric speciation by creating reproductive isolation through chromosome duplication. In polyploidy, an organism has more than two complete sets of chromosomes. This can occur through autopolyploidy, where chromosome duplication happens within a single species, or allopolyploidy, where hybridization between species is followed by chromosome duplication. Polyploid organisms often cannot reproduce with their diploid counterparts, leading to reproductive isolation. This mechanism is more common in plants, where self-fertilization can occur, allowing polyploid individuals to reproduce and form a new species within the same geographic area.

What are the mechanisms of reproductive isolation in sympatric speciation?

In sympatric speciation, reproductive isolation mechanisms include polyploidy and disruptive selection. Polyploidy involves chromosome duplication, creating individuals with different chromosome numbers that cannot interbreed with the original population. Disruptive selection favors divergent phenotypes within the same population, leading to reproductive isolation through mate choice or niche differentiation. For example, organisms may prefer mates with similar traits or occupy different microhabitats, reducing gene flow between groups. These mechanisms ensure that even within the same geographic area, populations can diverge and form new species.

What is the role of dispersal and vicariance in allopatric speciation?

Dispersal and vicariance are key mechanisms in allopatric speciation. Dispersal involves individuals moving from one location to another, such as when a group colonizes a new habitat, leading to geographic isolation. This isolation prevents gene flow between the original and new populations, allowing them to evolve independently. Vicariance, on the other hand, occurs when a physical barrier, like a mountain range or river, splits a population into separate groups. This physical separation also prevents gene flow, leading to independent evolution and eventually, the formation of new species. Both mechanisms create the geographic isolation necessary for allopatric speciation.

Why is sympatric speciation less common than allopatric speciation?

Sympatric speciation is less common than allopatric speciation because it requires reproductive isolation to occur within the same geographic area, which is more challenging. In sympatric speciation, mechanisms like polyploidy or disruptive selection must create strong reproductive barriers to prevent gene flow between diverging groups. These barriers are harder to establish and maintain when populations are not physically separated. In contrast, allopatric speciation benefits from geographic isolation, which naturally reduces gene flow and facilitates independent evolution. Therefore, the conditions for sympatric speciation are less frequently met, making it a rarer occurrence.