Sympatric Speciation

Can populations diverge into new, distinct species without being geographically separated? Sympatric speciation is a type of speciation that specifically excludes geographic separation as a factor. In the following, we will discuss the definition of sympatric speciation, compare it to allopatric speciation, and then elaborate on its causes and provide some examples.

What Is the Definition of Sympatric Speciation?

Speciation occurs when populations of a species, due to certain factors, evolve to become new, distinct species. It involves the divergence of one lineage to two or more lineages.

Sympatric speciation refers to a type of speciation that occurs when two groups of the same species live in the same geographical area but no longer interbreed. It is a controversial alternative to allopatric speciation, in which speciation occurs due to the geographic isolation of members of a population.

Allopatric vs. Sympatric Speciation: What Is the Difference?

Allopatric speciation and sympatric speciation are two important processes that lead to the formation of new species.

Sympatric Speciation Specifically Excludes Geographic Isolation

Allopatric speciation is a type of speciation where the formation of new species is due to the geographic isolation of two or more populations of the same species via a physical barrier or migration.

When populations of a species are geographically isolated, gene flow across the groups is restricted, and the allele frequencies between the groups become more different over time. Suppose the groups are separated for a long enough period. In that case, the differences between their alleles become greater due to the differences in their environment (climate, predation, food sources, and other factors). Eventually, new species may form. This process is illustrated in Figure 1 below.

This diagram shows how a physical barrier could split a population and lead to allopatric speciation.

Figure 1. This diagram shows how a physical barrier could split a population and lead to allopatric speciation. Source: Andrew Z. Colvin, CC BY-SA 4.0, via Wikimedia Commons.

In contrast, sympatric speciation occurs even when populations are not physically separated. It is rarer than allopatric speciation and requires the formation of a reproductive barrier while members of the population are still in contact with each other. This process is illustrated in Figure 2 below.

This diagram shows how sympatric speciation occurs in a population even without geographic isolation.

Figure 2. This diagram shows how sympatric speciation occurs in a population even without geographic isolation. Source: Andrew Z. Colvin, CC BY-SA 4.0, via Wikimedia Commons.

As opposed to allopatric speciation, sympatric speciation is controversial because there must be enough evidence to show that the descendant species originated from a common ancestral species, that the group formed reproductive isolation, and that speciation was not caused by geographic isolation.

Additionally, allopatric speciation occurs more frequently in animals than in plants, while sympatric speciation occurs more frequently in plants than in animals. Plants can form polyploidy and reproduce asexually, while not many animals can tolerate the anatomical changes accompanying these processes. To reproduce, a tetraploidy animal needs to look for another animal of the same species but of the opposite sex that has also gone through polyploidy at random.

What Are the Causes and Some Examples of Sympatric Speciation?

Sympatric speciation requires the formation of a reproductive barrier that will restrict gene flow. How does a reproductive barrier form when members of the population live in the same geographical area?

The formation of reproductive barriers can be caused by polyploidy, sexual selection, and habitat differentiation. We will discuss these causes and cite an example of each.

Polyploidy

Polyploidy is when one or more individuals are born with an abnormal number of chromosomes due to an error during cell division. Figure 3 shows how normal individuals with two copies of each chromosome (diploid) can undergo a failed meiosis and produce offspring with four copies of each chromosome (tetraploid).

Polyploidy can be considered a reproductive isolation mechanism because a polyploid individual cannot mate with a normal individual. Polyploid individuals can only interbreed with population members with the same abnormal number of chromosomes.

Polyploidy rarely occurs in animals, but it is quite common in plants. An estimated 80% of extant plant species descended from ancestors formed through polyploid speciation.

This diagram shows how diploids can produce a tetraploid as a result of failed meiosis.

Figure 3. This diagram shows how diploids can produce a tetraploid due to failed meiosis.

Source: Ilmari Karonen, Public domain, via Wikimedia Commons.

There are two forms of polyploidy:

1. Autopolyploidy: an individual has more than two sets of chromosomes that are all derived from a single species.
2. Allopolyploidy: a fertile polyploid is produced from the asexual reproduction (e.g., self-pollination) of a sterile hybrid offspring of two different species.

Examples of Polyploidy Observed in Different Plant Species

The piggyback plant (Tolmiea menzisii) and the white sturgeon (Acipenser transmontanum) are natural autopolyploids. In modern agriculture, autopolyploidy--specifically autotriploidy (an autoploid with three copies of chromosomes) is used to produce seedless watermelons and bananas and sterile salmon and trout.

On the other hand, the wheat species Triticum aestivum is an example of an allopolyploid. It has six sets of chromosomes, two sets derived from three different parent species. The first instance of polyploidy–a spontaneous hybrid of a domesticated wheat species and a wild grass species that occurred over 8,000 years ago–eventually led to the emergence of the modern wheat species.

Sexual selection

Sexual selection is a type of natural selection where a species has mating preferences that lead to changes in the traits in the population over time. Mate preferences can act as a reproductive barrier that prevents gene flow between members of a population with specific traits.

An Example of Sexual Selection Observed in Cichlid Fish

Sexual selection leading to sympatric speciation was observed in cichlid fish. African cichlids found in the African Great Lakes are one of the most diverse fish on Earth. Lake Victoria alone was home to around 600 species of cichlid. Genetic evidence shows that these species emerged within the last 100,000 years from a small ancestral population from rivers and other lakes. It is unlikely that allopatric speciation caused the diversity of the African cichlid fishes because the cichlid fishes tend to assemble in large flocks.

Evidence points to a different reproductive barrier: a mating preference. Female cichlids have a preference for mates that have extreme phenotypic traits. For instance, Pundamilia pundamilia and P. nyererei are closely related species whose males differ only in coloration: P. pundamilia males have blue dorsal fins, while P. nyererei males have red dorsal fins (Figs. 4-5). Because female cichlid fishes prefer certain coloration, the gene pools that lead to these particular traits are kept apart, leading to genetic divergence and eventually sympatric speciation.

Figures 4-5. Pundamilia pundamilia (left) and P. nyererei (right) are very similar in size and shape but differ in coloration. Source (both images): Oliver Selz, CC BY-SA 3.0, via Wikimedia Commons.

Habitat Differentiation

In addition to polyploidy and sexual selection, there is evidence that habitat differentiation can cause sympatric speciation. Habitat differentiation is when a population segment uses a habitat not used by the main population.

An Example of Habitat Differentiation Observed in the Apple Maggot Fly

The apple maggot fly (Rhagoletis pomonella) is a parasitic insect that lays its eggs in the fruit of apples (Malus domestica). The fly used to lay its eggs only in the fruit of the hawthorn (Crataegus), but with the introduction of apple trees in the same geographic area, a portion of the population began to lay its eggs in the fruit of the apple tree.

When members of the population switched to a host species that was different from the rest of the population, the succeeding generations chose a different host as well: flies that grew on hawthorn fruits would lay eggs only on hawthorns. In contrast, flies that grew on apple fruits would lay eggs only on apple fruits. While the two types of maggot flies have not yet fully diverged into two different species, it is believed that this is an ongoing process of sympatric speciation.