Sympatric Speciation

Sympatric Speciation Definition

Sympatric speciation is speciation that occurs when two groups of the same species live in the same geographic location, but they evolve differently until they can no longer interbreed and are considered different species. It is different from other types of speciation, which involve the formation of a new species when a population is split into groups via a geographic barrier or migration. Sympatric speciation can be seen in many different types of organisms including bacteria, cichlid fish, and the apple maggot fly, but it can be difficult to tell when sympatric speciation is occurring or has occurred in nature.

Types of Speciation

To understand sympatric speciation, one must first understand the other types of speciation. There are four types of speciation: sympatric, allopatric, parapatric, and peripatric. The other three types of speciation involve the physical separation of two populations of the same species, while sympatric speciation does not.

In allopatric speciation, two different species can form when one species is separated into different groups due to population dispersal or a natural geologic event such as a mountain formation. Like all forms of speciation, the process is usually very gradual.
Parapatric speciation is when speciation occurs in subpopulations of the same species that are mostly isolated from each other, but have a narrow area where their ranges overlap.
Peripatric speciation occurs when members of a population on the border of that population’s habitat separate off from the main group and evolve over many generations to become a different species.

Sympatric speciation is unique because it takes place while two subpopulations of the same species are occupying the same range or in a range that highly overlaps. Even though the territory that the organisms live in is the same, they are able to split into two different groups that eventually become so genetically different from one another that they can no longer breed with each other. When one group can no longer breed with another, it is a separate species.

It can be difficult to tell whether speciation that has taken place is sympatric, another type, or even a mix of both during the speciation process. This has led to much discussion among evolutionary biology researchers as to what species have truly evolved sympatrically. For example, it was originally thought that two closely related stickleback species evolved via sympatric speciation, but further research suggests that the two different species actually colonized the lake independently. The first colonization led to the rise of one species of stickleback, while the other species evolved from the second colonization.

Jerry Coyne and H. Allen Orr have developed four criteria for inferring whether species have arisen sympatrically:

The species’ ranges must overlap significantly.
There must be complete speciation (i.e., the two species cannot interbreed).
The species must be sister species (most closely related to each other) or part of a monophyletic group, which includes an ancestor and all its descendants; in other words, all the descendant species have to be included if there are more than two, not just some of them.
The history of the species’ geographic range and evolution must make allopatry seem very unlikely, as allopatric speciation is much more common than sympatric speciation.

Examples of Sympatric Speciation

In Bacteria

True examples of sympatric speciation have rarely been observed in nature. Sympatric speciation is thought to occur more often in bacteria, because bacteria can exchange genes with other individuals that aren’t parent and offspring in a process known as horizontal gene transfer. Sympatric speciation has been observed in Bacillus and Synechococcus species of bacteria, and in the bacterioplankton Vibrio splendidus, among others. Subgroups of species that are undergoing sympatric speciation will show few differences since they have been diverging for a relatively recent time on the slow timescale at which evolution takes place. It is thought that one important factor in cases of sympatric speciation is adaptation to environmental conditions; if some members are specialized for living in a certain environment, that subgroup may go on to occupy a different environmental niche and eventually evolve into a new species over time.

In Cichlids

Another example of sympatric speciation is found in two species of Midas cichlid fish (Amphilophus species), which live in Lake Apoyo, a volcanic crater lake in Nicaragua. Researchers analyzed the DNA, appearance, and ecology of these two closely related species. The two species, though overall very similar, do have slight differences in appearance, and they cannot interbreed. All available evidence suggests that one species evolved from the other, which is the species of Midas cichlids that originally colonized the lake. The newer species evolved relatively recently, but in evolutionary terms, this means that it is thought to have evolved less than 10,000 years ago.

This is a photo of a Midas cichlid.

In Apple Maggot Flies

An extremely recent example of sympatric speciation may be occurring in the apple maggot fly, Rhagoletis pomonella. Apple maggot flies used to lay their eggs only on the fruit of hawthorn trees, but less than 200 years ago, some apple maggot flies began to lay their eggs on apples instead. Now there are two groups of apple maggot flies: one that lays eggs on hawthorns and one that lays eggs on apples. Males look for mates on the same type of fruit that they grew on, and females lay their eggs on the same type of fruit that they grew up on. Therefore, flies that grew up on hawthorns will raise offspring on hawthorns, and flies that grew up on apples will raise offspring on apples. There are already genetic differences between the two groups, and over a long period of time, they could become separate species. This shows how speciation can occur even when different subgroups of the same species have the same geographic range.

Species – A group of organisms that can interbreed.
Speciation – The evolution of a new species.
Range – The geographic area where members of a species live.
Niche – The function of a species in an ecosystem based on many factors such as the species’ habitat and diet.

Quiz

1. Which type of speciation does not involve geographic separation?
A. Allopatric speciation
B. Sympatric speciation
C. Peripatric speciation
D. Parapatric speciation

Answer to Question #1

B is correct. Allopatric, peripatric, and parapatric speciation all involve some form of geographic separation, either partial or complete. Sympatric speciation is the only type of speciation that takes place while subgroups of a species occupy the same range, and therefore, it is thought to be very rare.

2. Which organism mentioned in this article is NOT thought to have evolved or be evolving sympatrically?
A. Apple maggot fly
B. Midas cichlid
C. Bacillus bacteria
D. Stickleback

Answer to Question #2

D is correct. Some stickleback species were once thought to have evolved sympatrically, and were an oft-cited example of sympatric speciation. However, further research suggests that the same lake with two closely related stickleback species was actually colonized by sticklebacks two separate times.

3. The apple maggot fly may be in the process of speciation. How would one know whether speciation is complete?
A. The two groups of apple maggot flies would look different.
B. Some apple maggot flies would lay eggs only on apples, while others would lay eggs only on hawthorns.
C. The apple maggot flies would have different geographic ranges.
D. The two groups of apple maggot flies would not be able to interbreed.

Answer to Question #3

D is correct. The biological definition of a species is a group of organisms that can interbreed. Organisms are considered different species if they cannot interbreed. Just having different physical characteristics, as in choice A, would not be enough; also, some organisms look extremely similar but are actually different species. Choice B is happening presently, but members of the two groups can still interbreed. Choice C is incorrect because organisms can live in different areas and still be the same species, and because apple maggot flies are in the process of speciation even though the two groups live in the same geographic range.