New species do not always need a physical barrier to evolve. In sympatric speciation, populations living in the same geographic area can gradually become so genetically and reproductively different that they stop successfully interbreeding.
What is sympatric speciation?
Sympatric speciation is the evolution of new species from a single population without geographic separation. The word sympatric comes from Greek roots meaning “same homeland.”
In the simplest version of the process, members of one species live in the same place but begin using different resources, habitats, or reproductive strategies. If those differences cause individuals to mate mainly with others like themselves, gene flow between the groups can decline. Over many generations, genetic differences can accumulate until the groups become reproductively isolated.
Reproductive isolation means that two populations no longer exchange genes effectively. They may rarely mate, may not recognize each other as suitable mates, or may produce offspring that are unable to survive or reproduce successfully. Once reproductive isolation becomes strong enough, the populations can be considered separate species even though they still occupy overlapping geographic territory.
This makes sympatric speciation fundamentally different from allopatric speciation, in which a physical barrier such as a mountain range, river, island, or other geographic separation divides a population before the isolated groups diverge.
How can populations split while living together?
The central challenge for sympatric speciation is gene flow. If individuals from two emerging groups continue mating freely, their genes are repeatedly mixed together, making it difficult for substantial differences to develop.
For sympatric speciation to occur, some form of assortative mating or another mechanism must reduce that mixing. Assortative mating occurs when individuals preferentially mate with others that share particular traits.
One possible route begins with differences in how individuals use the environment. Suppose members of a population begin specializing on different resources that occur in the same geographic area. Individuals that specialize on one resource may spend more time in a particular part of the environment, encounter different potential mates, or reproduce at different times from individuals using another resource.
Those ecological differences can reinforce mating differences. If individuals increasingly mate within their own group, the genetic differences associated with the two lifestyles can become more common within each group.
The important point is that the population does not have to move to different places for gene flow to decline. Different ways of living in the same place can sometimes create the reproductive separation needed for speciation.
Natural selection can help drive the split
Natural selection can play a major role when different parts of a population face different ecological conditions.
Imagine a population whose members can exploit two different resources. Individuals better adapted to one resource may survive and reproduce more successfully when using it, while other individuals are better suited to the alternative resource. If those differences are inherited, natural selection can push the two groups in different evolutionary directions.
The process becomes especially powerful when ecological adaptation also affects mating.
For example, a trait that helps an animal exploit a particular resource might also influence which potential mates it encounters or chooses. Alternatively, individuals may prefer mates that use the same resource because they occupy the same environment or share similar traits.
This creates a feedback loop: ecological differences can produce mating differences, and reduced mating between the groups allows their genetic differences to increase.
What role does sexual selection play?
Sexual selection can also contribute to sympatric speciation when mate choice differs within a population.
Members of the same population may begin preferring different mating signals, appearances, behaviors, or other characteristics. If those preferences are linked to inherited traits, individuals can increasingly choose mates from their own subgroup.
At first, the difference may be small. But if individuals that share a particular trait consistently prefer one another, the association between the trait and mating preference can strengthen over generations.
Eventually, the groups may become sufficiently different in their mating behavior that they rarely reproduce with one another, even though they continue to live in the same geographic area.
Polyploidy can produce a new species unusually quickly
One of the clearest ways sympatric speciation can occur involves polyploidy, a condition in which an organism has more than two complete sets of chromosomes.
Polyploidy is particularly important in plants. Errors during cell division can sometimes produce cells with duplicated chromosome sets. If those cells contribute to reproduction, plants with a new chromosome number can arise.
A polyploid individual may be able to reproduce successfully with other polyploid individuals but have difficulty producing fertile offspring with the original population. In effect, the chromosome difference itself can create reproductive isolation.
This can make polyploid speciation much faster than the gradual accumulation of many small genetic differences. A new chromosome arrangement can immediately reduce successful reproduction between the new form and its ancestral population.
Polyploidy therefore provides a particularly strong example of how a new species can arise without a geographic barrier. A population does not necessarily need to be physically separated before reproductive incompatibility develops.
Is sympatric speciation common?
Sympatric speciation is biologically possible, but it is generally considered more difficult to achieve than speciation caused by geographic isolation.
Geographic separation naturally stops or greatly reduces gene flow. Once populations are isolated, mutations, natural selection, genetic drift, and other evolutionary processes can cause them to diverge independently.
In sympatric speciation, the populations remain in contact. Individuals from different emerging groups may continue to mate, constantly mixing their genes. That makes reproductive isolation harder to establish and maintain.
For this reason, scientists have been particularly interested in cases where ecological specialization, strong assortative mating, sexual selection, or chromosome changes can overcome the homogenizing effect of gene flow.
How do scientists know when speciation has occurred?
Species boundaries are not always defined by a single test. In many organisms, scientists consider several kinds of evidence.
One important question is whether populations interbreed successfully. If two groups living in the same area rarely mate or produce few viable, fertile offspring when they do mate, that is evidence of reproductive isolation.
Genetic evidence can reveal whether the groups have accumulated substantial differences despite living together. Ecological evidence can show whether they occupy different niches or specialize on different resources. Behavioral observations can reveal differences in mating preferences, breeding times, or courtship.
No single characteristic is necessarily decisive in every organism. Species can be separated by different combinations of reproductive, genetic, ecological, and behavioral differences.
Sympatric speciation is a process, not a sudden event
The formation of a species is usually not an instant transformation from one kind of organism into another. It is a process in which populations gradually become more distinct.
Early in the process, two groups may still exchange many genes. As ecological preferences or mating behaviors become more different, gene flow can decline. Natural selection and other evolutionary forces then act on increasingly separate gene pools.
Eventually, reproductive isolation may become strong enough that the two groups function as independent evolutionary lineages.
This also explains why scientists can encounter populations that appear to be partway through the process. Evolution does not always produce clean boundaries immediately. Populations can exist along a continuum from freely interbreeding groups to strongly isolated species.
Why sympatric speciation matters
Sympatric speciation demonstrates that geographic isolation is not the only route to biodiversity.
Species can diverge because organisms living in the same environment do not necessarily live in exactly the same way. Differences in resource use, ecological preferences, mating behavior, reproductive timing, chromosome number, or other inherited traits can reduce gene flow within a population.
Once that reproductive separation becomes strong enough, evolutionary change can continue independently within the emerging groups.
The result is a striking possibility: two species can originate from the same ancestral population without first being separated by a physical barrier. In the right circumstances, living in the same place does not mean evolving together.
