Species are not fixed forever. Over generations, populations can accumulate differences until some of them become so genetically and reproductively distinct that they form new species. This process is called speciation.
Speciation is one of the central processes of evolution because it explains how the diversity of life increases. It does not usually happen because a single organism suddenly becomes a new species. Instead, it generally unfolds across many generations as populations become separated, evolve different traits, or stop exchanging genes with one another.
What is a species?
The word species can have slightly different meanings in biology, but one of the most widely used definitions is based on reproduction.
Under the biological species concept, a species is a group of organisms that can naturally interbreed and produce fertile offspring. Members of the same species share a gene pool because their genes can pass between populations through reproduction.
This definition works well for many animals, but it has limitations. It is difficult to apply to organisms that reproduce asexually, and it cannot easily be used for fossils. Closely related species can also sometimes produce hybrid offspring, even though they remain separate species.
For these reasons, biologists also use other approaches, including differences in physical characteristics, evolutionary history, and genetic relationships. In practice, determining whether populations represent separate species can sometimes be complicated.
How does speciation happen?
The basic requirement for speciation is a reduction or eventual loss of gene flow between populations.
Gene flow is the movement of genes between populations, usually through migration and reproduction. When populations regularly interbreed, genetic differences tend to be mixed together. But when gene flow is reduced, populations can begin following different evolutionary paths.
Mutations introduce new genetic variation. Natural selection can favor different traits in different environments, while genetic drift can cause random genetic changes, particularly in small populations. Over many generations, these processes can make isolated populations increasingly different.
Eventually, the populations may become reproductively isolated. At that point, even if they come back into contact, they may no longer successfully exchange genes. That is the key step in the formation of separate species.
Geographic isolation can split a population
One of the clearest routes to speciation is allopatric speciation, which occurs when a physical barrier separates a population.
A mountain range, river, canyon, island, or other geographic change can divide a population into groups that can no longer easily mate with one another. Once separated, each population experiences its own mutations, natural selection, genetic drift, and environmental conditions.
Imagine a species spread across a large region that becomes divided by a newly formed geographic barrier. The two populations may encounter different climates, foods, predators, or habitats. Natural selection can therefore favor different characteristics in each population.
Even without major environmental differences, the populations can diverge through genetic drift and the accumulation of mutations.
At first, the differences may be small. Over many generations, however, they can accumulate. If the populations eventually meet again, they may be genetically and reproductively different enough that they no longer function as a single species.
Speciation does not always require geographic separation
Geographic isolation is important, but it is not essential in every case.
Sympatric speciation occurs when new species arise while populations occupy the same geographic area. This can happen when some individuals within a population begin using different resources, habitats, or mating behaviors, reducing reproduction between the groups.
Sympatric speciation is particularly important in plants because changes in chromosome number can sometimes create reproductive isolation very quickly.
A common mechanism is polyploidy, in which an organism possesses extra sets of chromosomes. If a plant develops a chromosome-number change that prevents it from producing fertile offspring with the original population, it may become reproductively isolated even while living alongside it.
Other forms of reproductive isolation can develop more gradually, including differences in mating behavior, breeding time, or habitat preference.
Reproductive isolation is the key to becoming separate species
For populations to become separate species, differences must eventually interfere with reproduction and gene flow.
Reproductive isolation can occur before fertilization or after it.
Prezygotic barriers prevent mating or fertilization from producing a viable embryo. Examples include differences in mating behavior, breeding seasons, preferred habitats, or the physical compatibility of reproductive structures.
Postzygotic barriers act after fertilization. Hybrids may fail to develop normally, survive poorly, or be unable to reproduce.
These barriers do not necessarily appear all at once. A population can gradually accumulate several forms of reproductive isolation until successful gene exchange becomes rare or impossible.
Natural selection can push populations apart
Natural selection can contribute to speciation when populations face different evolutionary pressures.
Suppose two populations of the same species become separated and encounter different environments. A trait that improves survival in one environment may be less useful in the other. Over generations, natural selection can increase different traits in the two populations.
Natural selection can also directly affect mating. If individuals prefer mates with particular colors, songs, behaviors, or other characteristics, those preferences can contribute to reproductive isolation.
In some situations, the same evolutionary pressures that cause populations to adapt to different environments can also make them less likely to reproduce with one another.
Genetic drift can also drive divergence
Not every difference between populations is produced by natural selection.
Genetic drift is the random change in the frequency of genetic variants within a population. Its effects are especially strong in small populations, where chance events can substantially alter which genetic variants are passed to future generations.
A small group that becomes isolated from a larger population may therefore evolve in genetically different ways simply because it carries a different sample of the original population’s genetic variation.
Two isolated populations can experience both natural selection and genetic drift at the same time. Their divergence is often the combined result of these and other evolutionary processes.
New species can form through changes in mating
Speciation can sometimes be closely tied to sexual selection.
If individuals in a population develop different preferences for mates, groups may increasingly mate within themselves rather than randomly across the population. Over time, those mating preferences can reduce gene flow.
Differences in courtship signals, songs, coloration, scents, or other mating traits can therefore contribute to reproductive isolation.
This does not mean that every difference in appearance creates a new species. The important question is whether the difference contributes to sustained reproductive separation.
Speciation can happen gradually or relatively quickly
Speciation is often portrayed as a slow, steady process, but evolutionary change does not always proceed at a constant rate.
Some populations may remain relatively similar for long periods and then diverge more rapidly when environmental conditions, geographic barriers, genetic changes, or other factors alter the course of evolution.
In other cases, reproductive isolation develops through the gradual accumulation of many small differences.
There is therefore no single timetable for speciation. The process can vary greatly among organisms and circumstances.
What is adaptive radiation?
Sometimes speciation produces not just one new species but many.
Adaptive radiation occurs when a lineage diversifies into multiple species that occupy different ecological roles. This can happen when organisms encounter new environments or resources that provide opportunities for populations to specialize.
As different populations adapt to different ways of living, reproductive isolation can develop between them. Over time, one ancestral lineage can give rise to numerous descendant species with different traits and ecological roles.
Adaptive radiation is therefore a particularly striking example of how evolutionary divergence can generate biological diversity.
How can scientists tell that speciation has occurred?
Scientists use several kinds of evidence because no single measurement always provides the complete answer.
Genetic data can reveal how closely related populations are and whether they continue to exchange genes. Observations of mating behavior can show whether populations preferentially reproduce within their own groups. Researchers can also compare physical characteristics, reproductive compatibility, geographic distributions, and ecological differences.
For living organisms, evidence of reproductive isolation is especially important when applying the biological species concept.
Scientists may also study populations that are at different stages of divergence. Some are clearly part of the same species, while others are clearly distinct. Populations in between can provide evidence about how reproductive barriers develop.
Speciation is the result of accumulated evolutionary change
A new species does not generally appear because evolution suddenly produces a completely different organism. Speciation is usually the outcome of populations becoming increasingly independent over generations.
Mutations provide genetic variation. Natural selection can favor different traits. Genetic drift can change populations by chance. Sexual selection can alter mating patterns. Geographic separation or differences in behavior and habitat can reduce gene flow. As these processes continue, reproductive barriers can strengthen.
Once populations become sufficiently reproductively isolated, their evolutionary histories can proceed independently.
That process—populations diverging until they become separate evolutionary lineages—is how speciation helps produce the extraordinary diversity of life.
