A new species forms when a population of organisms becomes different enough from other populations that the groups can no longer freely exchange genes. This process, called speciation, usually unfolds over many generations as populations accumulate genetic differences. Changes in geography, natural selection, sexual selection, mutation, genetic drift, and other evolutionary forces can all contribute.
The key step is reproductive isolation: members of the diverging populations eventually stop producing fertile offspring together, either because they no longer mate, cannot successfully reproduce, or produce offspring that cannot reproduce themselves.
What is a species?
A species is often defined as a group of organisms whose members can naturally interbreed and produce fertile offspring. This is known as the biological species concept.
The definition works well for many animals, but it does not apply universally. Some organisms reproduce asexually, and different species can sometimes hybridize. For that reason, biologists also use other ways of defining species, including differences in physical characteristics, ecological roles, ancestry, and genetic relationships.
Regardless of the definition being used, speciation involves populations becoming evolutionarily distinct.
How does speciation begin?
Speciation starts with variation within a population. Individuals are not genetically identical. Mutations create new genetic variants, while recombination reshuffles existing genetic variation during reproduction.
As long as individuals from different parts of a population continue to mate and reproduce freely, their genes can continue to mix. That gene flow tends to keep the population genetically connected.
Speciation becomes more likely when gene flow between groups is reduced or stopped.
A common route begins when a population is divided by a physical barrier such as a mountain range, river, changing habitat, or another geographic separation. Once separated, the populations evolve independently.
Allopatric speciation: when geography separates populations
Allopatric speciation occurs when populations become geographically separated.
Imagine a single population spread across a large region. A new geographic barrier divides it into two groups. Individuals on one side can no longer regularly reproduce with individuals on the other side.
The two populations now face different environmental conditions and evolutionary histories. Mutations arise independently in each population. Natural selection may favor different traits, while genetic drift can cause gene frequencies to change by chance, particularly in smaller populations.
Over many generations, the populations can become increasingly different.
If the barrier eventually disappears and the populations come back into contact, they may still be able to interbreed. But if their differences have become substantial enough to prevent successful reproduction, they have become separate species.
Geographic isolation therefore does not automatically create a new species. It creates the conditions in which populations can diverge.
Reproductive isolation is the crucial step
The defining feature of speciation is not simply that two populations look different or live in different places. What matters is whether they remain genetically connected through reproduction.
Reproductive isolation occurs when genetic exchange between populations is prevented or greatly reduced.
This isolation can develop in several ways.
Before fertilization
Some reproductive barriers prevent mating or fertilization from happening in the first place.
Two populations may begin breeding at different times of year, occupy different habitats, or respond to different courtship signals. Even if they live in the same general area, these differences can keep them from mating.
In other cases, individuals may mate but fail to transfer or receive sperm successfully because their reproductive structures or processes are incompatible.
These are called prezygotic barriers because they act before a fertilized egg, or zygote, forms.
After fertilization
Other barriers appear after mating has occurred.
Two organisms may produce hybrid offspring, but the hybrids may be less viable, fail to develop normally, or be unable to reproduce. Such barriers reduce the flow of genes between the parent populations.
These are known as postzygotic barriers because they occur after fertilization.
Reproductive isolation can therefore result from several barriers acting together rather than from a single dramatic evolutionary change.
Natural selection can push populations apart
Once populations are separated, natural selection can drive them in different directions.
Suppose two populations live in environments with different conditions. A trait that improves survival or reproduction in one environment may provide little benefit—or even be disadvantageous—in another.
Over generations, natural selection can increase different traits in the two populations. Differences can accumulate in anatomy, physiology, behavior, development, or other characteristics.
Those differences may eventually contribute to reproductive isolation.
Sexual selection can also play an important role. If individuals in different populations develop different preferences for mates or respond to different courtship signals, mating between the populations may become less common. Over time, those differences can strengthen reproductive isolation.
Genetic drift can also contribute
Not every evolutionary difference is produced by natural selection.
Genetic drift is the random change in the frequency of genetic variants within a population. Its effects can be especially strong in small populations.
When isolated populations experience genetic drift independently, variants can become common in one population and rare or absent in another simply because of chance.
These differences can accumulate alongside changes produced by natural selection and mutation. Eventually, enough genetic divergence may develop to contribute to reproductive isolation.
Can a new species form without geographic separation?
Yes. Speciation does not always require a physical barrier.
Sympatric speciation occurs when new species arise while populations occupy the same geographic area.
This can happen when genetic differences create strong reproductive barriers within a population. A particularly important mechanism in plants is polyploidy, in which an organism has extra sets of chromosomes.
Polyploid individuals can sometimes be reproductively isolated from their original population because chromosome differences interfere with reproduction between them. If the new genetic condition persists and the population becomes established, it can provide a route to a new species.
Other forms of sympatric speciation can involve ecological differences or changes in mating preferences that reduce gene flow even though the populations remain geographically close.
Speciation can happen gradually
There is usually no single moment when an ordinary population suddenly becomes a new species.
Instead, speciation is generally a process. Populations may first show modest genetic differences. Gene flow may then decrease, allowing differences to accumulate. Reproductive barriers can become stronger as the populations continue to diverge.
During this process, determining exactly when populations have become separate species can be difficult.
This is particularly true when reproductive isolation is incomplete. Two populations may differ substantially but still produce some viable, fertile offspring. Evolutionary divergence can therefore occur along a continuum rather than following a simple before-and-after boundary.
What happens when diverging populations meet again?
Separated populations can sometimes come back into contact before reproductive isolation is complete. This creates an important test of whether speciation has progressed.
If the populations freely interbreed, gene flow can continue to connect them.
If they rarely mate, produce few viable offspring, or produce hybrids with reduced fertility, reproductive isolation is stronger.
In some cases, natural selection can favor traits that reduce interbreeding when hybrid offspring are less successful. This can reinforce differences between populations and strengthen reproductive isolation.
Other populations may continue exchanging genes while remaining partly distinct. This shows that the evolutionary process is not always a simple split into two completely isolated groups.
The main routes to new species
Several evolutionary processes can therefore contribute to speciation:
- Geographic isolation can separate populations and reduce gene flow.
- Mutation introduces new genetic variation.
- Natural selection can favor different traits in different environments.
- Genetic drift can produce random genetic differences, especially in small populations.
- Sexual selection can cause populations to develop different mating preferences or signals.
- Changes in chromosome number, particularly polyploidy in plants, can produce rapid reproductive isolation.
- Ecological specialization can reduce mating between populations even when they live in the same region.
These processes often interact rather than operating independently.
How long does it take to form a new species?
There is no universal timetable for speciation.
Some cases of reproductive isolation can develop relatively rapidly, particularly when major genetic or chromosomal changes are involved. In other cases, populations may diverge gradually over many generations.
The pace depends on factors such as the amount of genetic variation, the strength and direction of natural selection, population size, mutation and recombination, the amount of gene flow, and the nature of reproductive barriers.
The important point is that speciation is an evolutionary process in which populations become increasingly independent until genetic exchange is sufficiently restricted for them to function as distinct evolutionary lineages.
