Speciation occurs when a single population becomes separated into two groups that, over many generations of independent natural selection, accumulate enough genetic differences to become reproductively isolated — unable to interbreed and produce fertile offspring. Understanding speciation explains why there are millions of species on Earth rather than just one.
What is a species?
Before studying speciation, it is essential to be clear about what a species is. The biological species concept defines a species as a group of organisms that can interbreed with each other and produce fertile offspring, but cannot do so successfully with members of other species.
Two closely related species may be able to mate and produce offspring — for example, a horse and a donkey produce a mule — but the offspring is sterile, so the two parent types are still classified as separate species.
The UK National Curriculum expects students to understand that organisms are classified into species based on shared characteristics and, for GCSE, on reproductive compatibility.
What is speciation?
Speciation is the evolutionary process by which one ancestral species gives rise to two or more new, reproductively isolated species. It is the mechanism that generates biodiversity.
For speciation to occur, two conditions must be met:
- A population must be divided so that the two sub-populations cannot interbreed.
- Sufficient genetic divergence must accumulate in each sub-population that, even if the barrier were removed, they could no longer interbreed successfully.
How does geographic (allopatric) speciation happen?
Allopatric speciation — speciation caused by a physical geographical barrier — is the type GCSE biology focuses on. The sequence of events is:
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Isolation: A physical barrier — a mountain range, a river, an ocean, a glacier — splits one population into two sub-populations that can no longer interbreed.
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Variation: Both sub-populations already carry genetic variation (from pre-existing mutations and sexual reproduction). New mutations continue to arise in both groups, but independently.
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Different selective pressures: Each sub-population faces its own environment with its own food sources, predators, pathogens, and climate. Natural selection acts differently on each group.
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Genetic divergence: Over many generations, the advantageous alleles in each environment become more common in that sub-population; different alleles are advantageous in the other environment. The two groups become genetically more different over time.
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Reproductive isolation: Eventually, the genetic differences become so great that even if the barrier is removed and members of the two groups meet, they cannot interbreed to produce fertile offspring. Two separate species now exist.
| Stage | What happens | Timescale |
|---|---|---|
| 1. Isolation | Physical barrier separates population | Geological event |
| 2. Variation | Independent random mutations and sexual recombination | Every generation |
| 3. Selection | Different environments favour different alleles | Ongoing |
| 4. Divergence | Allele frequencies diverge in the two groups | Thousands of generations |
| 5. Speciation | Reproductive isolation becomes complete | Varies widely |
What role does natural selection play in speciation?
Natural selection is the engine of genetic divergence. In each isolated sub-population:
- Individuals with alleles that make them better adapted to their local environment survive and reproduce more successfully.
- Their offspring inherit those advantageous alleles.
- Over many generations, advantageous alleles become more common and disadvantageous alleles become rarer.
Because the two environments differ — different climate, different predators, different food — the alleles that are advantageous also differ. Sub-population A evolves in one direction; sub-population B evolves in another.
A classic example is Darwin's finches on the Galápagos Islands. An ancestral finch species arrived and populations became isolated on different islands. Each island's food sources differed, so different beak shapes were selected for. Over millions of years, the isolated populations diverged to the point where they became reproductively isolated — new species had formed.
What is reproductive isolation?
Reproductive isolation is the inability of two populations to interbreed to produce fertile offspring. It can arise in several ways:
- Geographic isolation — they live in different places and never meet (this is the initial barrier during allopatric speciation).
- Behavioural isolation — different mating displays, songs, or breeding seasons.
- Anatomical isolation — reproductive organs are no longer compatible.
- Genetic incompatibility — chromosomes cannot pair correctly at meiosis, so any hybrid offspring are sterile (as with mules).
GCSE students need to understand that speciation is complete only when reproductive isolation is established — meeting the physical barrier is not enough on its own.
Is speciation always gradual and slow?
In the classic allopatric model, speciation is a slow process — typically taking thousands to millions of years. However, some mechanisms can be much faster:
- Polyploidy (duplication of the entire genome, common in plants) can produce reproductive isolation in a single generation. Many crop species, such as bread wheat (hexaploid — six sets of chromosomes), originated this way.
- Rapid environmental change can accelerate selection, speeding up divergence.
GCSE biology does not require knowledge of polyploidy in depth, but students should be aware that speciation rates vary.
Frequently asked questions
What is the difference between speciation and evolution?
Evolution is the change in allele frequencies in a population over time. Speciation is a specific outcome of evolution — the point at which two populations become reproductively isolated and can be considered separate species. All speciation involves evolution, but most evolutionary change does not lead to speciation; a single species can evolve considerably (adapting to a new environment) without splitting into two.
Why do the two sub-populations not simply interbreed again if the geographical barrier is removed?
If the barrier is removed before full reproductive isolation has developed, the two groups may interbreed and exchange genes — this is called gene flow, and it would slow or prevent speciation. However, if genetic divergence has already made the groups reproductively incompatible (different chromosome number, different mating behaviours, sterile hybrids), then removing the physical barrier makes no difference — they are already separate species.
Does speciation require a long time?
Usually yes — for most sexually reproducing organisms, speciation requires thousands to millions of years because it depends on the slow accumulation of random mutations and the gradual spread of advantageous alleles. However, polyploidy in plants can produce a new species in one generation. The famous example is Spartina anglica, a hybrid cord-grass that became a new polyploid species in Britain within decades of its hybrid parents meeting.
How does speciation link to biodiversity?
Speciation is the mechanism that generates new species and therefore increases biodiversity. Each speciation event that is maintained (rather than reversed by extinction) adds to the total number of species on Earth. Understanding speciation explains why biodiversity is highest in regions that have been geographically diverse and stable over long periods — such as tropical rainforests — where many isolated populations have had time to diverge.
For Socratic GCSE biology with Professor Darwin — tracing variation through populations to the origin of new species — visit aitutors.me.