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Speciation is the process by which new species arise. It occurs when populations of a single species become reproductively isolated from each other and diverge genetically to the point where they can no longer interbreed to produce fertile offspring. Understanding the mechanisms of speciation is essential for explaining the diversity of life on Earth.
Speciation is the formation of a new species. Recall that a species is defined (using the biological species concept) as a group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other groups.
For a new species to form, a population must be split into two or more groups that are reproductively isolated — they can no longer exchange genes. Once isolated, the groups accumulate different genetic changes over time (through natural selection and genetic drift) until they become so genetically different that they can no longer interbreed, even if they come back into contact.
Allopatric speciation occurs when populations are separated by a geographical barrier — a physical obstacle that prevents gene flow between the populations.
The following diagram illustrates the process of allopatric speciation:
flowchart TD
A["Original Population"] --> B["Geographical Barrier<br/>(Allopatric)"]
B --> C["Population A"]
B --> D["Population B"]
C --> E["Different Selection<br/>Pressures"]
D --> E
E --> F["Genetic Divergence"]
F --> G["Reproductive Isolation"]
G --> H["Two New Species"]
The Galapagos finches are perhaps the most famous example of allopatric speciation. An ancestral finch species from mainland South America colonised the Galapagos Islands. Different populations became isolated on different islands, each with different food sources:
| Finch Type | Beak Shape | Food Source |
|---|---|---|
| Ground finches | Large, strong, crushing beaks | Hard seeds and nuts |
| Tree finches | Grasping beaks | Insects in bark |
| Warbler finch | Thin, pointed beak | Small insects |
| Cactus finch | Long, probing beak | Cactus nectar and seeds |
| Woodpecker finch | Strong beak; uses tools | Insect larvae in wood |
Natural selection favoured different beak shapes on different islands because of the different food sources available. Over time, the populations diverged so much that they became separate species — there are now 13–18 recognised species of Darwin's finch.
Exam Tip: Darwin's finches are an example of adaptive radiation — the rapid diversification of a single ancestral species into many species, each adapted to a different ecological niche. Be prepared to link this to allopatric speciation.
Sympatric speciation occurs when new species arise within the same geographical area, without a physical barrier separating populations. This is less common than allopatric speciation but is well-documented, especially in plants.
Polyploidy is the condition of having more than two complete sets of chromosomes. It can lead to instant speciation because polyploid individuals are often reproductively isolated from the original diploid population.
Autopolyploidy — polyploidy within a single species:
Allopolyploidy — polyploidy involving hybridisation between two different species:
| Type | Mechanism | Example |
|---|---|---|
| Autopolyploidy | Chromosome doubling within one species | Some potato varieties |
| Allopolyploidy | Hybridisation between species + chromosome doubling | Triticum aestivum (bread wheat) — a hexaploid (6n) derived from three different ancestral grass species |
Exam Tip: Polyploidy is by far the most important mechanism of sympatric speciation for A-Level exams. Be able to explain both autopolyploidy and allopolyploidy, and explain why the polyploid is reproductively isolated from the parent species.
Populations within the same area may specialise on different microhabitats or food sources. Over time, they may become reproductively isolated due to differing selection pressures. For example, apple maggot flies in North America: one population specialises on native hawthorn fruit, another on introduced apple trees. They mate on their preferred fruit, so gene flow between the populations is reduced.
Populations may breed at different times (different seasons, different times of day), preventing gene flow even when they share the same habitat.
Reproductive isolation is the key requirement for speciation. Mechanisms that prevent interbreeding are classified as pre-zygotic (before fertilisation) or post-zygotic (after fertilisation).
These prevent the formation of a hybrid zygote:
| Mechanism | Description | Example |
|---|---|---|
| Geographical (habitat) isolation | Populations live in different areas and do not meet | Squirrels on opposite sides of the Grand Canyon |
| Temporal isolation | Populations breed at different times | Two species of toad: one breeds in early spring, the other in late summer |
| Behavioural isolation | Different courtship behaviours, songs, or pheromones mean individuals do not recognise each other as mates | Firefly species have different flash patterns; bird species have different songs |
| Mechanical isolation | Reproductive organs are physically incompatible | Different flower shapes prevent cross-pollination between plant species |
| Gametic isolation | Gametes are incompatible — sperm cannot fertilise eggs of a different species | Sea urchin species release gametes into the water, but species-specific proteins on egg surfaces prevent cross-fertilisation |
These act after fertilisation has occurred:
| Mechanism | Description | Example |
|---|---|---|
| Hybrid inviability | The hybrid embryo fails to develop properly and dies before reaching maturity | Crosses between certain sheep species produce embryos that die early in development |
| Hybrid sterility | The hybrid offspring is viable but sterile — it cannot reproduce | A mule (horse × donkey) is sterile because the chromosomes from each parent cannot pair during meiosis |
| Hybrid breakdown | First-generation hybrids are fertile, but their offspring (F2) have reduced fitness or are sterile | Some crosses between rice varieties produce fertile F1 hybrids but weak, infertile F2 plants |
Exam Tip: Exam questions often ask you to classify reproductive isolation mechanisms as pre-zygotic or post-zygotic. A good strategy is to ask: "Has a zygote formed?" If yes, it is post-zygotic. If no, it is pre-zygotic.
Adaptive radiation is the rapid diversification of a single ancestral species into many new species, each adapted to a different ecological niche. It typically occurs when:
| Example | Detail |
|---|---|
| Darwin's finches | 13–18 species evolved from one ancestral species on the Galapagos Islands |
| Cichlid fishes | Over 500 species in Lake Victoria alone, diverging in body shape, jaw structure, diet and colour in just 15,000 years |
| Hawaiian honeycreepers | Over 50 species evolved from a single ancestral finch-like bird, diversifying into seed-eaters, nectar-feeders, insect-eaters, and snail-eaters |
| Mammals after dinosaur extinction | The extinction of non-avian dinosaurs 66 million years ago freed ecological niches that mammals rapidly diversified to fill |
Geographical isolation is fundamental to allopatric speciation and is the most common pathway to new species formation. The key steps can be summarised as:
A ring species is a connected series of populations that can interbreed with adjacent populations, but populations at the two "ends" of the ring cannot interbreed with each other. Ring species provide a rare snapshot of speciation in progress.
Example: Ensatina salamanders in California's Central Valley. Populations form a ring around the valley. Adjacent populations can interbreed, but the populations that meet at the southern end of the valley cannot — they behave as separate species.
Ring species challenge the biological species concept because there is no clear boundary between "one species" and "two species."
| Key Concept | Detail |
|---|---|
| Speciation | Formation of new species through reproductive isolation and genetic divergence |
| Allopatric speciation | Geographical barrier separates populations; different selection pressures cause divergence |
| Sympatric speciation | New species arise in the same area, often through polyploidy |
| Polyploidy | More than two sets of chromosomes; autopolyploidy and allopolyploidy |
| Pre-zygotic isolation | Barriers before fertilisation: geographical, temporal, behavioural, mechanical, gametic |
| Post-zygotic isolation | Barriers after fertilisation: hybrid inviability, sterility, breakdown |
| Adaptive radiation | Rapid diversification of one species into many, each filling a different niche |
| Ring species | Speciation in progress — adjacent populations interbreed but endpoints cannot |
Exam Tip: Speciation questions often carry extended-response marks. Structure your answer as a clear narrative: describe the isolating mechanism, explain how different selection pressures cause divergence, and conclude by stating that reproductive isolation means the populations are now separate species.
The Edexcel 9BI0 specification places speciation within Topic 4: Biodiversity and Natural Resources, where it follows directly from Lesson 6 (Natural Selection and Evolution) — speciation is the long-term outcome of selection plus drift acting on populations whose gene pools have been severed by reproductive isolation. Synoptic ties run forwards to Lesson 8 (Evidence for Evolution) — the fossil record, biogeography and molecular phylogenetics document past speciation events — and backwards to Lesson 1 (Classification and the Species Concept), which defines the unit whose formation this lesson explains. Cross-topic, the lesson depends on Topic 8 (Modern Genetics) — DNA mutation as the source of variation, with chromosomal mutations (polyploidy, inversions, translocations) supplying the rare instant-speciation route, and on Topic 6 (Immunity) — horizontal gene transfer, which blurs species boundaries in prokaryotes and is one reason the biological species concept fails for bacteria. Specification statements concern: the biological species concept; geographical and reproductive isolation as the two requirements for speciation; the distinction between allopatric and sympatric routes; the classification of isolating mechanisms as prezygotic versus postzygotic; and the role of selection plus drift in driving genetic divergence between isolated populations (refer to the official Pearson Edexcel 9BI0 specification document for exact wording).
Question (8 marks):
A panmictic population of beetles inhabits a continuous river-valley grassland. A tectonic-driven mountain uplift over ∼10,000 generations divides the valley into northern and southern halves with no possibility of beetle migration across the new range. After the uplift, the northern half becomes cooler and wetter while the southern half becomes warmer and drier. Field workers returning 5,000 generations later collect beetles from both sides via an ice-free pass and attempt laboratory crosses. F1 hybrids form but produce no offspring.
(a) Identify the mode of speciation, naming the geographical barrier and the gene-flow consequence at the moment of barrier formation. (2)
(b) Explain how selection plus drift cause genetic divergence in the two sub-populations during the 5,000 generations of isolation. (3)
(c) Classify the F1-hybrid-produces-no-offspring observation as a prezygotic or postzygotic isolating mechanism, naming the specific category and one plausible cellular cause. (3)
Solution with mark scheme:
(a) M1 (AO1.2) — this is allopatric speciation; the geographical barrier is the uplifted mountain range physically separating the two halves of the original valley.
A1 (AO2.1) — at the moment of barrier formation, gene flow between the northern and southern populations falls to zero; the previously panmictic gene pool is divided into two independent gene pools.
(b) M1 (AO1.2) — independent mutation introduces new alleles in each sub-population at ∼10−8 per base per generation; the two pools accumulate different novel alleles.
M1 (AO2.1) — directional selection acts differently in each environment: the cooler-wetter north favours alleles for, e.g., greater cold tolerance and water-balance variants; the warmer-drier south favours desiccation-resistance and heat-shock variants. Allele frequencies diverge.
A1 (AO2.1) — genetic drift randomly fixes neutral or near-neutral alleles independently in each pool, with effect size ∼1/Ne per generation; over 5,000 generations cumulative drift is substantial even in large populations.
(c) M1 (AO1.2) — F1 hybrids form, so a zygote did form; the isolation is therefore postzygotic.
M1 (AO2.1) — F1 hybrids producing no offspring is hybrid sterility (rather than hybrid inviability — they reach reproductive age — or hybrid breakdown, which would require reduced F2 fitness).
A1 (AO3.1a) — plausible cellular cause: chromosomal rearrangements (inversions, translocations, fusions/fissions) accumulated independently in the two sub-populations now disrupt homologous pairing during meiosis I in the F1 hybrid, producing unbalanced gametes. The mule (horse × donkey, 2n=64 vs 2n=62) is the classic textbook analogue.
Total: 8 marks.
Question (6 marks): Outline the conditions required for speciation, distinguish between allopatric and sympatric routes, and apply the framework to the Galapagos finch radiation, evaluating why allopatric speciation is more frequent than sympatric in animals.
Mark scheme decomposition by AO:
| Marking point | AO | Credit-worthy content |
|---|---|---|
| 1 | AO1.1 | States the conditions: an ancestral population must be split into sub-populations whose gene flow is reduced or eliminated, with selection and/or drift then driving genetic divergence to the point that reproductive isolation evolves and the sub-populations meet the biological species concept. |
| 2 | AO1.2 | Distinguishes allopatric (geographical barrier precedes reproductive isolation — the standard mode) from sympatric (reproductive isolation evolves without geographical separation, typically via polyploidy in plants, host-race specialisation, or strong assortative mating). |
| 3 | AO2.1 | Applies allopatric framework to Galapagos finches: a colonising ancestor from mainland South America reached the archipelago; island-by-island isolation by ocean barrier reduced gene flow; different food sources on different islands imposed different selection pressures (large hard-seed beaks, fine insectivore beaks, cactus-probing beaks, tool-using woodpecker-finch). |
| 4 | AO2.1 | Applies the radiation outcome: ∼13 modern species occupying distinct ecological niches; reproductive isolation is reinforced both prezygotically (different beak morphology and song affecting mate choice) and postzygotically where hybrids do form. |
| 5 | AO3.1a | Evaluates: allopatric speciation is more common in animals because mobility and active mate choice mean that without a geographical barrier, gene flow rapidly homogenises any incipient divergence. Sympatric routes require either instant chromosomal isolation (polyploidy — common in plants but reduced gametic viability in vertebrates) or unusually strong assortative mating (apple maggot fly Rhagoletis pomonella mating on its host fruit). |
| 6 | AO3.2a | Concludes that the Galapagos radiation satisfies all four conditions of the allopatric framework, that the multiple-island geography is critical because each new colonisation event resets a small founder gene pool with strong drift potential, and that the ongoing observation of beak-size selection (Grants' work since 1973) confirms the framework is empirically tractable. |
Total: 6 marks split AO1 = 2, AO2 = 2, AO3 = 2. Section B "outline and apply": Edexcel rewards candidates who map the allopatric/sympatric framework onto a named radiation (AO2) and evaluate why one route is more common than the other (AO3) rather than retelling the finch story.
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