Speciation

Speciation is the evolutionary process by which a single ancestral species splits into two or more descendant species — the engine by which biodiversity is generated. Whereas natural selection (lesson 2) and drift (lesson 3) describe microevolution (within-species allele-frequency change), speciation is the canonical macroevolutionary event: a population genealogy bifurcates, the daughter lineages diverge, and reproductive isolation between them becomes complete. Understanding the modes of speciation, the species concept itself, and the reproductive isolating mechanisms that maintain species boundaries is foundational to evolutionary biology and a routine examination focus.

Spec mapping: This lesson sits in AQA 7402 Section 3.7.3 (species and taxonomy — speciation as the formation of new species; allopatric and sympatric mechanisms; reproductive isolation). Refer to the official AQA specification document for exact wording. It builds on the gene-flow and drift content from lesson 3 and the disruptive-selection content from lesson 2.

Connects to: Genetic drift and gene flow (Section 3.7.2, lesson 3 — restricting gene flow is the enabling condition); natural selection (Section 3.7.2, lesson 2 — divergent selection drives the morphological differences between forming species); classification and taxonomy (Section 3.7.3, lesson 5); molecular evidence for evolution (Section 3.7.3, lesson 6).

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What Is a Species?

The species concept is itself a contested topic in evolutionary biology — at least 25 distinct species concepts have been proposed in the academic literature. At A-Level, the standard concept is the biological species concept (BSC):

Key Definition — Biological species concept: A species is a group of organisms that can interbreed in the wild to produce fertile offspring and that are reproductively isolated from other such groups.

The BSC was formalised in 1942 by the German-American biologist Ernst Mayr, whose framing emphasised reproductive isolation as the defining criterion (paraphrasing his framework rather than quoting verbatim). The intuition is that, however different two populations look, if they freely interbreed and produce fertile offspring in nature, they are members of a single gene pool — a single species.

Limitations of the Biological Species Concept

The BSC is the operationally useful definition for sexually reproducing animals, but it has well-recognised failure modes:

• It does not apply to asexual organisms. Bacteria, archaea, many fungi, asexually reproducing plants and animals do not interbreed; the "interbreed to produce fertile offspring" criterion is meaningless. For these, alternative concepts are needed (e.g. the phylogenetic species concept based on monophyletic lineages, or pragmatic operational concepts based on sequence similarity in microbiology).
• It cannot be tested for fossils. We cannot assess whether two extinct organisms could have interbred. Palaeontologists use the morphospecies concept — defining species by shape — which is necessarily approximate.
• Some "good" species hybridise occasionally. Lions × tigers (ligers), polar bears × grizzlies, horses × donkeys produce hybrids; in some cases (e.g. wolf × dog, lion × tiger) the hybrids are fertile. Strict application of the BSC would force a reclassification.
• Ring species. As discussed below, a chain of populations may interbreed pairwise along its length but not between its terminal forms. There is no clean point at which "one species" becomes "two".
• Cryptic species. Sometimes molecularly distinct species are morphologically indistinguishable (e.g. various Anopheles mosquito species). The BSC catches these correctly but only after molecular analysis.

Despite these limitations, the BSC remains the most widely used definition for sexually reproducing organisms and is the only definition required at AQA 7402 A-Level.

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How Speciation Occurs

Speciation has two essential ingredients:

1. Reproductive isolation — gene flow between two populations is reduced or eliminated, so they evolve independently.
2. Genetic divergence — the isolated populations accumulate different alleles (through mutation, divergent selection, and drift) until they can no longer interbreed productively, even if the barrier to gene flow is later removed.

The mode of speciation is classified by how the initial reproductive isolation arises — physically (allopatric), within the same geographic area (sympatric), or partially across overlapping ranges (parapatric).

flowchart TD
  A["Single ancestral species"] --> B{"Reproductive isolation type?"}
  B -- "Geographical barrier" --> C["Allopatric speciation"]
  B -- "Same area, no geographical barrier" --> D["Sympatric speciation"]
  B -- "Adjacent ranges with limited overlap" --> E["Parapatric speciation"]
  C --> F["Independent evolution: divergent selection + drift"]
  D --> F
  E --> F
  F --> G["Genetic and morphological divergence"]
  G --> H["Reproductive isolation becomes complete"]
  H --> I["Two distinct species"]

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Allopatric Speciation

Key Definition: Allopatric speciation ("different homelands") occurs when a population is physically divided by a geographical barrier — mountain range, river, ocean, glacier, road, motorway — preventing gene flow between the separated groups.

Allopatric speciation is the most common mode of speciation in animals. It requires only that the geographical barrier be substantial enough to prevent gene flow; the divergence then follows automatically from drift and divergent selection.

Steps of allopatric speciation

1. A population of one species occupies a continuous geographical range.
2. A geographical barrier arises (uplift of a mountain range, formation of a river, sea-level rise creating an island), or — equivalently — some individuals disperse across an existing barrier and found a daughter population.
3. Gene flow between the now-separate populations ceases (cross-link to lesson 3 — once Nm drops well below 1, the populations evolve independently).
4. Each subpopulation experiences potentially different environmental conditions and therefore different selection pressures (e.g. one side of a mountain range may be wetter than the other).
5. Different mutations arise in each population (the mutation pool is small and stochastic; both populations will accumulate unique mutations).
6. Natural selection and genetic drift act independently on each population.
7. Over many generations, the populations diverge genetically — allele frequencies shift in different directions.
8. Eventually, the populations become so genetically different that they can no longer interbreed productively, even if the barrier is later removed. They are now separate species.

Example — Darwin's finches (Geospiza and related genera)

The 14 (sometimes 15 or 17, depending on the authority) finch species of the Galápagos Islands and Cocos Island are the canonical example of allopatric speciation combined with adaptive radiation. The ancestral colonisers reached the Galápagos from the South American mainland a few million years ago. Each volcanic island offered different food sources, predators and habitats; populations on different islands were geographically isolated from each other.

Natural selection favoured different beak morphologies on different islands: heavy bills for cracking large hard seeds (Geospiza magnirostris), slender bills for probing cactus flowers (Geospiza scandens), curved bills for stripping bark in search of insects (Camarhynchus pallidus, the woodpecker finch). Beak-size genetics has been worked out in molecular detail (the ALX1 and HMGA2 loci, for example, are major contributors). Over evolutionary time, the populations diverged sufficiently in beak morphology, body size, and behavioural mate-choice cues that pre-zygotic isolation became complete. This is adaptive radiation — the rapid diversification of one ancestral species into many species, each adapted to a different ecological niche.

Example — Drosophila on Hawaiian volcanic islands

The Hawaiian islands are an even more extreme adaptive radiation: an estimated 800+ endemic Drosophila species, all descended from a single ancestral colonising lineage. New volcanic islands appearing through hotspot volcanism created repeated opportunities for founder events and allopatric divergence.

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Sympatric Speciation

Key Definition: Sympatric speciation ("same homeland") occurs when a new species evolves from a population without geographical separation. The populations occupy the same area but become reproductively isolated through other mechanisms.

Sympatric speciation is rarer and more contested than allopatric speciation, because it requires reproductive isolation to evolve in the face of ongoing gene flow — gene flow between would-be incipient species tends to homogenise them and prevent the divergence from completing. Nevertheless, several well-documented mechanisms are known.

Mechanism 1 — Polyploidy (the major mechanism, especially in plants)

Polyploidy is the condition of having more than two complete sets of chromosomes — for instance triploid (3n), tetraploid (4n), or hexaploid (6n). Polyploidy generates instant reproductive isolation because polyploid individuals cannot produce balanced gametes when crossed with diploid individuals; the resulting hybrids are sterile.

• Autopolyploidy: An error during meiosis or mitosis produces cells with extra chromosome sets from the same species. A tetraploid (4n) individual paired with a diploid (2n) produces triploid (3n) offspring which are typically sterile because chromosomes cannot pair properly in meiosis — instant reproductive isolation between the 2n and 4n forms.
• Allopolyploidy: Hybridisation between two different species produces a sterile diploid F1 hybrid. If the hybrid undergoes chromosome doubling (via meiotic non-disjunction or colchicine treatment in horticulture), it becomes a fertile allopolyploid that can reproduce with other allopolyploids but is reproductively isolated from both parent species.

Example — Bread wheat (Triticum aestivum). Modern bread wheat is an allohexaploid (6n = 42 chromosomes), AABBDD, derived from three ancestral diploid grasses. The lineage involved sequential hybridisation and chromosome-doubling events: an AA-genome wild grass crossed with a BB-genome wild grass produced sterile AB hybrids that doubled to fertile AABB tetraploid emmer wheat; AABB emmer then crossed with a DD-genome wild grass to produce sterile ABD triploids that doubled to fertile AABBDD hexaploid bread wheat. The whole lineage represents two successive sympatric speciation events through allopolyploidy.

Polyploidy is dramatically more common in plants than animals. Estimates suggest 30–70% of all flowering-plant species have a polyploid history; in animals the rate is lower (~ 5%) because polyploidy often disrupts sex-determination mechanisms.

Mechanism 2 — Temporal (seasonal) isolation

Two sub-populations within the same area can become reproductively isolated by shifting their reproductive timing. Two sympatric frog populations breeding in the same pond but in different months will not interbreed even though they occupy the same physical space.

Mechanism 3 — Behavioural isolation

Sub-populations may evolve different courtship displays, mating songs or pheromonal signals. Drosophila species are often discriminated by sex-specific cuticular hydrocarbons; bird species by song; fireflies by flash patterns. Mate-choice divergence can be remarkably rapid.

Mechanism 4 — Habitat specialisation

Two sub-populations within the same broad geographical area may specialise on different microhabitats — for example, Rhagoletis pomonella (apple maggot fly) populations specialising on native hawthorn versus introduced apple trees. Mating typically occurs on the host plant, so host specialisation generates habitat isolation that operates as a pre-zygotic barrier.

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Reproductive Isolation Mechanisms

Once speciation is underway, reproductive isolation is maintained by various barriers. These are classified as pre-zygotic (operating before a zygote can form) or post-zygotic (operating after fertilisation).

Pre-zygotic barriers

These barriers prevent fertilisation and therefore prevent the wasteful production of hybrid offspring:

Barrier	Description	Example
Geographical (habitat) isolation	Populations live in different areas or different microhabitats	Different islands; different host-plant specialisation
Temporal isolation	Populations breed at different times of day or year	Spring-breeding vs autumn-breeding frogs
Behavioural isolation	Differences in courtship displays, songs, or signals	Firefly flash patterns; Drosophila courtship dance
Mechanical isolation	Reproductive structures physically incompatible	Different flower-pollinator relationships; Drosophila genitalia
Gametic isolation	Gametes incompatible — sperm cannot fertilise the egg	Sea urchin species-specific bindin proteins on sperm

Post-zygotic barriers

These barriers operate after fertilisation has occurred:

Barrier	Description	Example
Hybrid inviability	Hybrid embryo fails to develop properly and dies	Many sheep × goat crosses
Hybrid sterility	Hybrid is viable but infertile (cannot produce gametes)	Mule (horse × donkey) — chromosome pairing fails in meiosis
Hybrid breakdown	F1 hybrids are fertile but F2 or backcross generations have reduced fitness	Some rice and cotton hybrid lineages

Pre-zygotic barriers are evolutionarily favoured because they avoid the energetic waste of producing inviable or sterile offspring. Once post-zygotic incompatibility evolves, reinforcement can drive the evolution of pre-zygotic barriers (selection against hybridising individuals favours those whose mate-choice cues exclude the other species).

Exam Tip: In extended-response questions on speciation, the standard structure to use is: (1) reproductive isolation arises (name the mechanism); (2) different selection pressures act in each population (or polyploidy creates instant isolation); (3) genetic differences accumulate; (4) reproductive isolation becomes complete; (5) name the resulting species pair and the type of speciation. Marks are awarded for each numbered point.

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Adaptive Radiation

Key Definition: Adaptive radiation is the rapid diversification of a single ancestral lineage into many descendant species, each adapted to a different ecological niche. It typically occurs when organisms colonise an environment with abundant unoccupied niches.

Conditions favouring adaptive radiation

• Arrival in a new environment with unoccupied ecological niches — colonisation of a new island chain; freshwater colonisation of a new river system.
• Mass extinction removes competitors, opening niches for surviving lineages. The post-Cretaceous (post-K/Pg) mammalian radiation occurred after the non-avian dinosaurs went extinct ~66 Ma.
• Evolution of a key innovation — a morphological or physiological change that opens access to new resources. The vertebrate jaw, the angiosperm flower, and the avian wing are classic examples.

Documented adaptive radiations

• Darwin's finches on the Galápagos — 14–17 species from one mainland coloniser, divergent beak morphologies.
• Cichlid fish in the African Great Lakes — Lake Victoria, Lake Malawi and Lake Tanganyika together host over 1,500 cichlid species, many endemic to single lakes. Lake Victoria's flock evolved in approximately 15,000 years — extraordinarily rapid. Diversification involves divergent jaw morphology, diet specialisation, habitat specialisation, and sexual selection on coloration.
• Hawaiian honeycreepers (Drepanidinae) — 50+ species from a single finch-like ancestral coloniser, with extreme beak diversification.
• Australian marsupials — radiation into many ecological niches in geographic isolation from placental mammals.
• Post-K/Pg mammalian radiation — explosive diversification of mammalian lineages (primates, ungulates, cetaceans, carnivorans) following dinosaur extinction.

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Ring Species

A ring species is a series of neighbouring populations arranged around a geographical barrier, where adjacent populations can interbreed but populations at the two ends of the ring cannot. Ring species provide a unique "snapshot" of speciation in progress.

Example — Larus gulls (the classic textbook case). A chain of gull populations forms a ring around the Arctic, beginning with herring gulls (Larus argentatus) in Britain, through various intermediate forms across North America, Asia and Europe, ending with lesser black-backed gulls (Larus fuscus) — also in Britain. Adjacent populations in the chain interbreed; the British herring gull and British lesser black-backed gull do not. Recent molecular work has complicated the simple textbook picture (the ring may have been broken historically and re-formed), but the conceptual point stands.