7.3.4Genetics, Populations, Evolution and Ecosystems (A Level only)

Types of Speciation — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 12 of 25 1 quiz

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7.3.4 Types of Speciation — AQA A Level Biology 7402 notes: Explain how reproductive isolation leads to allopatric and sympatric speciation.Speciation is the formation of a new species from an existing one.A species is a group of organisms that can interbreed to produce fertile o…

Allopatric speciation
geographical barrier
reproductive isolating mechanisms
same geographical area
Ecological or habitat isolation
Temporal isolation

Section 1 of 9What is speciation?

Speciation is the formation of a new species from an existing one.

A species is a group of organisms that can interbreed to produce fertile offspring.

Reproductive isolation

Speciation therefore requires reproductive isolation - the populations can no longer exchange genes - followed by divergence of their gene pools by natural selection and genetic drift until they are no longer able to interbreed successfully, even if brought back together.

Isolation reduces or stops gene flow. Over many generations each isolated population accumulates different mutations and experiences different selection, so allele frequencies diverge.

Section 2 of 9Allopatric speciation

Allopatric speciation occurs when populations are separated by a geographical barrier (a river, mountain range, sea or land bridge formed by tectonic or climatic change). The steps are:

Allopatric speciation

An original population is split into two or more populations by a geographical barrier.

There is no gene flow between them.

Different selection pressures (climate, food, predators) operate in each habitat, and random mutations arise independently.

Allele frequencies change in each population by natural selection and genetic drift, so the gene pools diverge.

Reproductive isolating mechanisms

Eventually the populations cannot interbreed to produce fertile offspring: they have become separate species. This often depends on reproductive isolating mechanisms such as changed courtship behaviour or incompatible gametes.

Example

Darwin's finches on the Galapagos Islands, where birds colonised different islands and evolved different beaks according to the food available.
Another example is the speciation of fish in separate lakes.

Section 3 of 9Sympatric speciation

Sympatric speciation occurs in the same geographical area. Reproductive isolation arises without a physical barrier, through:

1Ecological or habitat isolation

Ecological or habitat isolation populations specialise on different food or niches (e.g. different host plants in one area).

2Temporal isolation

Temporal isolation different breeding or flowering seasons.

3Behavioural isolation

Behavioural isolation changes in courtship signals or mating calls so individuals only recognise their own type.

4Mechanical isolation

Mechanical isolation differences in reproductive structures.

5Gametic isolation

Gametic isolation gametes cannot fuse.

6Polyploidy

Polyploidy in plants - a failure of meiosis gives gametes with the diploid number; fusion of two such gametes produces a tetraploid that cannot breed with diploid parents, because triploid offspring are sterile (no homologous pairing in meiosis).

Many crop plants, such as wheat, evolved this way.

Section 4 of 9Comparison

Feature	Allopatric	Sympatric
Location	Different geographical areas	Same area
Cause of isolation	Geographical barrier	Ecological, behavioural, temporal or genetic (e.g. polyploidy)
Speed	Usually slow	Can be rapid, esp. polyploidy
Example	Island finches	Tetraploid plants

In both cases, new species arise only after divergence of the gene pool. The evidence for evolution includes fossil records, molecular comparisons (DNA and protein sequences) and the geographical distribution of related species.

Section 5 of 9Link to evolution and classification

The more recently two species shared a common ancestor, the more similar their DNA and proteins. Speciation is therefore the process that produces the branching pattern of the phylogenetic tree. Disruptive selection (7.3.3) acting in a single population can start sympatric speciation, while genetic drift and the founder effect are particularly important in small isolated populations on islands.

In exam questions, make sure you name the type of isolation, state that gene flow stops, describe different selection pressures or mutations, and end by stating that the populations can no longer interbreed to produce fertile offspring.

Section 6 of 9Evidence and examples

Evidence that speciation has occurred comes from several sources.

Evidence and examples

Hybrid sterility, as in the mule (a horse x donkey cross), shows that two species cannot produce fertile offspring even though they can mate.

Molecular evidence, such as DNA base sequences and amino acid sequences in proteins like cytochrome c, shows that recently diverged species have fewer differences.

Biogeography shows that island groups contain clusters of related species derived from a colonising ancestor, and the fossil record documents intermediate forms.

Ring species, where neighbouring populations can interbreed but populations at the ends of the ring cannot, show speciation as a gradual process. Exam answers should show an understanding that the boundary between one species and the next is crossed gradually, which is why classification by interbreeding has limits - it cannot be applied to extinct species or to organisms that reproduce asexually.

Section 7 of 9Exam-style questions

Explain how allopatric speciation could occur in a population of rodents separated by the formation of a new river. [5]5 marks
Suggest why polyploidy can cause sympatric speciation in plants. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Speciation	Formation of a new species from an existing species.
Reproductive isolation	Barrier preventing two populations from interbreeding to produce fertile offspring.
Allopatric speciation	Speciation following geographical isolation.
Sympatric speciation	Speciation within the same geographical area.
Gene flow	Movement of alleles between populations.
Polyploidy	Having more than two complete sets of chromosomes.
Niche	The role of a species in its habitat including resources used.

Section 9 of 9Exam technique

Saying that species form just because populations are separated; they must also diverge genetically so that they cannot interbreed to produce fertile offspring.

Confusing allopatric (different places) with sympatric (same place).

Omitting 'no gene flow' or 'fertile' from the answer.

Stating that individuals evolve to suit the new habitat rather than allele frequencies changing through selection.

Forgetting that sympatric speciation still needs some form of reproductive isolation.

Last updated: 18 October 2026