Speciation: Allopatric and Sympatric

What Is Speciation?

Speciation is the process by which one species evolves into two or more distinct species. It occurs when populations of the same species become reproductively isolated — they can no longer interbreed to produce fertile offspring. Over time, different selection pressures, genetic drift, and mutation in the separated populations lead to the accumulation of genetic differences until they become separate species.

A species can be defined (using the biological species concept) as a group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other such groups.

Reproductive Isolation

Reproductive isolation prevents gene flow between populations, allowing them to diverge genetically. Isolating mechanisms are classified as:

Pre-Zygotic Mechanisms (Prevent Fertilisation)

Mechanism	Description	Example
Geographic (spatial)	Populations are separated by a physical barrier	Mountain range, river, ocean separating populations
Temporal	Populations breed at different times	Two plant species flower in different months
Behavioural	Differences in mating behaviour prevent interbreeding	Different birdsong, courtship displays, or pheromones
Mechanical	Reproductive organs are physically incompatible	Differently shaped genitalia in insect species
Gametic	Gametes are chemically incompatible and cannot fuse	Sperm cannot penetrate egg of another species

Post-Zygotic Mechanisms (After Fertilisation)

Mechanism	Description	Example
Hybrid inviability	Hybrid embryo fails to develop	Sheep-goat hybrid embryos do not survive
Hybrid sterility	Hybrid offspring are sterile	Mules (horse × donkey) are sterile because chromosomes cannot pair properly in meiosis
Hybrid breakdown	F1 hybrids are fertile but F2 or later generations have reduced fitness	Some plant hybrids show decreased vigour in subsequent generations

Allopatric Speciation

Allopatric speciation ("other homeland") occurs when populations are separated by a geographical barrier that prevents gene flow.

The Process

1. Geographic isolation — a physical barrier divides a population into two or more groups. Barriers include mountain ranges, rivers, oceans, deserts, glaciers, or simply large distances. Events that can create barriers include continental drift, volcanic eruptions, rising sea levels, or colonisation of islands.

2. Different selection pressures — the separated populations experience different environmental conditions (climate, food sources, predators, disease). Natural selection favours different alleles in each population.

3. Genetic drift — in smaller populations especially, random changes in allele frequency cause the gene pools to diverge further. The founder effect and bottleneck effect can accelerate this.

4. Mutation — new mutations arise independently in each population, adding unique alleles to each gene pool.

5. Reproductive isolation — over many generations, the accumulated genetic differences mean that even if the barrier were removed, the populations could no longer interbreed successfully. They have become separate species.

Examples of Allopatric Speciation

Darwin's finches (Galápagos Islands):

• A single ancestral finch species colonised the Galápagos archipelago from mainland South America
• Populations on different islands experienced different food sources (seeds, insects, cacti)
• Natural selection favoured different beak shapes and sizes on each island, optimised for the available food
• Over time, populations diverged into at least 13 recognised species, each with distinct beak morphology
• When populations occasionally come back into contact, behavioural differences (e.g. song) maintain reproductive isolation

Squirrels of the Grand Canyon:

• The Kaibab squirrel (north rim) and Abert's squirrel (south rim) are thought to have diverged after the Grand Canyon formed a barrier
• They show differences in coat colour and tail markings
• The canyon prevents gene flow between the populations

Sympatric Speciation

Sympatric speciation ("same homeland") occurs when populations diverge into separate species without geographic separation — they live in the same area but become reproductively isolated by other mechanisms.

Mechanisms of Sympatric Speciation

Polyploidy (most common in plants):

Polyploidy is the condition of having more than two complete sets of chromosomes. It can cause instant speciation:

• Autopolyploidy — a failure of meiosis or mitosis produces an individual with multiple sets of chromosomes from the same species (e.g. 4n instead of 2n). A tetraploid (4n) individual cannot produce fertile offspring with diploid (2n) individuals because meiosis cannot pair chromosomes correctly. However, it CAN breed with other tetraploids, creating a new species in a single generation.

• Allopolyploidy — hybridisation between two different species produces a sterile hybrid (chromosomes cannot pair in meiosis). If the hybrid undergoes chromosome doubling (polyploidy), it becomes fertile — each chromosome now has a homologue to pair with during meiosis. The resulting allopolyploid is a new species, reproductively isolated from both parent species.

Example: Bread wheat (Triticum aestivum) is a hexaploid (6n = 42 chromosomes) that arose through two rounds of allopolyploidy involving three different wild grass species over thousands of years.

Ecological/habitat isolation:

• Within the same geographic area, populations may adapt to different microhabitats or ecological niches
• Example: apple maggot fly (Rhagoletis pomonella) — originally parasitised hawthorn fruits; when apples were introduced to North America, some flies began using apples as hosts. Apple-feeding and hawthorn-feeding populations mate at different times (because the fruits ripen at different times), creating temporal isolation. They are diverging genetically and may eventually become separate species.

Behavioural isolation:

• Changes in mating preference (e.g. through sexual selection) can reproductively isolate subpopulations
• Example: cichlid fishes in African lakes — females preferentially mate with males of specific colour patterns, driving rapid speciation within a single lake. Experiments showed that when colour differences were masked by red lighting, females no longer distinguished between males of different "species"

Comparing Allopatric and Sympatric Speciation

Feature	Allopatric	Sympatric
Geographic barrier?	Yes — essential	No — populations share the same area
Most common type overall	Yes — most animal speciation	Less common in animals; common in plants
Key mechanism of isolation	Physical separation prevents gene flow	Polyploidy, ecological, temporal, or behavioural isolation
Speed	Usually gradual (thousands to millions of years)	Can be very rapid (especially polyploidy — one generation)
Examples	Darwin's finches, Grand Canyon squirrels	Bread wheat (polyploidy), apple maggot fly, cichlids

Adaptive Radiation

Adaptive radiation is the rapid diversification of a single ancestral species into many new species, each adapted to a different ecological niche. It often follows colonisation of a new environment with many available niches (e.g. islands, lakes) or a mass extinction that opens up niches.

Examples:

• Darwin's finches on the Galápagos
• Cichlid fishes in the Great Lakes of Africa (~500 species in Lake Victoria alone, evolving in <15,000 years)
• Hawaiian honeycreepers — ~50 species from a single finch-like ancestor
• Marsupials in Australia — diversified to fill niches occupied by placental mammals on other continents

Exam Tips

• AQA commonly asks you to describe the stages of allopatric speciation — use the sequence: geographic isolation → different selection pressures → genetic divergence → reproductive isolation → new species
• Always mention that speciation requires reproductive isolation — without it, gene flow would prevent divergence
• For sympatric speciation, polyploidy is the most commonly examined mechanism — be able to explain both autopolyploidy and allopolyploidy
• Know the difference between pre-zygotic and post-zygotic isolating mechanisms and give named examples
• When discussing adaptive radiation, link it to the availability of unoccupied niches