AP Syllabus focus:
‘Sympatric speciation occurs with geographic overlap, whereas allopatric speciation involves geographically isolated populations.’
Speciation can occur either when populations are physically separated or while they still share the same geographic area.
Process overview of sympatric speciation. The figure depicts divergence within a shared geographic region, where subgroups become reproductively isolated even though they remain in contact. Conceptually, it highlights why sympatric speciation requires strong isolating mechanisms (e.g., habitat preference, assortative mating, or timing differences) to counteract ongoing gene flow. Source
The key distinction is how gene flow is reduced enough for populations to diverge genetically and reproductively.
Ring species as a gene-flow-and-divergence model. The schematic shows a chain of neighboring populations exchanging genes locally, while the terminal populations become reproductively isolated when they meet again after spreading around a geographic barrier. It’s a helpful visual reminder that speciation evidence can reflect complex histories of partial gene flow, not just a simple “complete isolation vs. complete overlap” binary. Source
Core idea: two geographic contexts for speciation
Allopatric speciation: formation of new species after populations become geographically isolated, preventing (or greatly reducing) gene flow.
Allopatric speciation is associated with a physical barrier (or dispersal to different places) that separates populations into different environments or isolates them by distance.
Sympatric speciation: formation of new species without geographic separation, within the same area, when gene flow is reduced by biological factors.
In sympatry, populations overlap in location, so reproductive separation must arise from mechanisms like ecological differences, timing differences, or mate choice.
Allopatric speciation (geographic isolation)
How geographic isolation reduces gene flow
Barrier formation: mountains, rivers, glaciers, newly formed islands, habitat fragmentation.
Dispersal events: a small group colonises a new area; the original population remains elsewhere.
Distance isolation: individuals are too far apart to interbreed effectively (even without a clear barrier).
Divergence after separation
Once isolated, populations can diverge because:
Different selection pressures act in different environments, favouring different alleles.
Genetic drift can shift allele frequencies, especially if the isolated population is small.
Mutations arise independently in each population, increasing genetic differences over time.
What indicates allopatric speciation
Closely related species are found in different geographic regions.
The timing of separation often aligns with a geological or climatic event.
If brought back into contact, populations may show reduced interbreeding due to accumulated differences (e.g., mismatched signals, incompatible gametes).
Sympatric speciation (geographic overlap)
How gene flow is reduced in the same area
Because individuals can potentially encounter one another, sympatric speciation requires non-geographic isolation to limit mating between subgroups. Common pathways include:
Ecological (habitat) differentiation: subpopulations use different microhabitats or resources in the same region, reducing encounters and favouring different adaptations.
Temporal isolation: subgroups reproduce at different times (daily, seasonal, or multi-year shifts).
Behavioural isolation: differences in courtship signals or mate preferences cause assortative mating (individuals mate with similar individuals).
Chromosomal changes: especially in plants, changes in chromosome number can instantly reduce successful mating between groups.
Autopolyploidy as a mechanism of sympatric speciation. The diagram shows how a chromosome-duplication event can generate a polyploid (e.g., 4n) lineage from a diploid (2n) ancestor, creating immediate reproductive isolation because gametes no longer match in chromosome number. This is a classic example of sympatric speciation driven by chromosomal incompatibility rather than physical separation. Source
Why divergence can be rapid or difficult
With ongoing overlap, any mating between groups restores gene flow, so reproductive separation must be strong.
Strong selection tied to resource use or mating preference can maintain separation even with overlap.
Small initial differences can become self-reinforcing if individuals preferentially mate within their subgroup and produce offspring better suited to that niche.
What indicates sympatric speciation
Closely related species occur in the same geographic area with limited interbreeding.
Reproductive separation correlates with resource use, mating signals, or breeding time, rather than physical barriers.
Genetic data may show divergence despite shared location, consistent with restricted gene flow within the area.
Distinguishing sympatric vs allopatric in evidence
Focus first on geography: do the diverging groups occupy different regions (allopatry) or overlap (sympatry)?
Then identify the likely gene flow barrier:
Allopatry: external physical separation is primary.
Sympatry: internal biological mechanisms reduce mating despite contact.
Be cautious: current overlap does not guarantee sympatric origin; populations can diverge allopatrically and later expand ranges to overlap.
FAQ
Yes, if polyploidy occurs (e.g., $2n \rightarrow 4n$). Polyploid individuals may be fertile with each other but have reduced fertility when mating with the original $2n$ population, creating rapid reproductive separation.
Look for divergence tied to within-area factors (resource use, mate choice, breeding time) plus genetic signatures consistent with continuous overlap, rather than a historical period of geographic separation.
If individuals prefer mates with similar traits, mating becomes non-random. This reduces gene flow between trait groups, allowing selection and drift to drive divergence even in the same location.
No. They can overlap geographically but use different microhabitats (depth zones, host plants, soil types). Fine-scale habitat choice can strongly reduce encounters and mating.
Physical separation can reduce gene flow immediately and strongly, so divergence can proceed without being “diluted” by interbreeding. Sympatric speciation typically needs stronger or more complex isolating forces to persist.
Practice Questions
State the difference between sympatric and allopatric speciation. (2 marks)
Allopatric speciation involves geographic isolation reducing gene flow (1)
Sympatric speciation occurs with geographic overlap and gene flow is reduced by non-geographic mechanisms (1)
A fish population lives in one lake. Over time, two groups begin breeding at different times of year and rarely interbreed, despite living in the same lake. Explain which type of speciation is most consistent with this pattern and why. (5 marks)
Identifies sympatric speciation (1)
States populations remain in the same geographic area / no physical barrier (1)
Explains temporal isolation (different breeding times) reduces interbreeding (1)
Links reduced interbreeding to reduced gene flow between groups (1)
States reduced gene flow allows genetic divergence to accumulate, potentially leading to separate species (1)
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