Biology · Species, Speciation & Macroevolution
Allopatric, sympatric and other routes to divergence
A river can divide a land animal's breeding range while barely interrupting the pollen exchange of a wind-pollinated plant. Geographic distance is not itself a universal reproductive barrier. Follow the organism's route of inherited exchange before deciding what a map implies.
Evolutionary Processes & Adaptation supplies allele frequencies, selection, drift, gene flow and the distinction between observation and explanation. Heredity, Variation & Evolution introduces branching ancestry. Meiosis & Patterns of Inheritance helps with the chromosome-set model; Development, Differentiation & Stem Cells helps with constraints and historical context. The examples require proportions, weighted averages and reading simple trees; each rule is explained where it is used. Species definitions for sexual organisms are introduced with their limits rather than assumed to classify every form of life.
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Distinguish geographic settings from causal mechanisms, then show how migration and selection can act in different directions at one locus.
Geographic terms describe settings
Allopatric divergence involves geographically separated populations. Separation can follow a barrier dividing a range, called vicariance, or colonisation of a new location followed by limited exchange. Selection, mutation and drift can then change populations along different trajectories. Isolation creates conditions for divergence; it does not guarantee that every separated population becomes a species.
Sympatric divergence occurs without an initial geographic separation of the relevant population. Ecological differences, mating associations or chromosome changes can limit exchange within overlapping space. The task is to explain how those differences arise and persist despite opportunities for inherited exchange.
Parapatric settings involve adjacent populations with contact across part of a range. Divergent selection and restricted dispersal may maintain differences across that contact. Natural histories can change among these settings through time, so the terms organise comparisons rather than prescribe three complete mechanisms.
The three maps show separated, adjacent and overlapping ranges. Their arrows mark possible reproductive exchange, not a measured amount. A physical obstacle may restrict exchange for one organism and allow it for another; overlapping points do not imply random mating.
Geographic settings describe opportunities
Read the diagram in words
The first map puts three P locations left of a barrier at x=5 and three Q locations right of it, with no exchange edge. The second puts adjacent ranges across a contact zone x=4 to 6, with one possible exchange edge. The third overlaps ranges and includes one possible exchange edge. Bounds are 0–10 by 0–6 in arbitrary map units; no dates or amount of gene flow are assigned.
Separated ranges
| Group | Map coordinates |
|---|---|
| Group P | (1, 2); (2, 4); (3, 2) |
| Group Q | (7, 2); (8, 4); (9, 2) |
Barrier limits reproductive exchange: x = 5
Adjacent ranges with a contact zone
| Group | Map coordinates |
|---|---|
| Group P | (1, 2); (3, 4); (4.5, 2) |
| Group Q | (5.5, 3); (7, 4); (9, 2) |
Contact zone: x = 4–6
| From location | To location | Relationship |
|---|---|---|
| (4.5, 2) | (5.5, 3) | Possible reproductive exchange |
Overlapping ranges
| Group | Map coordinates |
|---|---|
| Group P | (2, 2); (4, 4); (7, 2) |
| Group Q | (2, 3); (5, 4); (7, 3) |
| From location | To location | Relationship |
|---|---|---|
| (2, 2) | (2, 3) | Possible reproductive exchange |
All panels use the same arbitrary map bounds: x = 0–10; y = 0–6; Present overlap does not establish the origin
Three conceptual geographic settings; coordinates are arbitrary map units and colours/patterns distinguish groups, not dated historical observations. Barrier effects depend on the organism's dispersal and inherited contribution. Present-day overlap alone does not establish a sympatric origin.
Present overlap does not reveal the starting geography
Two groups now found together could have diverged in isolation and later met. That is secondary contact. To support a sympatric origin, investigate lineage relationships, past distributions, timing, ecology and alternatives involving earlier separation. A current map alone cannot recover that history.
Research on Howea palms of Lord Howe Island combined phylogenetic, ecological, flowering-time and genetic evidence to support a sympatric-divergence interpretation. The paper discussed uncertainty about the particular links between substrate adaptation, flowering differences and inheritance. A supported geographic history need not settle every molecular step.
Likewise, an isolated present-day population might have received gene flow earlier. Geographic labels need a time interval and a relevant spatial scale. They should not replace the evidence for reproductive barriers or for the processes changing allele frequencies.
Exchange and divergence can operate together
Consider two haploid gene pools of equal size. A starts at frequency 0.80 in P and 0.20 in Q. Replace 10% of each pool with contributions from the other pre-migration pool. P becomes 0.90×0.80+0.10×0.20 = 0.74. Q becomes 0.90×0.20+0.10×0.80 = 0.26. The frequency difference falls from 0.60 to 0.48.
Now assign opposing selection: A contributes twice as much as a in P, while a contributes twice as much as A in Q. In P, A's weighted share is 2×0.74/(2×0.74+0.26) = 74/87, about 0.851. In Q, it is 0.26/(0.26+2×0.74) = 13/87, about 0.149. The difference rises to 61/87, about 0.701.
Migration reduced the difference in this step, while selection increased it. The result demonstrates that positive exchange does not logically require immediate homogenisation. It does not prove stable long-term divergence, reproductive isolation or species status: the model covers one locus, one step and assigned fitness values. Other loci and later conditions can behave differently.
Migration reduces a difference; selection can increase it
Read the diagram in words
Initially P has A frequency 0.80 and Q 0.20, a difference of 0.60. Replacing 10% from the other initial pool gives 0.74 and 0.26, a difference of 0.48. Weighting A:a by 2:1 in P and 1:2 in Q, then renormalising, gives 74/87≈0.851 and 13/87≈0.149, a difference of 61/87≈0.701. This deterministic one-locus step is not an equilibrium calculation.
| Assigned step | Pool P | Pool Q | Between-pool frequency difference |
|---|---|---|---|
| Before migration | 0.8 | 0.2 | 0.6 |
| After 10% symmetric migration | 0.74 | 0.26 | 0.48 |
| After opposing selection | ≈ 0.8506 | ≈ 0.1494 | ≈ 0.7011 |
| Pool | Allele | Assigned selection weight |
|---|---|---|
| Pool P | A | 2 |
| Pool P | a | 1 |
| Pool Q | A | 1 |
| Pool Q | a | 2 |
A frequency: 0–1; One step at one locus; no species verdict
Two infinite deterministic haploid gene pools at one two-allele locus. Equal pool sizes; each migration step replaces fraction 0.10 with contributions from the other pre-migration pool simultaneously. Then selection weights A/a by 2/1 in P and 1/2 in Q. No drift, mutation, mating or whole-genome isolation is represented; one step is not an equilibrium or a speciation test.
Worked example
Keep the stages and their starting values in order
Both migrations use the initial frequencies simultaneously. Using Q's already updated 0.26 to calculate P would change the model. After migration, the sum of the two equally sized pool frequencies remains 1.00, consistent with symmetric exchange at this step.
Selection then changes reproductive contributions within each local pool. It does not simply add 0.10 or double a frequency without renormalising. For P the total weight is 1.74, so its A share is 1.48/1.74. The denominator includes both alternatives, which keeps the answer between zero and one.
Check your understanding
If migration occurs but the two variants have equal contribution within each pool, what are the ending frequencies for this step?
Show reasoning
They remain 0.74 in P and 0.26 in Q after migration. Equal weights do not change the relative shares; the difference is 0.48.
Why do two species living on the same island today not automatically establish sympatric speciation?
Show reasoning
They might have diverged elsewhere or in separated parts of the range and later met. Current overlap is an observation; the origin requires historical, genetic and ecological evidence and checks of alternatives.
Does the model's final difference of about 0.701 demonstrate that the pools are separate species?
Show reasoning
No. It is a conditional result at one locus after one migration-selection step. Species delimitation requires broader lineage evidence, and reproductive barriers or equilibrium were not included.
What to carry forward
Use maps to describe opportunities and history; test how exchange, selection and other processes affect the actual lineages.
References
Resources supporting this lesson’s explanations and examples.
- Biology 2e — formation of species, reconnection and ratesMary Ann Clark, Matthew Douglas and Jung ChoiOpen textbook — selected sections18.2 Formation of New Species18.3 Reconnection and Speciation Rates
- Sympatric speciation in palms on an oceanic islandVincent Savolainen and colleaguesOriginal research — selected article text · PDF
- Species concepts and species delimitationKevin de QueirozResearch perspective — selected original article text