AHL: Speciation
IB Biology Higher LevelΒ· D4.2 Speciation (AHL)Β· 45 min read
1. Speciation and Reproductive Isolationβ β ββββ± 10 min
Speciation
The evolutionary process by which new, distinct species form from an existing ancestral population, completed when reproductive isolation develops between new and parent populations.
Example:
Divergence of Darwin's finches into 15 distinct species from one common ancestor.
For speciation to occur, gene flow between two populations of the same species must stop, leading to genetic divergence. The end point of speciation is permanent reproductive isolation: the populations can no longer interbreed to produce fertile offspring.
Prezygotic isolation: Barriers that prevent fertilization (e.g. different mating seasons, incompatible pollinator preferences)
Postzygotic isolation: Barriers that act after fertilization (e.g. hybrid inviability, hybrid sterility like in mules)
Two populations of fruit flies live in the same forest. One mates only in the morning, the other mates only in the evening. What type of reproductive barrier is this, and is it prezygotic or postzygotic?
- 1
Recall that prezygotic barriers prevent fertilization, while postzygotic barriers act after fertilization.
- 2
Different mating times mean the two populations do not encounter each other to mate, so fertilization can never occur.
- 3
This is temporal isolation, a prezygotic reproductive barrier.
2. Allopatric Speciationβ β ββββ± 12 min
Allopatric Speciation
Speciation that occurs when a geographic barrier splits a population, blocking gene flow between subpopulations.
Example:
Divergence of marine snail populations separated by the Isthmus of Panama.
Geographic barriers can include mountain ranges, rivers, canyons, or distance between isolated islands. Over generations, different selection pressures and genetic drift cause the two populations to diverge genetically. If the barrier is removed later, the populations are too different to interbreed, resulting in new species.
A population of lizards is split by a hurricane that creates a wide new river channel through their habitat. Outline how this can lead to allopatric speciation.
- 1
- The river acts as a geographic barrier that prevents lizards from moving between the two sides, stopping all gene flow.
- 2
- Each population experiences different selection pressures (e.g. different food sources, predators) on each side of the river.
- 3
- Random mutations accumulate independently in each population, leading to gradual genetic divergence over thousands of generations.
- 4
- Eventually, the populations are so genetically different that even if the river dries, they cannot interbreed to produce fertile offspring, forming two new species.
3. Sympatric Speciation and Polyploidyβ β β ββHL onlyβ± 15 min
Sympatric Speciation
Speciation that occurs within the same geographic range, with no physical barrier blocking gene flow between populations.
Polyploidy
A condition where an organism has more than two complete sets of chromosomes, caused by nondisjunction during cell division.
Example:
Common bread wheat is a hexaploid (6 sets of chromosomes) formed by polyploid speciation.
Sympatric speciation is very common in plants, almost always driven by polyploidy. Polyploidy causes instant reproductive isolation because a polyploid individual cannot produce fertile offspring when mating with a diploid parent, so a new species can form in a single generation.
Explain how a tetraploid plant can form a new species in one generation from a diploid parent species.
- 1
- Nondisjunction during meiosis produces diploid (2n) gametes instead of haploid (n) gametes.
- 2
- Two diploid gametes fuse to form a tetraploid (4n) zygote, which develops into a mature tetraploid plant.
- 3
- The tetraploid can self-pollinate to produce more tetraploid offspring, forming a new population.
- 4
- If a tetraploid (4n) mates with a diploid (2n) parent, the resulting offspring is triploid (3n), which is sterile because chromosomes cannot pair during meiosis.
- 5
- The tetraploid population is reproductively isolated from the parent, so it is a new species.
4. Models of Speciation Rateβ β β βββ± 10 min
Fossil evidence shows two distinct patterns of speciation rate over geological time, leading to two accepted models.
Both models are consistent with evolution by natural selection; they only describe the typical rate of speciation.
Gradualism
Speciation occurs slowly and steadily, with small genetic changes accumulating gradually over millions of years.
+ Pros: Matches slow divergence of lineages in many fossil groups
β Cons: Does not explain long periods of no change followed by sudden appearance of new species
Punctuated Equilibrium
Speciation occurs rapidly (over tens of thousands of years) during periods of environmental change, followed by long periods of stasis (no change).
+ Pros: Matches the common fossil pattern of sudden new species appearance
β Cons: Often misinterpreted as opposing Darwinian evolution
After a volcanic eruption creates a new empty island, a single bird species colonizes it and 12 new species evolve in less than 1 million years. Does this match gradualism or punctuated equilibrium?
- 1
- Gradualism describes slow steady speciation over millions of years, while punctuated equilibrium describes rapid speciation after change or colonization.
- 2
- The empty island has many unoccupied ecological niches, leading to rapid divergent selection and rapid speciation (adaptive radiation).
- 3
- This pattern matches the punctuated equilibrium model.
5. Common Pitfalls
Wrong move:
Stating geographic isolation alone causes speciation
Why:
Geographic isolation only stops gene flow; speciation requires genetic divergence leading to reproductive isolation
Correct move:
Always mention that after geographic isolation, selection and drift cause genetic divergence that leads to reproductive isolation
Wrong move:
Confusing prezygotic and postzygotic isolation
Why:
Students often mix up the timing of the barrier, leading to lost classification marks
Correct move:
Remember pre = before fertilization, post = after fertilization. Prevents mating vs reduces hybrid fitness
Wrong move:
Claiming all speciation is allopatric
Why:
Sympatric speciation via polyploidy is a required AHL topic, and very common in plants
Correct move:
Recognize sympatric speciation occurs without geographic separation, most often via polyploidy in plants
Wrong move:
Stating punctuated equilibrium contradicts natural selection
Why:
Punctuated equilibrium describes the rate of speciation, not the mechanism
Correct move:
Both models rely on natural selection as the mechanism of divergence; only the rate differs
Wrong move:
Claiming polyploidy can never occur in animals
Why:
While extremely rare in animals, it is not impossible, and it always causes instant speciation when it does occur
Correct move:
When discussing polyploidy, note it is the most common driver of sympatric speciation in plants
6. Quick Reference Cheatsheet
Characteristic | Allopatric Speciation | Sympatric Speciation |
|---|---|---|
Geographic barrier required | Yes | No |
Main driver of isolation | Geographic separation stops gene flow | Polyploidy/ecological separation |
Most common in | All multicellular organisms | Flowering plants |
Time to form new species | Thousands to millions of years | Can form in one generation |
Example | Darwin's Galapagos finches | Hexaploid bread wheat |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· 1
Compare speciation modes
- 2024 Β· 2
Explain polyploidy speciation
- 2023 Β· 1
Identify allopatric speciation example
Going deeper
- syllabusIB Biology 2025 D4.2 SpeciationAligns with AHL learning outcomes
What's Next
Speciation is the core process connecting microevolution (changes in allele frequency within populations) and macroevolution (large-scale evolutionary patterns across geological time). Understanding how new species form explains the origin of Earth's biodiversity, linking common ancestry to the diversity of life we see today. This topic builds on your understanding of natural selection and the species concept, and prepares you to study large-scale evolutionary patterns, extinction, and modern classification. IB exam questions frequently combine speciation with evidence for evolution and cladistics, so connecting these concepts is key for exam success.