1.8.5Inquiry Process
Types of Speciation — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 30 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6Forms of Speciation
Speciation
- Speciation The formation of new species when populations become reproductively isolated and no longer exchange genes.
Allopatric speciation
- Allopatric speciation Populations separated by a geographic barrier diverge by mutation, selection and genetic drift.
Sympatric speciation
- Sympatric speciation New species arise in the same area, for example through polyploidy or by specialising on different resources.
Hybrid zones
- Hybrid zones Where diverging populations meet, hybrids may form and show how complete isolation has become.
Section 2 of 6Reproductive Isolation
Reproductive Isolation
Temporal
- Temporal Species that breed or flower at different times of year or day do not mate.
Behavioural
- Behavioural Differences in courtship signals mean that individuals do not recognise each other as mates.
Gametic
- Gametic Gametes of different species cannot fuse, because of incompatible surface proteins.
Hybrid sterility
- Hybrid sterility Zygotes form but the offspring are sterile, so alleles do not flow between species.
| Type | Barrier | Example |
|---|---|---|
| Allopatric | Geographic | Squirrels on opposite rims of the Grand Canyon |
| Sympatric | Genetic or behavioural | Polyploid plants |
Section 3 of 6Steps in Allopatric Speciation
1
A physical barrier such as a mountain range or a new sea separates one population into two.
2
There is no gene flow between the populations.
3
Mutations and different selection pressures cause allele frequencies to diverge.
4
Over many generations the populations become so different that they cannot interbreed.
5
If the barrier is removed they no longer exchange genes, so they have become separate species.
Section 4 of 6Isolation Before and After Fertilisation
Pre-zygotic barriers
- Pre-zygotic barriers Prevent mating or fertilisation, such as different habitats, breeding seasons or courtship songs.
Post-zygotic barriers
- Post-zygotic barriers Zygotes form but hybrids are weak or sterile, such as the mule.
Reinforcement
- Reinforcement Selection against costly hybrid matings strengthens pre-zygotic barriers where species overlap.
Genetic drift
- Genetic drift In small isolated populations, chance changes in allele frequency can speed up divergence.
| Barrier type | Stage | Example |
|---|---|---|
| Temporal | Before mating | Flowering in spring or autumn |
| Hybrid sterility | After fertilisation | Mule |
Section 5 of 6Worked Example
Question
- Two populations of mice are separated by a newly formed river.
- Over thousands of years they change, and when brought together they no longer mate.
- Name the type of speciation and outline the sequence.
- The river is a geographical barrier that stops gene flow between the populations.
- Each population experiences different mutations and different selection pressures.
- Allele frequencies diverge until the groups become reproductively isolated.
- They are now separate species, formed by allopatric speciation.
Answer This is allopatric speciation, driven by geographic isolation followed by divergence.
Section 6 of 6Exam Checklist & Tips
1.8.5 – Key Points for the Exam
- Speciation Formation of new species.
- Allopatric Occurring in different geographical areas.
- Sympatric Occurring in the same geographical area.
- Reproductive isolation A barrier preventing gene flow between groups.
Common Mistakes
- Thinking a geographic barrier alone creates a new species; divergence must follow.
- Saying sympatric speciation needs a physical barrier.
- Mixing up allopatric and sympatric.
- Forgetting that speciation takes many generations.
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