1.6.7Inquiry Process
Biological Species Concept: Challenges — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 21 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.
Part 1 of 5Where the Concept Struggles
The concept is useful but not universal, because it depends on tests that cannot always be made.
Asexual organisms
- Asexual organisms Bacteria and many protists reproduce without mating, so the test of interbreeding cannot be applied.
Fossils
- Fossils Extinct organisms cannot be tested for interbreeding, so palaeontologists must rely on shape and structure.
Fertile hybrids
- Fertile hybrids Some plants and animals that look distinct produce fertile hybrids, challenging a strict definition.
Gene transfer in bacteria
- Gene transfer in bacteria Plasmids and other DNA can pass between very different bacteria, so boundaries between groups are fuzzy.
Geographic separation
- Geographic separation Populations that never meet cannot be tested, so it is unknown whether they could interbreed.
Part 2 of 5Ring Species
Ring Species
What it is
- What it is A chain of neighbouring populations around a barrier, where adjacent groups interbreed but the two ends cannot.
Example
- Example Greenish warblers around the Tibetan plateau vary gradually, and the two northern ends do not interbreed.
Problem
- Problem Whether the chain forms one species or two depends on which populations are compared.
Evolutionary meaning
- Evolutionary meaning Ring species show speciation caught part-way through, with gene flow declining step by step.
Part 3 of 5Alternative Species Concepts
1
The morphological concept groups organisms by shared physical features, which works for fossils.
2
The ecological concept defines species by the niche occupied.
3
The phylogenetic concept uses a unique shared ancestry and DNA sequences to define a species.
4
DNA barcoding supports these by comparing short standard gene sequences.
5
Most biologists use several lines of evidence together rather than one concept.
| Concept | Basis | Strength |
|---|---|---|
| Biological | Interbreeding | Clear for sexual species |
| Morphological | Appearance | Works for fossils |
| Phylogenetic | Shared ancestry | Works for all organisms |
Part 4 of 5Worked Example
Question
- Greenish warbler populations form a ring around a barrier, and neighbours interbreed, but the two ends of the ring overlap without interbreeding.
- Discuss whether the ring is one species or two.
- Neighbouring populations interbreed, so under the concept each adjacent pair is one species.
- The end populations meet but cannot interbreed, so these two are separate species.
- Both statements cannot be true if species are fixed, so the case does not fit the definition cleanly.
- This shows that species are not always sharply defined and that speciation is a gradual process.
Answer The ring is neither clearly one species nor two, which is a limitation of the biological species concept and shows speciation in progress.
Part 5 of 5Exam Checklist & Tips
1.6.7 – Key Points for the Exam
- Ring species A series of linked populations whose end members do not interbreed.
- Hybrid Offspring of two different species or varieties.
- Horizontal gene transfer Movement of genes between organisms other than by reproduction.
- Morphological species A species defined by its form.
Common Mistakes
- Stating that the biological species concept works for every organism.
- Thinking that hybrids always show two organisms are the same species.
- Forgetting that fossils cannot be tested for interbreeding.
- Describing ring species as having no gene flow at all.
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