1.5.3Additional lessons
Convergent Evolution — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 6 of 142
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 4Convergent Evolution Explained
Definition
- Definition Unrelated lineages independently evolve similar traits because they face similar environmental challenges and selection pressures.
Analogous structures
- Analogous structures Features with a similar function and appearance but different evolutionary origin and different underlying anatomy.
Homologous contrast
- Homologous contrast Homologous structures share ancestry but may differ in function, while analogous structures share function but not ancestry.
Driver
- Driver Natural selection favours the same solution to the same problem, such as moving efficiently through water or surviving drought.
Genetic basis
- Genetic basis The similar traits arise from different alleles and developmental pathways, so the species stay genetically distinct.
Part 2 of 4Classic Examples
Classic Examples
Dolphins and sharks
- Dolphins and sharks A mammal and a fish both have a streamlined body, dorsal fin and flippers that reduce drag when swimming fast.
Cacti and euphorbias
- Cacti and euphorbias Cacti of the Americas and euphorbias of Africa both evolved fleshy water-storing stems and spines, despite belonging to different plant families.
Wings
- Wings Insect, bird and bat wings all allow flight but have different structures, so they are analogous rather than homologous.
Eyes
- Eyes Vertebrate and cephalopod eyes both form focused images with a lens, but they develop differently.
Divergent contrast
- Divergent contrast Divergent evolution is the opposite pattern,
- where related species from one ancestor become different as they adapt to separate habitats.
| Pair | Selection pressure | Shared trait |
|---|---|---|
| Dolphin and shark | Fast swimming in water | Streamlined body and fins |
| Cactus and euphorbia | Dry habitats | Water-storing stem and spines |
| Bat and insect | Flying | Wings |
Part 3 of 4Recognising Convergence
1
Identify two species with a similar trait and a similar function, such as fins in an aquatic mammal and a fish.
2
Compare internal anatomy and development to check whether the structures are built from the same underlying parts.
3
Use DNA sequence data or the fossil record to find whether the species share a recent common ancestor with that trait.
4
If the common ancestor lacked the trait, the similarity evolved independently, which indicates convergent evolution.
5
Link the trait to the shared environment to explain which selection pressure favoured it.
Part 4 of 4Exam Checklist & Tips
1.5.3 – Key Points for the Exam
- Convergent evolution Independent evolution of similar traits in unrelated species under similar selection pressures.
- Analogous structure A structure with similar function in different species that does not come from a common ancestor.
- Selection pressure An environmental factor that affects survival or reproduction and so favours certain heritable traits.
- Streamlined Having a smooth shape that reduces resistance when moving through water or air.
Common Mistakes
- Calling dolphin and shark fins homologous;
- they are analogous because their ancestors were not finned in this way.
- Saying unrelated species "copy" each other; each lineage evolved the trait separately by natural selection.
- Assuming similar appearance means close relationship, when DNA evidence can show distant relatives.
- Forgetting to state the shared environmental pressure when explaining a convergent trait.
- Mixing up convergent evolution with divergent evolution, which starts from a shared ancestor.
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