4.9.3Additional lessons
Selection Pressures: Skills — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 134 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.
Section 1 of 5The Guppy System
Endler's work in the 1970s tested these ideas using both a laboratory experiment and a field study.
Species
- Species Guppies live in streams of Trinidad and Tobago and show strong variation in colour and spot pattern.
Sexual dimorphism
- Sexual dimorphism Males are brightly spotted, but females are mostly dull.
Camouflage
- Camouflage Spot patterns can match the gravel of the stream bed and so reduce the chance of being seen by predators.
Hypothesis
- Hypothesis Predation selects for dull, camouflaged males, but female choice selects for bright, conspicuous ones.
Section 2 of 5Laboratory Experiment
1
Guppies bred freely for several months without predators, building up variation in spot number and size.
2
Spots were counted and measured, and guppies were allocated randomly to ponds in a greenhouse.
3
Half the ponds had coarse gravel and half fine gravel, to test camouflage against different backgrounds.
4
Ponds were set up with no predators, a weak predator such as the killifish, or a dangerous predator such as the pike cichlid.
5
After several months the spots were counted and measured again, and the populations were compared.
Section 3 of 5Findings
Findings
High predation
- High predation The mean number of spots decreased, so conspicuous males were removed.
Low or no predation
- Low or no predation The number of spots continued to rise, as bright males were favoured by females.
Gravel size
- Gravel size In ponds with predators, spot size tended to match the gravel, so coarse gravel favoured larger spots.
Field transfer
- Field transfer Dull males moved from a high-predation stream to a low-predation one became more colourful after about fifteen generations.
Section 4 of 5Evaluation
Trade-off
- Trade-off Natural selection from predators and sexual selection by females pull in opposite directions.
Strengths
- Strengths Random allocation, replicates and a field test supported the conclusions.
Limitations
- Limitations Ponds are simpler than streams, and only some of the factors were varied.
Exam skill
- Exam skill Describe a trend with data, state the selection pressure responsible, and explain the effect on allele frequency.
Section 5 of 5Exam Checklist & Tips
4.9.3 – Key Points for the Exam
- Camouflage Colour or pattern that makes an organism hard to see against the background.
- Conspicuous Easily seen.
- Trade-off A balance between opposing selective forces.
- Predation Killing and eating of one organism by another.
- Control group A group not given the treatment, such as ponds with no predator.
Common Mistakes
- Concluding that predators change individual guppies; they change survival, not the colour of any one fish.
- Ignoring the no-predator ponds when explaining the effect of female choice.
- Stating that bright males always have an advantage in all streams.
- Describing the data without saying which selection pressure explains them.
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