4.10.2Additional lessons
Selection Pressures — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 85 of 101
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 5Ingredients of Natural Selection
Natural selection explains how populations become better suited to their surroundings.
Variation
- Variation Individuals in a population differ in features, partly because of mutation and the shuffling of alleles in sexual reproduction.
Overproduction
- Overproduction Populations produce more offspring than the environment can support, so many do not survive to breed.
Struggle
- Struggle Limited resources and hazards create competition, so individuals with suited traits are more likely to survive.
Inheritance
- Inheritance Survivors pass beneficial alleles to the next generation, which changes allele frequencies over time.
| Selection pressure | Example of effect |
|---|---|
| Predation | Camouflaged prey survive more often |
| Disease | Resistant individuals live to reproduce |
| Competition | Better competitors gain more food or mates |
| Climate | Individuals tolerant of heat or cold survive extremes |
| Resource shortage | Efficient feeders leave more offspring |
| Toxic chemicals | Tolerant individuals survive and breed |
Part 2 of 5Natural Selection in Sequence
1
A change in the environment, such as a new predator or antibiotic, creates a new selection pressure.
2
Individuals with certain heritable traits have a higher chance of surviving the pressure than others.
3
These survivors reproduce more successfully and pass on the alleles that determine the helpful trait.
4
After many generations the proportion of the helpful trait in the population increases, and the population has evolved.
Part 3 of 5Types of Pressure
Types of Pressure
Abiotic
- Abiotic Temperature, water availability, salinity and pH act as pressures that favour tolerant genotypes.
Biotic
- Biotic Predators, parasites, pathogens and competitors act on populations, often producing evolutionary arms races.
Sexual selection
- Sexual selection Mate choice favours showy or competitive traits even if they slightly reduce survival.
Human activity
- Human activity Pesticides, antibiotics and hunting create strong pressures and drive rapid change.
Part 4 of 5Worked Example
Question
- A beetle population has 300 green and 200 brown beetles.
- Birds eat 20% of green beetles and 60% of brown beetles on a green background.
- Calculate the percentage of green beetles among the survivors.
- Green survivors = 300 × 0.80 = 240.
- Brown survivors = 200 × 0.40 = 80.
- Total survivors = 240 + 80 = 320, so green = 240 ÷ 320 × 100 = 75%.
- Green was 300 ÷ 500 = 60% before predation, so predation raised its share.
Answer 75% green (up from 60%), showing predation as a selection pressure.
Part 5 of 5Exam Checklist & Tips
4.10.2 – Key Points for the Exam
- Selection pressure An environmental factor that affects the survival or reproduction of individuals.
- Fitness The ability of an organism to survive and leave fertile offspring.
- Adaptation A heritable feature that increases the chance of survival and reproduction.
- Evolution Change in the heritable characteristics of a population over generations.
- Variation Differences in traits between individuals of one species.
Common Mistakes
- Writing that individuals evolve; populations evolve over generations.
- Saying that organisms change on purpose to meet a pressure, rather than pre-existing variation being selected.
- Treating acquired characteristics, such as larger muscles from training, as inherited.
- Describing fitness as strength rather than reproductive success.
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