4.9.2Additional lessons
Selection Pressures — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 133 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 5Abiotic Selection Pressures
A selection pressure is any factor that affects survival or reproduction and so changes allele frequencies over generations.
Temperature
- Temperature Extreme heat or cold affects survival, so individuals with suitable adaptations are favoured.
Rainfall and water
- Rainfall and water Drought favours plants and animals that conserve water, while flooding favours tolerant species.
Salinity and pH
- Salinity and pH Different tolerances of salts or acidity affect which individuals live in a habitat.
Density independence
- Density independence These factors affect survival regardless of how many individuals live in the area.
Section 2 of 5Biotic Selection Pressures
Biotic Selection Pressures
Predation
- Predation Predators remove individuals that are easier to catch, so camouflage, speed and warning signals are favoured.
Disease
- Disease Pathogens kill susceptible hosts, which favours alleles that give resistance.
Competition
- Competition Individuals compete for food, space, light or mates, and better competitors leave more offspring.
Intraspecific competition
- Intraspecific competition Members of the same species share a niche, so they compete directly and variation determines who succeeds.
Section 3 of 5Sexual Selection
The peacock's tail is a classic example, since it attracts mates though it makes the bird more likely to be caught by predators.
1
Variation exists in male traits that females can see, such as plumage or courtship display.
2
Females prefer males with certain traits, which may signal good health and fitness.
3
Preferred males mate more often and have more offspring.
4
Alleles for the preferred traits become more common over generations.
5
The population may then show sexual dimorphism, with differences in size or appearance between the sexes.
Section 4 of 5Natural and Sexual Selection Compared
Competition for
- Competition for Natural selection involves competition for resources and survival, while sexual selection involves competition for mates.
Outcome
- Outcome Natural selection tends to produce good adaptation to the environment, while sexual selection may produce traits that reduce survival.
Reproductive isolation
- Reproductive isolation If courtship traits diverge, groups may stop mating with each other, which can help a species split into two.
Speed
- Speed Variation in mating success can be large, so sexual selection can cause rapid evolutionary change.
| Feature | Natural selection | Sexual selection |
|---|---|---|
| Selected for | Survival | Mating success |
| Example | Camouflage | Peacock tail |
| Cost to survival | Usually improves it | May reduce it |
Section 5 of 5Exam Checklist & Tips
4.9.2 – Key Points for the Exam
- Selection pressure A factor that influences survival or reproduction of individuals.
- Abiotic factor A non-living part of the environment, such as temperature.
- Sexual selection Selection caused by the preference of one sex for traits in the other.
- Sexual dimorphism Difference in size or appearance between males and females of a species.
- Reproductive isolation Barrier that prevents groups of a species from interbreeding.
Common Mistakes
- Writing that sexual selection always makes the species better adapted; it can reduce survival.
- Listing disease as an abiotic factor; it is biotic.
- Saying that females choose a trait on purpose to improve evolution.
- Forgetting to link the pressure to differences in survival or reproduction.
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