4.9.1Additional lessons
Natural Selection and Evolution — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 132 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 5Evolution and Darwin's Argument
Natural selection is the main mechanism proposed for adaptive evolution.
1
Evolution is the cumulative change in the heritable characteristics of a population over successive generations.
2
Organisms of a species vary, and some of the variation is heritable because it is determined by alleles.
3
Populations produce more offspring than the environment can support, so individuals compete for limited resources.
4
Individuals with characteristics better suited to the environment are more likely to survive and reproduce.
5
They pass on advantageous alleles in greater numbers, so over generations the frequency of those alleles rises.
Part 2 of 5Sources of Variation
Mutation
- Mutation Random changes in DNA create new alleles, and are the original source of all genetic variation.
Crossing over
- Crossing over Exchange of segments between non-sister chromatids in meiosis I creates new combinations of alleles on chromosomes.
Random orientation
- Random orientation Homologous pairs line up independently at metaphase I, so gametes receive different mixtures of maternal and paternal chromosomes.
Random fertilisation
- Random fertilisation Any sperm may fuse with any egg, so offspring genotypes are very diverse.
| Source | Stage | Effect |
|---|---|---|
| Mutation | DNA replication | New alleles |
| Crossing over | Prophase I | New allele combinations |
| Random orientation | Metaphase I | New chromosome combinations |
| Random fertilisation | Fertilisation | New genotypes |
Part 3 of 5Example: Rabbit Fur
Example: Rabbit Fur
Variation
- Variation In a rabbit population one allele codes for brown fur and another for white fur.
Selection pressure
- Selection pressure Predators hunt in a snowless woodland, and white rabbits are easier to see against dark ground.
Differential survival
- Differential survival More brown rabbits survive to breed than white rabbits, producing more offspring.
Result
- Result The frequency of the brown allele rises across generations, and the white allele becomes less common.
Part 4 of 5Darwin and Lamarck
Lamarckism
- Lamarckism It claimed that characteristics acquired during life, such as stretched necks, were passed on to offspring.
Why rejected
- Why rejected Acquired characteristics do not change DNA in gametes, so they are not heritable.
Darwin's theory
- Darwin's theory Natural selection explained adaptation by differential survival of heritable variation, with strong supporting evidence.
Paradigm shift
- Paradigm shift Replacing Lamarck's ideas with natural selection changed the whole framework used by biologists,
- which is why it counts as a paradigm shift.
Part 5 of 5Exam Checklist & Tips
4.9.1 – Key Points for the Exam
- Evolution Cumulative change in the heritable characteristics of a population.
- Natural selection Differential survival and reproduction of individuals with heritable advantageous traits.
- Heritable Able to be passed to offspring through genes.
- Mutation A change in the base sequence of DNA.
- Paradigm shift A fundamental change in the accepted ideas of a scientific field.
Common Mistakes
- Writing that individuals evolve; populations evolve over generations.
- Implying that organisms change on purpose to survive.
- Saying well-adapted organisms are certain to survive; they are more likely to.
- Including acquired traits such as scars as examples of heritable variation.
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