4.10.4Additional lessons
Gene Pools — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 87 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 5Gene Pool Concepts
A population evolves when the allele frequencies in its gene pool change.
Gene pool
- Gene pool All the alleles of all the genes carried by the members of an interbreeding population at one time.
Allele frequency
- Allele frequency The proportion of one allele among all copies of that gene in the population, written as a number from 0 to 1.
Evolution
- Evolution In genetic terms, evolution is a change in allele frequencies in the gene pool over generations.
Population
- Population A group of individuals of one species living in the same area who can breed together.
| Process | Effect on gene pool |
|---|---|
| Mutation | Creates new alleles |
| Natural selection | Raises frequency of advantageous alleles |
| Gene flow | Adds or removes alleles by migration |
| Genetic drift | Random change, strongest in small populations |
| Natural selection and drift together | Drift can override weak selection in small populations |
Part 2 of 5Small Populations and Chance
1
A large population has many individuals, so chance events have little influence on allele frequencies.
2
In a small population, a few individuals may fail to reproduce by chance, taking their rare alleles out of the gene pool.
3
Over generations random changes accumulate and one allele may become fixed while another disappears.
4
The population loses genetic diversity, which can reduce its ability to adapt to a new selection pressure.
Part 3 of 5Special Cases of Drift
Special Cases of Drift
Bottleneck effect
- Bottleneck effect A sudden severe reduction in population size leaves survivors with only a sample of the original alleles.
Founder effect
- Founder effect A small group that colonises a new area carries only some alleles, so the new population differs from the source.
Gene flow
- Gene flow Movement of individuals or gametes between populations makes their allele frequencies more alike.
Isolation
- Isolation Barriers that stop gene flow allow populations to diverge, which may eventually lead to speciation.
Part 4 of 5Worked Example
Question
- A population has 10 diploid individuals with allele A at frequency 0.50.
- By chance only four individuals breed, with genotypes AA, Aa, aa and aa.
- Calculate the frequency of A among the alleles of these breeders.
- Total alleles = 4 × 2 = 8.
- A alleles = 2 (AA) + 1 (Aa) + 0 + 0 = 3.
- Frequency = 3 ÷ 8 = 0.375.
Answer
- 0.375;
- the fall from 0.50 shows genetic drift acting by chance in a small group.
Part 5 of 5Exam Checklist & Tips
4.10.4 – Key Points for the Exam
- Gene pool The total set of alleles in an interbreeding population.
- Allele frequency The proportion of a particular allele among all alleles of that gene.
- Genetic drift Random change in allele frequencies, important in small populations.
- Bottleneck Sharp reduction in population size that reduces diversity.
- Founder effect Reduced diversity in a population started by a few individuals.
Common Mistakes
- Describing allele frequency as the number of individuals with a trait rather than the proportion of alleles.
- Saying that genetic drift is caused by selection; it is random.
- Thinking drift matters equally in all population sizes.
- Forgetting that gene flow can add alleles as well as remove them.
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