Gene pool
p. 790A gene pool is all the genes and alleles in an interbreeding population. Allele frequency is the proportion of all copies of a gene that are one allele. Evolution is a change in allele frequencies over generations.
Guiding questions
- What can change allele frequencies in a population?
- What part does reproduction play in natural selection?
Slides
Notes
Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 790–797 of the course textbook. Image credits are on slide 13.
Gene pools and selection
Separated populations
pp. 791–792Separated populations differ in mutations, selection pressures and chance events. In humans, lactase persistence is common in northern Europe but rare in East Asia, while an ALDH2 variant, which causes alcohol flush, is common in East Asia.
Selection and allele frequency
pp. 793–794Individuals with favourable heritable traits survive and reproduce more, so they pass on more copies of their alleles and those alleles rise in frequency.
Three patterns of selection
pp. 794–795
Stabilizing selection favours intermediate phenotypes, directional favours one extreme and disruptive favours both extremes.
Hardy-Weinberg and artificial selection
Hardy-Weinberg equation
pp. 796–797p + q = 1 and p2 + 2pq + q2 = 1. Start from the homozygous recessive frequency: q is its square root, then p = 1 − q, and carriers are 2pq.
Conditions for equilibrium
p. 797Random mating, no selection, no mutation, no migration and a very large population. If observed frequencies do not fit, at least one condition is not being met.
Artificial selection
p. 797People choose which individuals breed, to increase wanted traits in crops and domestic animals. Antibiotic resistance is an unintended result of human action, but it is natural selection.
Videos
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Note-taking sheet
Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
Quick self-check
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Practice questions
A printable worksheet covering all seven statements, from quick recall to data questions and a longer answer. It has 40 marks and takes about 40 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Practical
Hardy-Weinberg with beads
You will need: Two colours of beads, a bag, calculator
- Draw pairs of beads at random, returning them each time, to represent genotypes
- Record 50 genotypes and calculate p and q
- Compare the observed genotype numbers with p squared, 2pq and q squared
- Remove one colour selectively, and observe how frequencies change
Safety: Keep beads off the floor to avoid slips.
Carry this out with your teacher's supervision and follow your school's lab rules.
Nature of science and TOK
Nature of science
Models
The Hardy-Weinberg principle describes a population that does not really exist, yet it is useful for detecting when evolution is occurring.
Theory of knowledge
How can a model known to be unrealistic be useful?
Its assumptions are almost never met in nature. Consider how departures from an ideal can themselves be informative.
Syllabus statements
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
- D4.1.9What a gene pool isp. 790
- D4.1.10Allele frequencies in populations that are geographically separatedp. 791
- D4.1.11How selection based on inherited differences changes allele frequencies in a gene poolp. 793
- D4.1.12How directional, disruptive and stabilizing selection differp. 794
- D4.1.13Using the Hardy-Weinberg equation to calculate allele and genotype frequenciesp. 796
- D4.1.14The conditions a population must meet to stay in Hardy-Weinberg equilibriump. 797
- D4.1.15Artificial selection: people choosing which traits to breed forp. 797
More of D4.1 in this course
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