Changing allele frequencies
Changing allele frequencies
- Natural selection isn't the only thing that changes a gene pool.
- Chance events can shift allele frequencies too.
- And we can calculate them with Hardy–Weinberg.
Founder, drift and bottleneck
- founder effect — a few individuals start a new population, so they carry only some of the original alleles.
- genetic drift — in a small population, allele frequencies change by chance each generation.
- bottleneck effect — a sudden fall in population size leaves few survivors, reducing the variety of alleles.
Practice
The founder effect happens when:
The few founders carry only a sample of the original gene pool, so the new population differs.
Practice
A population bottleneck reduces genetic variety because:
Few survivors carry only some alleles, so the recovered population has less variety.
Antibiotic resistance
- A chance mutation makes a few bacteria resistant to an antibiotic.
- The antibiotic kills the non-resistant ones; the resistant ones survive and reproduce.
- So resistance spreads — natural selection in action.
Practice
Antibiotic resistance is an example of natural selection because:
A chance mutation gives resistance; the antibiotic selects for it, so resistant bacteria increase.
Hardy–Weinberg
- The Hardy–Weinberg principle lets you calculate allele and genotype frequencies in a population.
- It only holds when: a large population, random mating, and no mutation, migration or natural selection.
Practice
The Hardy–Weinberg principle only holds true when there is:
Hardy–Weinberg assumes a large, randomly mating population with no mutation, migration or selection.
You've got it
Key idea
- founder effect (few start a new population), genetic drift (chance in small populations), bottleneck (disaster cuts variety)
- antibiotic resistance spreads by natural selection (mutation → survivors reproduce)
- Hardy–Weinberg calculates allele/genotype frequencies — needs a large population, random mating, no mutation/migration/selection