Changing allele frequencies
Introduced| English | Meaning |
|---|---|
| founder effect/ˈfaʊndə ɪˈfekt/ | Reduced genetic variety when a few individuals start a new isolated population. |
| genetic drift/dʒɪˈnetɪk drɪft/ | In a small population, allele frequencies change by chance each generation. |
| bottleneck effect/ˈbɒtəlnek ɪˈfekt/ | A sharp loss of genetic variety after a population crash. |
| Hardy–Weinberg principle/ˈhɑːdi ˈwaɪnbɜːɡ ˈprɪnsɪpl/ | The prediction that allele and genotype frequencies remain stable in an ideal population with random mating and no evolutionary forces. |
Evolution by the numbers
- 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.
Allele frequency change lab
See how a population can move from chance or selection to changed allele frequency.
A change in the allele frequencies of a population over generations is called ______.
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Selection and genetic drift change allele frequencies — that is evolution at the genetic level.
Founder, drift and bottleneck
- founder effect 奠基者效应 — a few individuals start a new population, so they carry only some of the original alleles.
- genetic drift 遗传漂变 — in a small population, allele frequencies change by chance each generation.
- bottleneck effect 瓶颈效应 — a sudden fall in population size leaves few survivors, reducing the variety of alleles.
The bottleneck effect reduces genetic variety
The founder effect happens when:
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The few founders carry only a sample of the original gene pool, so the new population differs.
A population bottleneck reduces genetic variety because:
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Few survivors carry only some alleles, so the recovered population has less variety.
Match each cause of changing allele frequency to its description.
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Selection is non-random (by fitness); drift, founder and bottleneck effects are random changes, strongest in small populations.
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.
Antibiotic resistance is an example of natural selection because:
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A chance mutation gives resistance; the antibiotic selects for it, so resistant bacteria increase.
Hardy–Weinberg
- The Hardy–Weinberg principle 哈迪-温伯格原理 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.
The Hardy–Weinberg principle only holds true when there is:
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Hardy–Weinberg assumes a large, randomly mating population with no mutation, migration or selection.
In a population the recessive phenotype (q²) makes up 0.04 of individuals. What is the recessive allele frequency, q?
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q² = 0.04, so q = √0.04 = 0.2. (Then p = 1 − q = 0.8.)
You've got it
- 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