Natural selection
Introduced| English | Meaning |
|---|---|
| adapted/əˈdæptɪd/ | Having characteristics suited to survival and reproduction in a particular environment. |
| peppered moth/ˈpepəd mɒθ/ | peppered moth |
| stabilising/ˈsteɪbəlaɪzɪŋ/ | Favours the average, removes the extremes (population stays the same). |
| directional/daɪˈrekʃənl/ | Favours one extreme, so the mean shifts that way. |
| disruptive/dɪsˈrʌptɪv/ | Favours both extremes, removes the average. |
| antibiotic resistance/ˌæntɪbaɪˈɒtɪk rɪˈzɪstəns/ | antibiotic resistance |
Survival of the fittest
- A population makes far more offspring than can survive.
- So they compete — and the best adapted 适应 win.
- Over generations, the helpful alleles spread.
Natural selection acts on the variation that already exists in a population.
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It cannot create variation; it selects from what is already there (mutation creates the new variation).
The process
- More offspring are born than can survive, so they compete for food and space — the "struggle for existence".
- The best-adapted individuals are most likely to survive, reproduce, and pass on their alleles.
- Over many generations, the helpful alleles become more common.
- The peppered moth 桦尺蛾 shows this: the form that is best camouflaged on the bark is eaten less, so it becomes more common.
Natural selection
Step through Darwin's idea: variation + a selection pressure means the best-adapted survive and pass on their alleles.
In natural selection, which individuals are most likely to pass on their alleles?
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The best-adapted individuals survive, reproduce, and pass on their helpful alleles, which spread over generations.
The peppered moth shows natural selection because:
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Birds eat the moth that stands out; the better-camouflaged form survives and increases in frequency.
Put natural selection in order.
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Variation → pressure → differential survival → reproduction → the allele frequency changes.
Three types of selection
- stabilising 稳定选择 — favours the average, removes the extremes (population stays the same).
- directional 定向 — favours one extreme, so the mean shifts that way.
- disruptive 分裂 — favours both extremes, removes the average.
Directional selection:
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Directional selection favours one extreme, shifting the population mean in that direction.
Match each type of selection to what it does to the distribution.
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Directional shifts the mean, stabilising narrows it, disruptive splits it — all by selecting from existing variation.
Disruptive selection:
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Disruptive selection favours both extremes, splitting the distribution into two peaks.
Selection we can watch
- Antibiotic resistance 抗生素抗性 is natural selection happening fast enough to observe.
Worked example. Antibiotics kill non-resistant bacteria; the few resistant ones survive and multiply. Within days a resistant population dominates — selection in real time.
The peppered moth: birds eat the form that stands out, so camouflage is selected for.
You've got it
- overproduction → competition → the best-adapted survive and reproduce → helpful alleles spread
- the peppered moth: better camouflage = eaten less = more common
- stabilising (favours average), directional (shifts mean), disruptive (favours both extremes)