Natural Selection
Introduced| English | Meaning |
|---|---|
| directional selection/daɪˈrekʃənl sɪˈlekʃn/ | directional selection |
| disruptive selection/dɪsˈrʌptɪv sɪˈlekʃn/ | disruptive selection |
| stabilizing selection/ˈsteɪbɪlaɪzɪŋ sɪˈlekʃn/ | stabilizing selection |
| fitness/ˈfɪtnəs/ | fitness |
Selection can push in different ways
- Natural selection does not always push a population the same direction.
- Sometimes it favours one extreme; sometimes the average; sometimes both ends.
- The pattern depends on which individuals leave the most offspring.
- Three named patterns describe how selection reshapes a trait.
Directional selection
- Directional selection 定向选择 favours one extreme of a trait.
- The whole population gradually shifts that way.
- The dark peppered moths spreading in a sooty city is a classic example.
- One end of the range is favoured; the population follows.
In biology, an organism's fitness measures its…
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Fitness is reproductive success — leaving more surviving offspring — not just physical strength.
Stabilizing selection
- Stabilizing selection 稳定选择 favours the average, middle form.
- Both extremes are selected against.
- Human birth weight is a good example — very small and very large babies fare worse.
- The population becomes more uniform around the middle.
Selection shifts a population
Watch the population change as the environment favours one form over another.
Directional selection shifts a population…
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Directional selection favours one extreme, shifting the whole population that way (like the dark moths).
Stabilizing selection favours…
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Stabilizing selection favours the middle and removes both extremes — like human birth weight.
Disruptive selection favours the average and works against both extremes.
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False. That is stabilizing selection. Disruptive selection favours both extremes and works against the average, which can split a population into two forms.
Disruptive selection and fitness
- Disruptive selection 分裂选择 favours both extremes and works against the average.
- It can split one population into two distinct forms.
- Which individuals win is measured by fitness 适合度 — reproductive success.
- Fitness is about leaving offspring, not simply being strong.
Select all true statements about selection patterns.
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Fitness is reproductive success, not just strength. The other three are correct.
Biological fitness does not mean being the strongest or fastest. It means leaving the most surviving offspring. A small, dull animal that reproduces successfully has higher fitness than a magnificent one that never breeds.
Birth weight and stabilizing selection:
- Very small babies historically had lower survival.
- Very large babies were harder to deliver safely.
- So selection favoured the middle weight — a clear case of stabilizing selection.
Natural selection can act in three patterns: directional (shifts toward one extreme), stabilizing (favours the average), and disruptive (favours both extremes). Which individuals succeed is measured by fitness — reproductive success, not just strength.
Write from stabilizing selection
- Babies of medium birth weight survive more often than very small or very large babies.
- The mean weight stays near the middle.
What mode of selection is this?
Stabilizing selection favours intermediate phenotypes.
What selection favours medium birth weight?
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The middle of the distribution is favoured.