Natural Selection
Introduced| English |
|---|
| directional selection/daɪˈrekʃənl sɪˈlekʃn/ |
| disruptive selection/dɪsˈrʌptɪv sɪˈlekʃn/ |
| stabilizing selection/ˈsteɪbɪlaɪzɪŋ sɪˈlekʃn/ |
| fitness/ˈfɪtnəs/ |
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…
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…
Directional selection favours one extreme, shifting the whole population that way (like the dark moths).
Stabilizing selection favours…
Stabilizing selection favours the middle and removes both extremes — like human birth weight.
Disruptive selection favours both extremes and works against the average.
Disruptive selection favours both extremes at once, 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.
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.