CAIE A Level Biology 9700 · Selection and evolution
A population of a plant species was studied over 20 generations. During this period, the environment became increasingly drier. The table shows the mean seed mass and standard deviation of seed mass in the population before and after 20 generations of selection.
| Trait: Seed mass (mg) | Before selection | After 20 generations |
|---|---|---|
| Mean | 4.2 | 5.8 |
| Standard deviation | 1.0 | 0.6 |
Describe how directional selection has changed the seed mass distribution in this population.
4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.
Describe means: Say what happens or what something is like, in order and with detail. You are not being asked why. If there is data, quote it: name the trend, then give figures with units at the points where it changes.
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