Say which type of selection acts in each case…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Say which type of selection acts in each case.
- In a field of wheat, the very tall stems fall in the wind, and the very short stems are shaded. Plants of medium height leave the most seeds.
- In a dry year, only large and hard seeds are left on an island. Finches with larger beaks survive, and the mean beak size rises.
- On another island, there are only very small seeds and very large seeds. Finches with small beaks or with large beaks feed well, and finches with medium beaks starve.
- After selection, the mean of a character is the same, but the spread is smaller. Which type is it?
Need a reminder? The key ideaShow
Stabilising selection favours the mean and narrows the curve. Directional selection moves the curve to one side. Disruptive selection favours both ends. When is selected against with a coefficient , the new frequency is . For it is .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) Stabilising selection. The mean height is favoured and both extremes are removed.
Step 2. (b) Directional selection. The curve moves to the side of larger beaks.
Step 3. (c) Disruptive selection. Both ends of the curve are favoured, and the middle is not.
Step 4. (d) Stabilising selection. The mean does not move and the spread falls.
Answer: (a) stabilising; (b) directional; (c) disruptive; (d) stabilising.
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