You can answer this question by reading in Chapter 7. There, Diamond lists a number of characteristics that would cause people to naturally selected some individual plants over others. Those individuals would then pass their genes on and later generations would show those characteristics more strongly.
Among the characteristics that Diamond says would be selected are:
- Size. Bigger berries and seeds and such are preferable to smaller ones.
- Taste. Of course, people would select sweet fruits over bitter ones.
- Fleshiness. People would rather pick a fruit with a lot of meat and only a little bit of seed.
- Oiliness in seeds. Oily seeds have more nutritional value than less oily ones.
- Length of fibers. It is easier to make cloth out of long fibers than short ones.