In Chapter 7, Diamond lists a number of factors that account for this difference.
The first set of factors consists of visible factors that would have determined which plants were picked by hunter-gatherers and thereby had more of a chance to be domesticated. These factors included:
- Size of fruit or seeds. People will want to pick things that are big and are therefore worth taking the time to pick. Related to this is the amount of flesh on the fruit. You don't want to pick something that's big but is all seed.
- Taste. No one wants to pick bitter fruits and seeds.
- Oily seeds or long fibers. These are other things that you want in a plant because they can yield oil for cooking or fibers for cloth.
Other factors were not so obvious. These included:
- Dispersal mechanisms. If a plant disperses its seeds (like by having them blown by the wind) they can't get picked.
- Timing of seeds sprouting. If you have a plant that sprouts all at once, its sprouts could get killed off by one bad weather day. This makes it harder to domesticate.
- Fertilization process. Plants that can fertilize themselves are better than ones that can't.
All of these kinds of qualities impacted plants' ease of domestication.