You can find the answer to this question by reading in Chapter 7. There, Diamond describes how some plants come to be tastier and bigger and to generally have more of the attributes that human beings prefer.
Diamond says that wild plants that were tastier would be the ones that people would try harder to pick. People would learn which fruits were tasty and would pick them and eat them or would bring them home and eat them later. The seeds of the fruits that were eaten would then be excreted and might sprout near latrines. Alternatively some of the fruits might spoil and be discarded or might be dropped. Again, their seeds would sprout. This would happen nearer to human settlements. People would then gather these fruits with good characteristics and the cycle would start again. With each cycle, the fruit would become more and more desirable. Diamond describes this process happening with such plants as strawberries, peas, and apples.