Chapter 7 outlines the beginnings of plant domestication, which heavy emphasis on genetic selection. It begins with the hunter-gatherers expending great energy to go out and forage. Diamond's theory is that these people would only eat the best plants available and deposit the seeds in their latrines. The plants that made it back to the latrines (and spitoons and garbage) were the first to be selected for in the proto-gardens. Selection while foraging and in these early gardens focused on visible factors like size, taste and fleshiness (from bitter to sweet was probably a mutation).
Invisible selection also occurred involving eliminating seed dispersal, promoting immediate germination and being able the self-pollinate (to prevent dilution of selecting favors). These were likely selected for because it made them easier to harvest, although it wasn't a conscious decision (but it did help during the border between hunter-gatherers and farmers). When humans started to provide artifical environments for cultivation, best growth in these environments was selected for (tilled soil vs. dry land).
Domestication of cereals was first in the Fertile Crest, allowing farmers to adopt and sedentary lifestyle and giving them more patience to domesticate more complex species (nuts and then fruit-bearing trees). All developments of food production eventually led to a stable supply of cereals and pulses for a balanced diet of carbohydrates and proteins.
Some plans defied domestication. Oaks take over a decade to reach to bear acorns and the selection by squirrels severely impedes artificial selection by humans. Also, bitterness is controlled by several genes (instead of one), making it hard for breeders to cultivate sweet varieties.
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