Preemptive domestication refers to how easily different crops developed genetically among separate regions of the globe.
Jared Diamond explores this concept by examining how many times crops were domesticated independently in different regions. If crops were domesticated more than once in different regions, it shows that the spread of that crop was extremely slow.
When a crop was domesticated only once it means that particular crop's spread was much more rapid. According to Diamond this theory makes sense because a society transitioning from hunter-gatherer to food-producer wouldn't dismiss a perfectly good crop to try to find another.
In the crops that spread more rapidly, those of preempted domestication, more crops could be obtained by finding similar species of the same crop. By examining the types of crops that were of the preemptive domestication category, Diamond concludes that most crops spread rapidly out of southwest Asia or the Fertile Crescent.
Find out more at Jareddiamond.org.