Larry Moran has a very good article on the genetic diversity found in the DNA of rice varieties. I won’t try to summarize what he has to say, because he says it so well. But I will emphasize something that he doesn’t. The biologists who looked at rice wanted to know what caused the pattern of diversity they see across rice vareties. One possibility is selection. The other is the founder effect, where a very small subset of a larger population gives rise to a new species (or variety) so that the new species contains less diversity than the ancestor population from which it sprang. In rice, neither explanation on its own is sufficient.
An agricultural scientist might not care one bit, or might simply assume that it was all due to selection by farmers. But the truth is that it needed a combination of the founder effect and positive selection to create modern rice diversity. The founder effect relates to the fact that Oryza sativa indica and Oryza sativa japonica were both domesticated independently from the wild Oryza rufipogon, maybe a few times. The results of those early events set up the foundation genes for rice. Then selection was brought to bear and created the diversity we now see. Luck and hard work, every time.