There's marker assisted breeding, QTL analysis, etc., but the hard limit is generation time. If you want to introgress a particular trait into a useful backgro

There's marker assisted breeding, QTL analysis, etc., but the hard limit is generation time. If you want to introgress a particular trait into a useful background, it's going to take ~5 generations to do it adequately. That generally means 5 years, or sometimes you can squeeze in a winter nursery in the southern hemisphere. Proper QTL analysis requires making suitable inbreds (several generations of selfing), creation of the RILs (another several generations of selfing after the making the F2s), and finally a couple years for gathering trait data. Oh, and this all requires generation of proper markers and so forth. Starting from scratch, this is a decade-long process if everything goes well. As for diversity, you collect wild accessions and those are generally maintained by a seed co-op, often something associated with a university.