An ag researcher who did our course two years ago explained how they created new varieties from hybrid corn:
They plant a field with as many types of hybrids as they can.
From those they select plants with desirable qualities ( early, late, tall , short, etc..)
They plant those separately.
They then select for those same traits.
She said the establishment of a new variety is surpiringly rapid- if I am not mistaken, already by the third generation...But then this is corn, which mixes so readily..
I also heard of a case where a new bean was introduced to the Philippines and then forgotten about, and some years later (20?) a curious researcher looked and found 60 new varieties that thie farmers had selected from this bean...