There are strains that are considered semi-autoflowering and will flower without a 12/12 light schedule. These are not strains that have been bred with ruderalis. Some of the Middle Eastern landrace strains are semi-autoflowering.
There is also a Siberian strain that contains levels of THC much higher than a standard ruderalis. Although it's likely that the Siberian is a hybrid with ruderalis genetics in it's lineage.
"This Siberian accession is a resinous, day-neutral (autoflowering) intermediate type direct from Chelyabinsk, close to Kazakhstan."
Syrian:
Characteristics: Early, hardy, Indica and Sativa types, aromatic, semi-autoflowering, CBD
Morrocan Ketama:
Characteristics: Resinous, short, very fast flowering, semi-autoflowering
Egyptian Sinai:
Characteristics: Early maturing, semi-autoflowering, hardy, unique terpenes, Indica- and Sativa-type morphology
Lebanese:
Characteristics: Early maturing, compact, resinous, columnar and spherical variants, semi-autoflowering, high CBD
I've grown Ketama and some of the plants started flowering under 18/6 while still in veg. Ruderalis is not the only cannabis that can autoflower. It's just what's used by breeders making autoflower seeds. The semi-autoflower strains may or may not exhibit early flowering which is likely why ruderalis is so widely used. But it should be possible through selection to breed a semi-autoflowering strain that flowers early consistently. I do know that some of these strains have been used by some to produce early finishing strains that can be harvested in northern areas with short growing seasons.
I currently have seeds from all of the strains I listed above. I have future plans to reproduce all of those seeds to have enough stock to go through and search for plants that exhibit the autoflowering tendency in order to use them in crosses instead of using ruderalis. One of the reasons I don't care for auto's is because of the ruderalis genetics. I don't grow auto's often but the ones I have grown have been lacking compared to the Photoperiod plants I grow. There also seems to be a high leaf to calyx ratio with many autos that I don't care for. So many of the auto's people post photos of look very unappealing with so much leaf matter comprising a good portion of the flower.