You and i are just going to have to disagree.
If a population is homozygous for a gene or series of genes it cannot produce the absent trait. Ie blue eyed parents cannot have brown eyed offspring.
The reduction in the variation of the Canis genome that yields ‘Beagles ‘ means that Beagles’ lineage cannot be bred to make wolves (morphology ) because those genes are not in the Beagles’ chromosomes.
Sans ‘magic’ mutations in the genome of course.
Taxonomy is a human construct not a genetic one.
On mobile pardon any errors.
Mutations don’t need to be magic. The gene for floppy ears is not in the wolf population. It is a mutation that was selected for by dog breeders. A throwback mutation of that gene would give you a beagle with upright ears. If that was selected for, rather than against, you would have beagles with upright ears. Wolves don’t have genes for short stubby legs in their population.... etc, etc.