Photographs of a breed taken a century apart often show animals that would be hard to identify as the same type. The mechanism producing that drift is written into how breeds are maintained.
What a standard actually is
A breed standard is a written description of the ideal animal, covering proportions, head shape, coat, movement and temperament.
It is prose rather than measurement, and most of it describes qualities in relative terms — deep chest, short back, ample coat — without numerical limits.
That looseness is deliberate, since it allows for variation, but it also means the standard is interpreted rather than applied.
How interpretation drifts
Judging compares dogs against each other in a ring, and a judge choosing between two animals that both meet a description tends to reward the more pronounced expression of it.
If a short muzzle is desirable, the shorter of two acceptable muzzles wins. Repeated across shows and generations, that preference moves the population.
Winning animals are used most in breeding, which compounds the effect. The written standard may not change at all while the dogs steadily do.
Where the consequences show
The clearest cases involve traits with direct physical consequences: facial shortening affecting airways, skin folds creating persistent moisture, and back length altering spinal loading.
Angulation of the hind limb has shifted markedly in some working breeds, changing how weight is carried during movement.
In each case the trait was originally described in moderate terms, and the change came from selecting repeatedly towards one end of an acceptable range.
The response from within the system
Several registries have revised standards to add explicit limits, requiring visible nostrils or stating that exaggeration is a fault to be penalised.
Some have introduced health checks at shows, where dogs in certain breeds must pass an assessment before an award is confirmed.
Whether these measures shift populations depends on judging practice rather than wording, since the same interpretive gap that produced the drift remains.
What it means for choosing a dog
Within any breed there is usually a wide range, and lines bred for work often differ substantially from those bred for the ring.
Asking what health screening the parents underwent, and seeing the adults move and breathe rather than only stand, gives more information than pedigree alone.
Where a breed carries recognised structural conditions, a veterinarian familiar with it can explain what to look for before a decision is made.