I appreciate this post and discussion... thank you Gregoire!

I'd like to talk a little bit about Breed Standards.

Being a member of an AKC breed club, I have observed there to be powerful forces within the fancy, including members of Judge's Education Committees, Boards of Directors and well respected successful breeders who resist the temptation to make even minor Breed Standard revisions, let alone sweeping changes - the frequency of breed standards revisions falls many years apart. Education of all factions within the fancy as to the reasons for and benefits of altering a breed standard must be done - this is no small task and requires considerable communication efforts on the part of people charged with creating an accepted and officially adopted breed standard revision. I have noticed that no project of this nature is accomplished with any great speed in dog clubs. At this time, taking into account the international aspects of dog breeding and showing, taking the lead in revising a breed's standard becomes quite an undertaking involving many vested stakeholders.

Identification of breed standard issues which contribute to functionality including health and aesthetics which require written clarification is the starting point. I have observed widely varying opinions and some considerable diversity in individual's interpretations of words used in a breed standard to describe breed characteristics. Vague language used in breed standards to describe traits, intended or not, seems to allow for a drift into 'extremes'. Breeding for those extremes and judges awarding them is almost an inherent aspect of the sport of purebred dogs.

The consensus that must be reached as to exactly which points in a breed standard need to be clarified or changed to aid breeder's and judge's understanding of proper breed type is challenging. Changing the breed standard to increase muzzle length (head shape) by inserting specific language to describe the proportional relationship of muzzle to back skull could involve completely revising all aspects of head type description i.e. eyes shape, set, ears set come to mind; but really, the entire shape of the skull and fore-face would likely come into play as well as expression, and perhaps balance, carriage and neck, even body shape. I would wonder... where within an existing brachycephalic breed would dogs with longer muzzles come from? The examples available I'd imagine would be somewhat limited and clubs putting a mechanism in place for an overall evaluation of dogs by breed experts and/or veterinarians for breed specimens which typically are not shown at dog shows and are instead placed as pets for the reason they lack correct breed type could be a significant hurdle.  

Performance evaluations and testing, participation in working competitions do tend to keep breeds functionally honest to some degree.      

I fully agree with you about the difficulty to change breed standards, I have only limited knowledge at it, but I heard that depending on breeds and their origins, the precision and place that is let for interpretation vary a lot, which would probably impact the efficiency of a change in standard.

Relatively to your last question on where could the dogs improving the dog morphology come from, as a geneticist, I would answer this would depend of the gene pool on the breed, as well as the genetic complexity of the trait. Just supposing the inheritance of your trait is polymorphic and you have a large population, it should be possible to select toward a better morphology, even if currently no dogs are showing the phenotype your require. In cattle, some breeds produce currently 10,000 litter per years, largely due to selection, while 50 years the production was less than half of this.

Yet the less simple in your trait inheritance, and the smallest in your population, the less chance you will have to make things move without relying on crossbreeding. In the case of skull shape, I think several candidate gene have been identified, which may help in selecting or introgressing some alleles of interrest in future, with development of adequate genetic tests...

Schoenebeck, J. J., & Ostrander, E. A. (2013). The genetics of canine skull shape variation. Genetics, 193(2), 317-325.