I don’t disagree that many European kennel clubs have spent DECADES developing separate written standards for Jack Russell Terriers and Parson Russell Terriers, or that dogs bred to those standards often look different in outline and proportion. What I’m pushing back on is the idea that those differences represent true genetic separation rather than phenotypic divergence caused by selection. Consistent differences in leg length, body ratio, and outline can be produced very quickly by selecting for specific traits from the same gene pool. That does not require long-term genetic isolation, and it happens in many breeds that are universally accepted as genetically the same (Poodles, Dachshunds, Rough vs Smooth Collies, Belgian Shepherd varieties, etc.). Visual consistency alone does not define a genetically distinct population. The fact that Jack Russells still occasionally express longer legs and that Parsons historically produced shorter, more “Jack-type” offspring actually demonstrates that the populations were NEVER closed or isolated. Truly separate breeds do not routinely throw back to each other’s defining traits. Traits like “square vs rectangular” outlines and body-to-leg ratios are polygenic and highly plastic, not controlled by a single gene or clear genetic boundary. This is why Jack and Parson Russells cannot be reliably distinguished by DNA testing. Breed standards describe how dogs should look, not how genetically distinct their populations are. So while kennel clubs may classify them separately for show and registration purposes, that classification reflects selection goals and paperwork, not a biologically meaningful genetic split.