Each breed of dog has been selectively bred for certain attributes. These may be physical attributes alone or primarily behavioural. For breeds based on physical attributes is appropriate to formalise these and expect members of the breed to conform to a highly specific set of narrow physical criteria. Where the breed is based primarily on behavioural characteristics, these take precedence over physical characteristics and depending upon the genetics of the breed, this can result in differing degrees of degrees of physical variation within the breed.
Jack Russell Terriers were bred as a working dog for fox hunting. Although primarily behavioural, this required physical attributes that enable it to follow foxes down earths and manoeuvre underground as freely as the fox does. They are genetically variable in physical features, with a broad standard but less so in their instinctive hunting behaviours. Based on the above quoted criterion. Jack Russell Terriers also cannot be considered as a breed.
From the start GSD’s were bred to guard the flock and stop it from straying into adjoining crops. Intelligence that allowed training to stop the flock wandering into specified areas was also selected for. These attributes of protection of home territory and its occupants, loyalty/responsiveness to its owner, and intelligence and trainability, have been selected for from the outset and are what suit it to the role of protection/guardian dog in modern situations, including families.