What is Selection Pressure and why does it matter? Selection Pressure is defined as those changes that occur when the environment selects which members of a species live long enough to reproduce and which do not. Selection Pressure can help enhance new traits. People are always changing the Selection Pressure on animals, often in ways they didn’t expect.
Dog emotions are likely the most obvious way that Selection Pressure presents itself. An example would be the thin collie face. Breeders, over time, have thinned out the collie face, leaving less space inside the skull for the brain. Look at old Lassie shows – Lassie was a dog with a broad, flat forehead. Thinner skulls = smaller, more distorted brains = a less intelligent dog. Causing collies to become dumb wasn’t the goal, but rather a side effect. Breeders probably just wanted to enhance a feature unique to the collie, which is its long thin nose. But in the process of breeding for long noses, they bred in a thinner skull (with a smaller brain) and an animal that is not as smart.
Selection Pressures on purebred dogs are primarily negative, as observed with the collie. Also, breeders are trying to meet American Kennel Club standards, which are heavily weighted towards the way a dog looks aesthetically, not emotionally or behaviorally. The AKC strongly influences the dog-buying decisions of people in this regard.
When it comes to mixed-breed dogs, however, selective pressure can be a good thing. A mutt who bites people has a high chance of being put to sleep. That means its genes will be removed from the gene pool. Therefore, the argument can be made that mixed-breed dogs that end up reproducing are those that are good at living with people.
In terms of behavior, purebreds are responsible for the large majority of serious dog bites. The fact that mutts are less aggressive is evidence that Selection Pressures on mixed breeds are more favorable. A conclusion can then be made that, in general, mixed-breed dogs are easier to live with.
I’m not trying to influence a potential dog owner away from a purebred. I’ve lived with both purebreds as well as mixed-breeds and have had great experiences with each. Just know that there are a lot of things to consider when looking for your new companion.