We, as humans, have a tendency to think we are only changing what we can see and immediately observe, when we select for a specific breed or trait, but we are always doing so much more than that. In her book, “Animals in Translation”, Temple Grandin describes a perfect example of selective pressure, she talks about how chicken breeders decided they wanted faster growing chickens, so they began to breed fast growing hens with fast growing roosters. The result was faster growing chickens, but these chickens tended to have weaker hearts and legs, which led to a greater instance of heart failure (or flip-over disease) in these chickens. The breeders thought they were simply improving their business, but they ultimately costs themselves a lot of money in loss of chickens.
When we put this type of selective pressure on domesticated animals we are rarely doing them a favor. Another really great example of selective pressure was brought to my attention by Dr. Liz Sufit, who showed me the results of selective pressure on the Bulldog. If you look at the way Bulldogs used to look (even the Bulldog mascot) you can see that there stature has evolved overtime for traits that we seem to think are cute, but are in no way helping the breed live an easier life. The below picture is a good example of the change in general stature and confirmation. You can see that the bulldog on the left has a dip in his back, more so that the bulldog on the right. This will put pressure on both his back and hips overtime making it difficult to walk and exercise at an older age. You can also see the slight change in the shape of the face. We tend to like animals with pushed in faces and round heads (the round heads and big eyes are due to neoteny, but I will discuss that at a later date). The smashed face of a bulldog causes plenty of problems all on its own. It causes them to have a hard time exercising or breathing which can lead to problems with the heat, and often times, they have a tendency to snore. To top it off, many bulldogs owners like their wrinkles. The wrinkles in their face and around their tail become a prime location for yeast and bacteria infections.
When I describe this breed to people, I always say, there is nothing ergonomic about them. What I mean is that there is nothing about the way that they look that is useful, or makes their life easier, it is a direct result of selective breeding and pressure.