My fiance C posted in her blog about the effects of evolution on the friendly pug, commenting on a humorous post from another blog suggesting that the pug is “proof that evolution doesn’t work”.
The obvious response is that pugs are not the result of evolution, i.e. natural selection, but rather the result of man’s breeding to enhance certain traits. Artificial selection, as opposed to natural selection, usually results in unintended consequences. In a pug, breeding for friendliness also resulted in attributes for a short snout (“how do you breathe through that thing?”).
Natural selection favours attributes that contribute to an animal’s ability to survive and thrive in their environment and pass on DNA to the next generation. In nature dogs were efficient and effective hunters, as wolves are today; after domestication and human-influenced breeding we have certain dogs that are great hunters but also other dogs that are for show or companionship and would probably not survive long in the wild.
I seriously doubt that pugs, as a breed, would last in the wild long enough to pass on their DNA. Nor would a lot of other human-created breeds that are incapable of hunting. How would they survive day to day without their human companion there to feed them? Certainly bulldogs requiring C-section births wouldn’t even get to the starting line. But as long as humans are there to feed them in exchange for companionship then the dog will be just fine. Return a domesticated hunting-capable dog to the wild, and within not too many generations it will regain attributes specialized for survival and loose those breed into it for domesticity.
So how did the pug get its short snout? It probably wasn’t bred specifically by humans for that, but rather acquired that as an attribute somehow connected to other traits found desirable by the human breeder. You want friendly you also get a short snout; other short snouted dogs are also friendly so we have to assume that somehow these attributes are connected in a dog’s DNA. But how did it happen? How does a dog “know” how to make a short snout?
Natural selection works because of “mistakes” in the inheritance of attributes. You get a some mix of attributes from both parents through sexual reproduction; you look like one or other or sometimes a mix of your parents, for example. But sometimes there’s a hiccup and you get something that came from neither parent, for example a tall child being born of short parents. If this unanticipated attribute is severe and negative we might call it a birth defect. But if that attribute makes the child more able to survive and thrive in their environment then the child will be more likely to pass on that attribute to the next generation.
All dogs are born with short snouts in order to facilitate sucking; a baby born unable to suck would starve, so only dogs with that (perhaps originally accidental) attribute have lived and passed on their DNA to the next generation. But a short snout is a huge disadvantage in an adult as it lessens the effectiveness of a bite in hunting. (Can you imagine a pug hunting?) So evolutionarily successful dogs were born with short snouts which after birth grew into long snouts. This happens because a “switch” in the dog’s DNA told the snout to start growing after birth. For some reason in pugs and other short-snouted dogs such as bulldogs, boxers, etc. this switch never turns on.
We see the effect of this switch in other animals as well. A human does not develop a long snout but a chimp does; they look similar at birth but develop differently afterwards. We share, it is said, 99% of our DNA with the chimp, but some of that DNA is composed of switches that while identical have different on/off settings.
Another example is in the skeletal structure of mammals. All mammals have the same bones, but the development of those bones is different. Comparing a human to a bat we see that all the same bones are there, but the finger bones in bats grow extremely long and have skin in between them that forms a wing. All mammals have five digits at the end of each arm/leg, but some have stunted growth to the point where they’re almost invisible. For example, a horse’s hoof is actually the overdeveloped middle digit, with the other digits suppressed. A whale has the bones for five digits in each of its flippers, and has rudimentary hip, leg, and feet bones hidden in its lower abdomen.
Very interesting stuff. I can’t claim credit for any of these ideas; I’ve just recently finished reading Richard Dawkins’ The Greatest Show on Earth, which is a great explanation of the proofs of natural selection. Highly recommended reading.