Artificial selection, also known as selective breeding, is a nice way of saying that humans have guided the evolution of other animals until they become mutants. The stories of these six human-created mutants offer a fascinating perspective on how evolution works.
The lowly pigeon. Flying Rat. Carrier of disease and dirt. They bob around our cities cooing and crapping everywhere, picking at whatever refuse the come upon. Mostly, we just ignore them, or send a wayward kick when they crowd the sidewalk. But if we took the time to look at these city specialists, we would notice a startling array of colors, body shapes, and beaks. This incredible diversity was driven by artificial selection — these wild pigeons are the descendants of carefully-bred birds that escaped from captivity to roam our cities. In fact, the diversity of city pigeons is what first inspired Charles Darwin to contemplate the theory of evolution.
But what is artificial selection doing to today's domesticated animals, which are still closely managed by human breeders?
One of the most universal hallmarks of domesticated animals is a childlike or "babyish" cast to their features, called paedomorphism. This set of characteristics gives adult animals (including humans) larger heads, ears, and eyes, smaller jaws, and juvenile behavior. They tend to arise alongside domestication. Animals that are friendlier and more accepting toward humans often exhibit these other characteristics, even when breeders select only for friendliness.
Humans don't stop at making friendly and babyish animals. We have spent thousands of years tailoring the looks and personalities of our domesticated animals to our preferences and tastes without ever really having an understanding of the genetic mechanisms at play. To this date, science doesn't really know what causes some of these extreme mutations. Some of these changes are functional, and some of them are downright monstrous. For example:
Someone, somewhere, considers this beakless wonder to be one of the most beautiful breeds of pigeon. A few decades of intense artificial selection a bird with an essentially non-existent beak, bulging white eyes, and a tiny skull. Not only has the human hand shaped the appearance of this bird, but breeders have also selected for a "tumbling" behavior. This behavior causes the birds to flip over backwards while flying or attempting to fly, and seems to be tied to serotonin abnormalities in the birds, leading to what is essentially seizure (scientists have controlled this behavior with anticonvulsants and antidepressants). Although pigeons aren't the most diverse species in terms of looks, breeders over centuries have managed to control everything from eye color to the size of the bird's crop to alter the looks of this species.
Again, someone did this on purpose. Like the domesticated pigeon, canaries have been selectively bred for a few hundred years. And in that time, breeders have taken a fairly nondescript bird with a nice singing voice, and created hundreds of varieties. Breeders can control everything from skeletal formation, feather number and hardness (very hard feathers, like those on the pictured bird, to very soft fluffy feathers), to the length and style of the bird's song. Breeders even managed to create a fertile hybrid between the canary and the red siskin, introducing red feather coloration into the species. This intensive breeding isn't without cost, as birds can suffer painful, deadly ingrown feathers and can display high susceptibility to infection and injury.
Chickens have been hanging around humans for a long time. And in that time, we have managed some pretty spectacular changes to this forest pheasant. But perhaps the most freaky is this featherless monstrosity. For once, science has a solid understanding of why this bird looks like it does. While the embryo is forming, the skin fails to form properly, and the bird never grows feathers, scales, or spurs Scientists have even pinned this one down to a single nonsense mutation on a single chromosome causing incomplete formation of an important signaling protein, a mutation unique to this pink wonder.
At first glance, this dog looks pretty decent. All fur accounted for, no bulging eyes. In fact, this breed is fairly functional. Bred specifically to hunt African lions, the dog is large, swift and strong, with excellent eye-sight. Unfortunately, and in keeping with the human need for adding a bit of useless flair, this dog has a serious problem. The breed is named for one of its most distinct features, a ridge of hair growing in the opposite direction along its back. Pretty cool and harmless, right? Wrong. That ridge is caused by a developmental deformity, one that can leave a small, deep hole in the dog's skin. That hole (called a dermoid sinus) can reach all the way into the dog's spinal column. This essentially leaves delicate nervous tissue exposed to bacteria and disease, and dogs can become life-threateningly ill from an infected dermoid sinus.
The goldfish has also experience a long relationship with humanity. This species has always been bred specifically for ornamentation, leading to an incredible array of strange shapes and sizes. The strangest by far is the Bubble Eye goldfish. Not only does this fish grow enormous, fluid-filled sacs under its eyes (which can and do pop and deflate), the fish has malformed, upward turned eyes, a severely curved spine, and no dorsal fin at all. Other varieties of goldfish grow deformed scales, no scales, large fleshy protrusions on their heads, and maybe weirdest, two tails.
Ironically, this breed of goat routinely wins "Most Beautiful Goat in the World" in Middle Eastern goat competitions. These goats have been selectively bred for extremely short, snubbed noses and undershot jaws, much like the modern English Bulldog. It is unclear exactly what causes this extreme mutation in the goat. Another unusual mutation in domestic goats causes "fainting spells", which are actually periods of extreme muscle rigidity brought on by being startled. This condition, called myotonia congenital, is caused by a mutation that alters the structure of chlorine channels in muscle cells allowing for disruption of muscle signaling , which in turn cause prolonged, intense contractions in muscles. Unsurprisingly, people breed for this mutation on purpose as a form of parlor trick, although most breeders raising goats for meat or milk avoid it.
Humans have been shaping animals to our aesthetics for almost as long as we have been domesticating species. Although it appears that physical changes to animals initially occurred mostly as an unintended consequence of selection for domesticity, it didn't take long for humans to start engineering some of the strangest looking creatures on planet earth.