Artificial Selection: How Humans Directly Influence Organismal Evolution
Anyone who has taken a life science course has at least a basic understanding of natural selection. Natural selection drives evolution and speciation; organisms must be able to overcome natural barriers, challenges and conditions in order to survive to pass on their successful genetics. Individuals that are unable to survive are unable to reproduce, and traits that hinder an organism are lost as the individual is unable to pass those genes onto its offspring. The species genetics will gradually shift to reflect those traits that are beneficial, or necessary, for survival, while those that are detrimental are lost. Evolution does not work toward the most complex form, but rather to the form that is best suited for survival.
Natural selection has driven evolution on Earth for almost four billion years, but another form of selection has arisen more recently, yet has radically changed the evolutionary course for many species. Artificial selection, the means of controlling the genetics, and thereby the evolution, of an organism to shape the species the way we choose, has been in practice for thousands of years. Artificial selection began long before humans had any concept of evolution or genetics, but our use to the practice led to not only the evolution of new species and breeds, but to the successful rise of mankind.
So how does artificial selection work? To put it simply, we choose what individuals will reproduce and pass on their genetics to future generations. We select the individuals that have the particular traits we desire, and cross them with a mate that has a similar pedigree, ensuring that offspring will more than likely bear the same traits. Horses, for example, have long been bred to serve a variety of purposes; some were artificially selected for their speed and agility, while others we bred to be beasts of burden with incredible strength and stamina. Over a long period of time, this selectivity gives rise to different breeds bearing different genetics and appearances, allowing humans to shape organisms to fit a needed form and function, whether it be for food, work, war or companionship.
Dog breeds are perhaps the greatest example of artificial selection at work, resulting in a huge number of breeds greatly ranging in size and shape, some bearing remarkably different traits. We began breeding dogs for different purposes not long after their domestication; certain dogs were bred for their aggression, to serve as early guard dogs, while others were bred for speed, assisting early humans in their hunts. Over time, dogs were bred for other purposes, including companionship or even just their appearance. We selected dogs for nearly every trait imaginable; some we chose because we liked their fur, others we selected for ears that stood up pointed or drooped over. We bred dogs for traits that were intimidating, useful, or even just because we thought it made them cute. Whatever the reason for each cross, this selection gave rise to today's numerous dog breeds.
Artificial selection became the only force acting upon many of these animals; under our care, dogs, horses, cats, cattle, and a host of other animals were safe from the forces of nature. Because of our efforts, these animals did not have to overcome the natural challenges, such as predation and climate, that they once had to face in the wild. Without natural selection to weed out the unfit, it became survival of the most useful (or even survival of the cutest). Their evolution was driven entirely by humans, and they began reflecting many traits that would not have been beneficial in the wild, but did not affect their ability to survive in captivity. Many domesticated animals simply could not survive in the wild because the traits they have been selected for for hundreds, or even thousands, of years would be detrimental to survival in wild conditions (imagine the likelihood of a chihuahua or dachshund surviving long in the wild).
Artificial selection does not have to necessarily be a purposeful endeavor; many species are accidentally selected for certain traits by humans without our awareness. We put a lot of stresses on the environment and its animals, and they often overcome these stresses by exhibiting, or lacking, certain traits. For example, in the southern United States, Rattlesnake Roundups are a popular event in many regions. Rattlesnake hunters spend weeks catching theses snakes en mass prior to the Roundup event. They do this by listening for the telltale sound of the snake's rattle; when an individual snake begins rattling, the hunters are able to find it and capture it. Some rattlesnakes are occasionally born with shortened, or even without their iconic rattles, and these snakes are often spared by the hunters because they are not heard like the snakes with rattles. This has already resulted in a higher number of snakes than usual being born without rattles, as the rattle-less snakes are more likely to avoid the hunters, and survive to pass on their traits to their offspring.
Artificial selection is a powerful tool that we have wielded for centuries. We have bred animals for work, war, food, looks and companionship. We have selected and cross-pollinated plants for hardiness, crop/fruit yield and pest resistance. Without this ability, it is unlikely that humans would be the remarkably successful species we are today. In our efforts to master the Earth, we have successfully mastered evolution itself, bending it to fit our own agenda. We can essentially design the "perfect" organism, based on our desire to see certain genetics exhibited; what breeds and species will we see in the future as we continue to artificially select for certain traits?
Note: This is an extremely simple look at artificial selection for the sake of brevity. It is a complex concept that could (and has) fill entire books.