The process of natural selection is the mechanism that drives biological evolution, a theory first described famously in the mid-1800s thanks to the independent work of Charles Darwin and Alfred Russel Wallace.
Evolution accounts for the genetic diversity of life on Earth, all of which is derived from a single common ancestor at the dawn of life on the planet itself around 3.5 billion years ago.
Evolution has occurred in nature thanks to a scheme described as descent from modification, which proposes that heritable traits (that is, characteristics that can be passed on via genes from one generation of organisms to the next generation) that are favorable, and that confer genetic "fitness," become more prevalent in a group or species of organisms over time.
This occurs because the genes in question are naturally selected by the pressures of environment in which given organisms live.
Artificial selection, or selective breeding, makes use of the principles of natural selection to create populations of animals or plants that align with the needs of human farmers, researchers or breeders of show or sporting animals.
In fact, it was the long-established practice of artificial selection that helped guide Darwin's ideas about natural selection, for it provided stark and rapid examples of how genes became more prevalent in populations given known inputs.
Natural Selection Definition
Natural selection must be understood in order to fully comprehend artificial selection. Natural selection works not on individual organisms but on genes – in other words, lengths of deoxyribonucleic acid (DNA) that carry the "code" for a specific protein product.
Formally, natural selection includes four aspects:
- Genetic variance in traits exists in a population of animals. If all animals in a species were genetically identical – that is, if they all had the same DNA and hence the same genes – then no traits could be selected for, naturally or intentionally, because none would create greater or lesser levels of genetic fitness.
- There is differential reproduction. Not all animals pass on their genes to the maximum number of offspring.
- The different traits are heritable. Traits that make an animal more likely to survive in a given environment can be passed to offspring to begin with.
- A shift in the ratio of organisms and their underlying genetic composition over time is the result. It would be expected that, depending on the strength of the selection pressures within a given environment, the ratio of fitter-to-less-fit organisms would increase over time. Often, extinction events will occur, and the less-fit organisms disappear from the ecosystem outright.
Natural Selection, Explained
As an example, let's say you start off with a species of animals that have either yellow fur or purple fur, and these animals have just been relocated to a purple jungle in some undiscovered part of the world. The purple animals are likely to reproduce at a higher rate because they could more easily hide from predators by hiding within the purple vegetation, whereas the yellow animals would be more easily "picked off."
Fewer yellow animals surviving would result in fewer yellow animals available to mate and reproduce. If fur color were random, then no set of parents would be any more likely than any other to produce purple, and thus fitter (in this environment) offspring. But here, purple animals are indeed more likely to produce purple offspring, and similarly for yellow animals.
In the context of natural (and by extension artificial) selection, "variation" is equivalent to "genetic variation." In our animal example, purple-fur genes become more prevalent in that purple-hued jungle.
Artificial Selection in Detail
You have probably heard about the use of performance-enhancing drugs in sports, or "doping," a practice that which in most cases is banned owing to a combination of ethical and safety concerns. These drugs allow the body to reach greater feats of strength and endurance thanks to enlargements of the muscles or other physical improvements that would not occur without the added drugs.
These drugs, however, only work because of processes that are in play: exercise, training and striving in practice in competition. In other words, the banned drugs do not create unprecedented physical traits, such as the growth of additional legs or arms; they "merely" hone and augment capabilities already in place.
Artificial selection may be viewed in much the same context. It is a form of genetic modification that plays on the fixed principles of natural selection listed previously and that intentionally amplifies one or more of the variables already in play to achieve a desired result.
Artificial selection is the intentional choosing of the parents, that is, the organisms that will reproduce, which is why it is also known as "selective breeding." This is done to create individual organisms (plants or animals) with beneficial or desired traits.
Selective Breeding: History and Mechanism
Artificial selection, which is actually a type of genetic engineering, has been practiced around the world for thousands of years. Even if people did not know exactly how farm animals with desirable traits were able to pass these traits on to offspring, they were aware that this occurred and shifted their farming accordingly.
If certain cows on a farm were larger and provided more meat, breeding cows in the immediate "family" of these robust specimens were likely to produce similarly large offspring and a greater beef yield. The same principles can be applied to crops, often more emphatically because of fewer ethical concerns in the area of breeding plants versus breeding animals.
In biology terms, artificial selection leads to an increase in genetic drift, or a change in the frequency of genes within a species over time. By selecting for desired genes and the traits they confer, humans to curate plant and animal populations in which both "good" genes have been increased and "bad" ones have been winnowed down or eliminated.
Darwin, Pigeons and Artificial Selection
By the 1850s, shortly before the publication of his groundbreaking work On the Origin of Species, Charles Darwin had already advanced a then-controversial idea to explain variation of "breeds" within species: that humans had manipulated the composition of species by mating them in programmed ways, a process that had relied on some as-yet unknown genetic mechanism to bring this about.
(At the time, humans knew nothing about DNA, and in fact the experiments of Gregor Mendel, which showed how traits were passed on and could be dominant or recessive, were just beginning in the mid-1850s.)
Darwin's many observations of a particular kind of pigeon popular in his native England at the time included the fact that pigeons that had been bred in a way that produced markedly different sizes, colors and so on could nevertheless be bred with each other. In other words, all were still pigeons, but different factors in the environment had systematically shifted the genetic picture in certain directions.
He proposed that natural selection acted in the same way, and on the same molecules, whatever they were, but over longer periods of time and without conscious manipulation by people or anyone else.
Examples of Artificial Selection: Agriculture
The entire purpose of farming is to produce food. The more food a farmer can produce per unit of effort expended, the easier his or her job will be.
In subsistence farming, the idea is to produce enough food for a given farmer and his or her immediate family or community to survive. In the modern world, however, farming is a business like any other, and people seek to profit from their farming by producing beef, crops, dairy products and other goods that consumers want.
The behavior and methods of farmers is therefore predictable. Farmers and growers select plants that, thanks to genetic changes, produce more fruit than others to get more fruit-bearing plants, choose plants that yield larger vegetables to get more mass of product per seed invested, choose plants to reproduce that are able to survive extreme temperatures during droughts and otherwise go about striving for maximum efficiency in the context of the range of challenges they face.
Examples of selective breeding in plants today are almost limitless. The creation of distinct species of cabbage plants to get more types of vegetables has given humankind cabbage, Brussels sprouts, cauliflower, broccoli, kale and other popular greens. Similar work has been done to make different kinds of gourds (e.g., pumpkins and other types of squash) available.
Animal Breeding: Livestock, Dogs, and Others
Like the artificial selection of certain plant varieties, the breeding of domesticated animals for desirable traits from wild species has been going on for thousands of years, and was carried out for centuries despite humans not knowing the genetic basis for why it works. This has been done in the area of livestock, or farm animals, where the aim is typically to create more meat or milk per organism.
Just as you would want every human worker on an auto-assembly team to be able to, say, assemble more cars, having more product per farm animal drives up farming profits, or in nonprofit settings, ensures that people will have enough to eat.
Dogs provide among the most startling examples of the effects of artificial selection. Various dog breeds have been created by humans over the past 10,000 or more years starting from the common ancestor of all dogs, the gray wolf.
Today, breeds of dogs with seemingly little or nothing in common, such as Dachshunds and Great Danes, exist in abundance, demonstrating the range of traits coded for in the dog genome. This is because the definition of "desirable traits" in domestic a dog varies considerably between god owners. Doberman Pinschers are smart, muscular and sleek and make great guard dogs; Jack Russell terriers are agile and can catch lots of animals that haunt farms.
The same principle spans other species and industries. Successful racehorses are bred together to create a higher likelihood of creating faster, stronger horses in subsequent generations, as having a winning horse in major events can be lucrative to the human owner or owners.
Also, in the genetic modification of food, an extensive topic in itself, humans modify food sources to enhance certain traits and then breed these together to form "superior" strains of these plants and animals. Examples include soybeans, corn, chickens that grow more breast meat and many more.
Adverse Consequences of Artificial Selection
Altering the natural course of things using the methods described here has unquestionably bettered the lives of human beings in various ways, such as by increasing crop yield, allowing for better and more meat to be produced, and even creating new dog breeds with genetically and behaviorally desirable characteristics.
When, however, people make us of artificial selection, this reduces overall genetic variance within the population by creating, in effect, an "army" of more similar animals. This results in a higher risk of mutations, greater vulnerability to certain diseases, and an increased incidence of physical problems that would otherwise be minimal or absent. For example, chickens bred to grow larger breasts (via their pectoral muscles) often spend their lives in substantially more discomfort because their frames and hearts have not adapted over time to carry the added mass.
In other scenarios, unforeseen mutations and traits can arise along with the selected traits. In bees, for example, "killer" breeds were bred to produce more honey, but in the process they also became more aggressive and thus became dangerous. Artificial selection can lead to sterility in organisms, and in certain pure-bred dogs, recessive traits that would otherwise diminish naturally are allowed to persist, such as hip dysplasia in Labrador retrievers.
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About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.