Artificial Selection – What is it?
Artificial selection is the breeding of certain traits over others. It was originally defined
by Charles Darwin in contrast to the process of natural selection, in which the differential
reproduction of organisms with certain traits is attributed to improved survival and
reproductive ability in the natural habitat of the organism. Artificial selection that
produces an undesirable outcome from a human perspective is sometimes called negative
selection (but note that this term has a better-established meaning as a type of natural
selection).
Historical development
Charles Darwin originally coined the term as an illustration of his proposed wider process
of natural selection. He noted that many domesticated animals and plants had special
properties that were developed by intentionally encouraging the breeding potential of
individuals who both possessed desirable characteristics, and discouraging the breeding
of individuals who had less desirable characteristics.
He then postulated that a similar process occurs naturally; individuals in the wild who
possess characteristics that enhance their prospects for having offspring would then
undergo a similar process of change over time; although in this case "desirable"
characteristics would be not those which specifically satisfy human needs, but those
which enhance survivability. This natural process forms the basis of the theory of
Darwinian evolution.
Contrast to natural selection
The difference between natural and artificial selection centers on the difference in
environment among organisms subject to the two processes. Essentially, in artificial
selection, the fitness which is the amount of offspring an individual contributes to a
population relative to other individuals in that same population of an organism is defined
in part by its display of the traits being selected for by humans. Since humans either
intentionally or unintentionally exert control over which organisms in a population
reproduce or how many offspring they produce, the distribution of traits in the organisms'
population will change.
It should be emphasized that there is no real difference in the genetic processes
underlying artificial and natural selection, and that the concept of artificial selection was
first introduced as an illustration of the wider process of natural selection. The selection
process is termed "artificial" when human preferences or influences have a significant
effect on the evolution of a particular population or species.
Examples of artificial selection
Most examples of artificial selection fall into the category of selective breeding, in which
particular individuals are selected for breeding because they possess desired
characteristics or excluded from breeding because their traits are undesirable. Both
processes have contributed to the domestication of animals and plants by humans.
The most obvious examples of artificial selection can be found in the range of specialized
body shapes and even personality types in bred in domesticated dogs. The wide range of
sizes and shapes, from Dachshund to Wolfhound, shows the power of artificial selection
through selective breeding. Systematic selective breeding has led to extreme traits such as
the large size and eating habits of the Great Dane versus the small size of the Chihuahua.
It is possible for traits to be selected for under artificial selection - for example,
aggressive behavior in small dogs - that would be selected against in the natural
environment absent human influence. An even more illustrative example is the
domestication of corn, which has been bred so that it no longer disperses its seeds,
instead relying on human intervention to disseminate them. Because both organisms
derive significant benefits from the other, this could be termed a symbiotic relationship.
Certain characteristics may unintentionally be encouraged while intentionally selecting
for a desired result. For example, the domestic chicken has been bred to reach a large size
relatively quickly (compared to its feral ancestors). The resulting changes in the chicken's
gut have come at the expense of a reduced brain size and relatively smaller leg bones;
these latter changes were not intentional artificial selections, but through a parallel
process sometimes called "unconscious selection".
It is also possible for humans to exert artificial selection pressures on our own species,
either unintentionally through social pressures, or intentionally. Eugenics efforts, in
which those with "undesirable" characteristics are prevented from reproducing and those
with "desirable" characteristics are encouraged to reproduce, form the most extreme such
example.
Artificial Selection - A heiroglyph depicting dog breeds
The domestic dog comes in a mind-boggling array of shapes and sizes. Whether you
prefer a Chihuahua you can tuck in your pocket or a 200-lb Saint Bernard that can carry
you, there's a dog for every human fancy. Over the years, humans have bred dogs for
work and whimsy, creating the hundreds of domestic breeds we know today. This
human-controlled breeding is called artificial selection.
From one ancestor, many breeds
Starting thousands of years ago, dogs of many sizes and shapes appear in the
archaeological and artistic records of almost all human cultures. Selected for hunting,
herding, protection, companionship, and looks, dogs were welcomed into households of
all kinds.
Most of today's almost 400 recognizable dog breeds were established after 1850. This is a
sign of those times, reflecting the rising popularity of purebred pets as status symbols,
specialized breeding, and dog shows.
Dog diversity depends on three key factors. Three processes are involved in the rapid
evolution of dog breeds.
1. Systematic selection involves carefully selecting certain dogs for inherited traits like
body type, coat characteristics, speed, herding, hunting, endurance, and size. Over time,
breeders' choices produce animals with distinctive looks and abilities. The saluki, basenji,
and greyhound are just a few examples.
2. Sports (a term Charles Darwin used for macromutations) refer to puppies with an
unusual trait, such as hairlessness or lack of a tail. This includes the mutation for
achondroplasic dwarfism that causes limbs to stop growing prematurely resulting in
short-limbed breeds like dachshunds and basset hounds. Sport mutations generally don't
help dogs survive in the wild but they can give a dog a look that appeals to breeders.
3. Cross-breeding produces new forms by crossing two breeds that differ in appearance or
behavior, followed by selective breeding of the offspring. Such repeated crosses unleash
a lot of genetic variation -- many shapes, sizes, and colors! The Australian cattle dog,
Doberman pinscher, and the whippet are a few examples of cross-breeding.
The first formal dog show, held in 1859 in Newcastle, England, presented an enormous
array of dogs. These shows have since become key for establishing bloodlines and breed
standards. These standards describe the ideal height, weight, coat color and texture, ear
type, and so on for individual breeds. Breed standards maintain the uniform appearance
of purebred dogs. Once established, the look of purebred dogs won't change much over
time. In contrast, wild species evolve constantly in response to changing natural
conditions.
Reading Questions to be answered
1. What is artificial selection?
2. How is it different from natural selection?
3. What did Charles Darwin say about artificial selection? Did he like it?
4. What are some good examples of artificial selection?
5. Why is artificial selection used and what animals are artificially selected?
6. What are Eugenics?
7. How many dog breeds are there in the world?
8. What are the 3 key factors of dog breeding?
9. What are some problems associated with purebred dogs?
10. What is the name of the famous dog show in NYC?