Artificial Selection
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.
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.
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 an exotic look that appeals to breeders.
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.
Breed standards help keep purebreds pure
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 pure bred dogs. Once
established, the look of pure bred dogs won't change much over time. In contrast, wild species
evolve constantly in response to changing natural conditions.
