From a teacup-size Chihuahua to a Great Dane, there is an incredible amount of variety among dog breeds. But all breeds belong to a single species, so scientists have studied the breeds to better understand the workings of evolution, and how such great variation could have arisen within one group.

The dog (Canis lupus familiaris) is far more variable in size, shape and behavior than any other living mammal, but most experts now believe that all dogs, no matter how different, originated exclusively from a single species: the gray wolf (Canis lupus) of central Asia, said James Serpell, professor at the University of Pennsylvania School of Veterinary Medicine and editor of "The Domestic Dog: Its Evolution, Behaviour, and Interactions With People" (Cambridge University Press, 1995).

It's also likely that there was just one domestication event, and all domesticated dogs today descended from an ancestral wolf-dog that became someone's best friend long ago. The evidence comes from a 2009 study in which a team of researchers at the Royal Institute of Technology in Stockholm analyzed samples of mitochondrial DNA (the DNA found in mitochondria, or energy-making structures within cells) from dogs around the world.

They found that all dogs belong to one lineage, which indicates that domestication occurred just one time. (If wolves were domesticated several times in various regions, the team would expect to find more than one lineage among modern dogs.)

Despite the fact that dogs were first domesticated about 14,000 to 17,000 years ago, most dog breeds were developed within the last few centuries. When ancient humans bred dogs for features such as a louder bark (for added protection of their owner's property) or a docile temperament (so it would be less likely to lash out at its owner), they were actually already tinkering with the selection of dog genes.

One of the earliest breeds believed to be purposefully selected for its preferred traits is still around today – the greyhound. Perhaps the first fully distinct breed was the Saluki, also called the Arabian greyhound, whose name translates to "noble," according to "Simon & Schuster's Guide to Dogs" (Fireside, 1980).

"Selective cross-breeding has been done since antiquity, but it really accelerated during the 19th century," said Leslie Irvine of the University of Colorado at Boulder, who is author of "If You Tame Me: Understanding Our Connection with Animals" (Temple University Press, 2004).

Over time, because of natural mutations, climate and human preferences, "breeds became ever more numerous and specialized until they reached the point of modern classification," according to "Guide to Dogs." This classification is based on the aptitude of a breed in five skills: hunting, shepherding, guarding, work and company.

Now, there are about 340 breeds recognized by the Fédération Cynologique Internationale (FCI), the world governing body of dog breeds, sometimes known as the World Canine Organization. But the standards for breed recognition vary from country to country – the American Kennel Club currently recognizes only 167 breeds.

Recently, the number of deliberately crossbred "designer dogs" has been growing. These include the labradoodle (a cross between a Labrador and a poodle), the cockapoo (a cross between a cocker spaniel and a poodle) and the puggle (the offspring of a pug and a beagle).

"Dogs are constantly evolving as we're continually building variants of dog breeds," said Stanley Coren, author of "The Modern Dog" (Free Press, 2008).

"The nature of humans is to want unique things, but a unique thing is not necessarily a better thing," Coren told Life's Little Mysteries.

Remy Melina is a Staff Writer for Life's Little Mysteries, a sister site to LiveScience.