How Dogs Work
Date______________________________ Per_______
Natural selection is the process that Darwin proposed as the mechanism behind evolution. There is genetic
diversity (variation) within any population, which is caused by slight mutations in the DNA of every organism,
as well as gene swapping during sexual reproduction. In animals, this genetic diversity includes physical and
behavioral differences. Animals may be slightly bigger or smaller, differently colored, faster or slower, or more
or less aggressive. Some of these characteristics are neutral -- they neither benefit nor harm the individual that
has them. However, some of these qualities affect the ability of the individual to survive and reproduce.
Animals that need to hide but are oddly colored and more visible than the rest of their species will probably die
young without offspring. When that happens, the genetic variations that cause that odd coloration will be lost.
This trait is selected against. Conversely, animals that have a beneficial quality will survive better and
reproduce more, increasing the proportion of those traits in the population. As those traits become more
common, the population overall changes as it becomes better suited to its environment. This is evolution.
Artificial selection is a similar process, but people select the traits that continue instead of "survival of the
fittest." Traits favored by people may or may not be directly beneficial to the animal, but it doesn't matter as
much because these are the animals that people choose to breed. For example, artificial selection for
increasingly large heads in bulldogs means that many bulldog puppies cannot be born on their own, and require
This is not a trait favored in nature, but with the aid of veterinary medicine, it's possible to select for an animal
with these qualities. Artificial selection is the process that allowed humans to naturally breed fruits, vegetables,
and livestock to be bigger, juicer, breed faster, etc.
Where did all the diversity in dogs come from? Studies conducted on Russian fox farms may reveal the answer.
In the 1950s, Russian scientist Dmitri Belyaev began selectively breeding captive silver foxes on a fur farm
with the idea of making them tamer and easier to handle. He carefully chose foxes that were more tolerant of
humans than the rest. Over a few generations of breeding, the foxes became tamer. However, they also
developed strange coat colors and other odd characteristics, such as floppy ears and curly tails. Belyaev's new
foxes barked more, and the females came into heat more often and younger than their ancestors. In fact,
Belyaev's foxes had exactly the same kinds of qualities we see in dogs, but never in wolves. Why should
selecting for tamer animals also create individuals with all these unusual physical qualities?
The theory is that by selecting for individuals that are friendlier and less suspicious of humans, you also affect
some aspects of the developmental process. You're breeding animals that are more puppy-like in their behavior.
Linked to the genes that control this extended puppyhood are other genes that affect coat color. In addition,
when you lengthen the amount of time behavioral development takes, you disrupt some other types of
Based on the research on foxes, some 20,000 – 30,000 years ago the natural selective pressure on early human
village wolves to be tamer might have simultaneously created a population of wolves with all kinds of odd
characteristics. Now you have a group of animals that are smaller and friendlier than wolves and that come in
lots of colors. This is the point at which researchers like the Coppingers say that humans began adopting
puppies and favoring some attributes over others, using artificial selection to create different types of dogs.
It is in this way that dogs have been both naturally and artificially selected. Naturally because of natural
genetic variations among wolves which lead to greater acceptance by humans, and artificially as humans
specifically bred dogs for their shape, color, size, and behavior for thousands of years. Humans are responsible
for over 200 purebred species of dog, from Chihuahua to Great Dane; the variation in traits among dogs are
many. The only thing that identifies dogs as the same species is their ability to create offspring through sexual
reproduction. When two dogs of a different pure-breed have offspring, the offspring are referred to as hybrids,
or mutts.
Reading Questions:
1. Summarize Natural Selection in your own words.
2. Why is genetic diversity (variation) important to Natural Selection?
3. In what ways do humans use artificial selection for our benefit?
4. Write a CEL paragraph that indicates whether you are for or against artificial selection. Make sure
to answer how the modern dog is a product of both natural selection and artificial selection? Cite
information from the text and the film.