Part A. Introduction: Reproductive Isolation and Speciation
If dogs and wolves can interbreed and produce fertile offspring, why are they considered different
species? Remember, many books give the definition of species as an organism that is morphologically
different from other organisms like it. Some other definitions say that species need to be reproductively
isolated from each other. Reproductive isolation means that because of physical, geographical, or
behavioral differences, two similar organisms do not reproduce fertile offspring.
The problem with deciding whether something is a separate species can be confusing because there is no
single accepted definition of the term (Wayne and Gittleman 37). The biological species concept by Ernst
Mayr takes into account different isolating mechanisms besides the ability to produce fertile offspring.
First of all, members of a species might not naturally mate and meet in nature. Their habitats are different,
so that even if they are physically capable of reproducing in nature, they do not because they usually do
not come in contact with each other. This could be called seasonal or habitat isolation. Two species could
also be behaviorally isolated. For example, two similar species of birds might use different mating calls.
Therefore, the two species might not mate because they do not recognize each other's mating signals. A
final type of reproductive isolation, mechanical isolation, is when two species cannot interbreed because
their sexual organs are not compatible. (Strickberger 486).
Morphologically, two species would be considered different if there are measurable differences between
many of their traits such as the size of the skull or width of the teeth. If the distribution of measurements
do not overlap those of another group, the two might be considered different species. This is commonly
called the typological species concept (Mayr 187). "Another widely discussed idea designates a species
based on a unique characteristic not found in other closely related organisms - for example, the upright
posture of humans" ( Wayne and Gittleman 37).
An example of an animal that does not fit in easily in any of these definition is the red wolf. The red wolf
was first described about two hundred years ago and can breed extensively with the coyote and the gray
wolf. It is also very similar physically to the coyote and the gray wolf. Many modern researchers believe
that the red wolf is actually a species produced by the mating of a coyote and the gray wolf. A species
produced by the mating of two other species is called a hybrid and is referred to as a product of
hybridization.
Part B: The Evolution of the Domestic Dog.
About 10,000 to 14,000 years ago the domesticated dog began to live with humans. This was during the
Pleistocene era, during the Ice Age. The earliest fossils of what is believed to be one of the first
domesticated dogs was found along with human remains in a cave in Iraq. During the Ice Age, humans
and dogs were competing for the same food, so it would not be too difficult to imagine an alliance formed
between them (Clutton-Brock 34). "Wolves and late Pleistocene hunters and gatherers undoubtedly came
into contact regularly, since both were social species who hunted many of the same prey items" (Morey
339).
Domestication is defined by Webster's Dictionary as tame or living with humans. There is much more to
the definition of the word as it is used in the phrase, "domesticated dog". Domestication is a "condition
wherein the breeding, care , and feeding of animals is more or less controlled by humans. The process of
domestication involves some biological change in an animal over a period of generations" (Fox 3). Some
scientists believe that dogs are really just wolves that have changed their habits and habitat through their
taming and association with man.
Many scientists like Darwin believe that the domestication of the dog was solely a human choice. But
some scientists believe it was natural selection brought on by a change in the environment, specifically
the scarcity of food. It probably was an adaptive advantage for wolves to be tame enough to associate
with Ice-Age man. If prey was scarce, a dog would benefit for being fed by man and associating with
them (Morey 339). During the Ice Age a new niche evolved for wolves, those who could live with man
and benefit from the arrangement.
Domesticated dogs are very similar to gray wolves. The gray wolf and the dog both have 78
chromosomes. Gray wolves and dogs have only a 0.2 % difference in their DNA and RNA sequences. In
addition, a study comparing thirteen different breeds of dogs and 26 different gray wolves found either
identical mitochondrial DNA sequences or the sequences are only different by the gain or loss of one site
(Wayne 220). The next most closely related animal to the gray wolf is the coyote. The coyote and wolf
differ in 4% of their mitochondrial DNA sequences. In fact, the dog and the wolf are so similar
genetically that most dogs can reproduce viable offspring with wolves.
Domesticated dogs also differ from their most common ancestor, the gray wolf. One difference is their
rate of maturation and another is the breeding season. Gray wolves only have one breeding period per
year, while dogs can go into heat two or three times in a year. In addition, dogs attain sexual maturation in
six to twelve months while it takes wolves and coyotes two years to attain sexual maturation. Finally,
male dogs can produce sperm and are potent all the time while male wolves will produce little or no
sperm outside the breeding season. (Fox 37)
There are other differences between dogs and gray wolves. The dog is smaller overall (with the exception
of a few breeds) and the length of the snout is shorter than in a wolf. The dog has a steeper forehead and a
wider cranial skull. These characteristics of dogs are juvenile characteristics present in wolves when they
are puppies. This phenomenon is called paedomorphosis by scientists (Morey 341). Paedomorphosis is
where juvenile characteristics are maintained in the adult animal. In addition to its puppy-like looks, the
dog has a more slender coronoid process of the lower jaw that has a noticeable overhang not present in
the wolf. (Hemmer 40).
Wolves and dogs are also different in the area of behavior. Wolves can be tamed, but they still have a
wild temperament. Wolves tend to react in an aggressive, defensive way to the unfamiliar while most
domesticated dogs do not. Dogs also vocalize more than wolves. "Wolves and coyotes will bark
occasionally in the wild and other members of the Canidae family will learn to vocalize in captivity but
the deep bark of the large breeds of dogs, the baying of the Bloodhound and the yapping of toy dogs are
products of domestication" (Clutton-Brock 37). In addition, domesticated dogs have learned to be
subordinate to dominant humans. (Morey 339). Finally, adult dogs display behavior that is considered
juvenile behavior in wolves. Therefore, dog behavior and attention seeking action can be considered
similar to the behavior of immature wolves.
In conclusion, dogs and gray wolves are very similar genetically. They can mate and produce viable,
fertile offspring. The domesticated dog has also undergone evolutionary change for the past ten thousand
years. These changes have made the dog morphologically different from the gray wolf and have produced
an animal that occupies a different habitat and employs different behavior than its predecessor.