Studying Species by Examining the Canidae Family and the Evolution of the Domesticated Dog
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Name________________________________________ Bio II --- December 2011 Studying Species by Examining the Canidae Family and the Evolution of the Domesticated Dog 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. Part C. After reading the assignment go back and answer these questions on another sheet of paper. They will display your understanding of the text. Terminology -- Many words that might not be in your vocabulary are defined contextually in the reading. Go back to the reading and find the contextual clues to arrive at a definition for each term provided. reproductive isolation mechanical isolation behavioral isolation fertile Pliocene Paedeomorphosis habitat isolation domesticated hybridization Questions: To help you cement some of the concepts discussed in this reading, answer the following questions in a few sentences. A. What is the biological species concept? B. Why is the idea of determining a species so difficult? C. What is the typographical species concept? D. How does the red wolf fit in the evolution of Canidae and why is it problematic? E. What similarities do dogs and wolves share? Part D: Finally, to show your understanding of the reading and its concepts, write a persuasive essay that explains why you consider dogs to be in the same or in a different species than the gray wolf. Explain what definition of species you are using to determine this and support your position with evidence from the reading. .