SAT Critical Reading - Question #3
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The following passage is an excerpt from an article written by a zoologist.
Ever since the giant panda was discovered in the middle of the nineteenth century, a controversy has raged over its relation to other species. While the general public tends to view the giant panda as a kind of living teddy bear, biologists have not been sure how to classify this enigmatic animal. At different times, the panda has been placed alternately with bears in the Ursidae family, with raccoons in the Procyonidae family, and in its own "Ailuropodidae" family. Biologists who classify animal species have tried to categorize the panda according to whether its traits are "homologous" or merely "analogous" to similar traits in other species. Homologous traits are those which species have in common because they have descended from a common ancestor. For instance, every species of cat has the homologous trait of possessing only four toes on its hind foot, because every member of the cat family descended from a common feline ancestor. The greater the number of such traits that two species share, the more closely they are related. A cat and a lion have more homologous traits between them than a cat and a human, for exampleso cats and lions are more closely related, biologically.
What appears to be a homologous trait may only be an analogous trait, however. An analogous trait is a trait that two species have in common not because they are descended from a common ancestor but because they have different ancestors that developed in similar ways in response to their environment. The eagle and the butterfly, for example, both possess the trait of wings.
It is often difficult to distinguish homologous from analogous traits, which is why analysis of the panda's traits has raised more questions than it has answered. The panda may look like a bear, for example, but its appearance could just be an analogous trait; the panda also has many traits that bears do not possess. It has a more massive jaw than a bear since its diet consists primarily of bamboo. Giant pandas also have thumbs which are used to strip leaves from bamboo stalks. Bears do not have a similar digit. Furthermore, most bears growl or roar, but giant pandas bleat. Progress has been made on the panda mystery only through examination of its genetic material. Using a technique known as DNA hybridization, biologists have demonstrated that the giant panda is indeed a relative of the bear, but the relationship is distant indeed. Their most recent common ancestor lived over fifteen million years ago.
The author refers to "the eagle and the butterfly" in the third paragraph chiefly to illustrate that
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