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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 example—so 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.
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The
primary purpose of the passage is to:
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