Video Transcript
Which of the following characteristics would be most useful for a scientist
classifying organisms into distinct groups? (A) Mode of nutrition, (B) method of excretion, (C) rate of growth, or (D) rate of
cellular respiration.
Biological classification is the organization of species into groups, called taxa,
based on meaningful shared characteristics. These taxa are ranked from the most general, typically a kingdom or domain, to the
most specific, which can be a species, subspecies, or strain.
Taxonomy is the study of classifying and naming organisms. Modern taxonomy is based upon natural classification methods, which are based on
genetic similarity and evolutionary relationships. These methods involve measuring the degree of genetic similarity between two or more
organisms by comparing their DNA sequences. The more DNA two organisms or taxa have in common, the more closely related they
are. For example, a turkey shares more DNA with a sea turtle than it does with a
rattlesnake. This demonstrates that birds are more closely related to turtles than to snakes. A higher degree of genetic similarity also indicates a more recent divergence from a
common ancestor.
In contrast, artificial classification refers to the classification of organisms
based only on observable physical characteristics, for example, the number and shape
of flower petals. It is called artificial because physical characteristics do not always guarantee that
organisms are closely related or that they’re not. Artificial classification is the basis of systems devised by both Carl Linnaeus and
Robert Whittaker. At the time these systems were created, scientists did not understand that the
genetic code within DNA can reveal evolutionary relationships between taxa.
Although they are not the most accurate, artificial classification methods can still
make useful contributions to taxonomy. One particularly useful artificial classification is the presence or absence of a
nucleus. Classifying living organisms with this criteria creates two groups: the nonnucleated
prokaryotes and the nucleated eukaryotes.
Now, let’s return to our question. Take a look at the answer options and see if you can determine whether each one is a
natural or artificial classification method. If you said they are all artificial classification methods, you are correct. When using artificial classification, the observations should be ones that are
relatively stable within an organism. They also should not be overly dependent on changing environmental variables. In light of this, we can eliminate options (C) and (D) because rate of growth and
rate of cellular respiration change frequently depending upon an organism’s age, the
time of day, the season, and whether the environment is ideal or stressful.
Method of excretion is a characteristic that is usually relatively constant in an
organism, but it is not specific enough. There are many different substances that organisms excrete, such as water, salts,
carbon dioxide, urea, and ammonia, just to name a few. So we can rule out option (B) as well.
Mode of nutrition describes how an organism obtains its energy. Although there are a few organisms that can use multiple modes of nutrition, this
criteria typically doesn’t change throughout an organism’s lifetime. It is also quite a specific trait and is well conserved among closely related
taxa.
Therefore, the correct answer is option (A). Mode of nutrition would be the most useful characteristic for classifying organisms
into distinct groups.
