Video Transcript
A student is going to use a dichotomous key to identify and classify organisms. How could they use it correctly? (A) They should use common observable characteristics when differentiating between
the organisms at the beginning. (B) They should avoid using morphological characteristics as much as possible. (C) They should consider more than two characteristics at a time. (D) None of the answers are correct.
This question asks us about dichotomous keys which are important for identifying
organisms from a defined group of specimens.
Here, the word “specimen” just means a whole or part of an organism that has been
collected for display or analysis. To illustrate the concept of dichotomous keys, we can talk about a theoretical one
that involves different vertebrates, which include a broad group of organisms that
contain a backbone. Then, at each juncture of the dichotomous key, we can separate our specimens by their
morphological characteristics, or observable traits, such as whether they possess
hair or not.
You’ll notice that we only focus on two characteristics at a time, which is easy to
remember since the prefix di- in dichotomous key means two. These characteristics are usually contrasting, meaning that one side of the juncture
could be the presence of a trait, while the other side is the absence of the same
trait.
So, in the first juncture, the vertebrate either has hair, which identifies it as a
mammal, or it doesn’t and we’re taken to the next set of descriptions. This could separate mammals from other vertebrates that do not have hair, such as
birds and fish. It’s best to start with common characteristics that are less specific, which then
become more specific as we work our way down the dichotomous key.
Now that we understand more about dichotomous keys and how they’re used to identify
different specimens, let’s try to answer our question about how dichotomous keys can
be used correctly.
Answer choice (A) states that they should use common observable characteristics when
differentiating between the organisms at the beginning. This is true and seems like the correct answer, but let’s make sure and eliminate the
other choices first.
Answer choice (B) states that they should avoid using morphological characteristics
as much as possible. This is incorrect because dichotomous keys are created using observable traits. Answer choice (C) states that they should consider more than two characteristics at a
time. This is also incorrect since dichotomous keys only focus on two traits.
Therefore, the statement that describes how to use a dichotomous key correctly is
given by answer choice (A). They should use common observable characteristics when differentiating between the
organisms at the beginning.
