Taxonomy

BIOL 123 Taxonomy & Dichotomous keys- Names & Identities

Objectives: Following this lab you should be able to…

Understand taxonomy and dichotomous keys
Learn how to build a dichotomous key for identification
Learn how to employ a dichotomous key for identification

Overview

Exercise I. Build a dichotomous key to identify your classmates
Exercise II. Build a dichotomous key of isopods
Exercise II. Use you key to identify unknown isopods from our colony

You might use a key to identify closely related taxa like these salamander species.

Exercise I
Exercise II
Exercise III

Carl Linnaeus, known as the "father of modern taxonomy."

Click for more practice.

Exercise I. Practice Creating a Key

In order for scientists to communicate with other scientists about particular species, it is important that the identity, or taxonomy, of the species in question be absolutely certain. The most robust way to ensure this is to perform a genetic analysis on the species. However, much of today's research is conducted in the field, away from sophisticated lab equipment. So how do scientists in the field determine the identity, or taxonomy, of species in question? They use dichotomous keys.

Dichotomous keys are diagnostic tools that use the process of circumscription to narrow down choices about the identity of a species. By using paired, mutually- exclusive choices, called couplets, a scientist can begin to eliminate possibilities for the identity of an unknown species. These couplets offer characteristics that can be observed by scientists. Observable characteristics must be objective, not subjective. For example, when building or using a dichotomous key, one should never encounter wording like, "If the specimen has big leaves, then it is species X", or "If the specimen has little leaves, then it is species Y". The problem with terms like big and little is that they are subjective; big to one person might be little to another. To build this couplet properly, one would use an exact size measurement to differentiate between "big" and "little", such as ".......leaves larger than 15cm, it is species X." What if someone used "green leaves" as a diagnostic tool in a dichotomous key? Would that include teal, turquoise, and lime colored leaves as well? Color is rarely used as a diagnostic tool because it depends entirely on the observer and is not standardized -- it is subjective. Use the table below to develop lists of objective and subjective characteristics.

When building a dichotomous key, it is important to check that your key has one less couplet then the number of species to identify. For example, if you are trying to build a key to help distinguish between 10 tree species in a forest, your dichotomous key should have 9 couplets. Your key should also reflect the true evolutionary relationships of the species involved. In this example, you will see how poorly chosen characteristics can yield poor results when building a dichotomous key for four animals: a black-widow spider, a wolf, a carrion beetle, and a bald eagle.

Notice that these four species were identified using three couplets, one less than the number of animals involved, which is correct. However, this key does not reflect the correct evolutionary relationships between the taxa in question. Also, it incorrectly identifies beetles as "wingless" simply because the wings are not visible from the outside. A closer examination would show that many beetles do, in fact, have wings; however they are tucked under protective covers called elytra, so they are hidden. The following dichotomous key represents the same four species of animals but does so in the correct evolutionary context and using proper diagnostic characteristics.

procedure. Practice making a key

Build a dichotomous key for every member in your classroom today.
A good starting point for your first couplet might be height.
We will work together as a class and write the couplets on the board.
Specific concerns with creating a key for human individuals: Assume that your classmate's physical features (e.g. height, head circumference, number of tattoos, etc.) are fixed and will not change.

Rules:
Trait selection: Real World vs. Best Case (Objectives vs. Subjective)
Group according to evolutionary history
Make big divisions first
Top of couplet (1 or 1a) gets a name (id) or # (go to 4)
Bottom of couplet (1’ or 1b) gets an id OR leave blank and come back
Finish “naming” all of one section before starting on the next
Sections will funnel from large to small…
Once a section is complete start the next one according to the closest couplet (from the bottom)
Total couplets = 1 less than number of objects

Exercsie II. Make your own key of isopods

An isopod is any member of the order Isopoda (class Crustacea), a group of diverse, widely occurring forms including marine, freshwater, and terrestrial species. Most are free-living, but a number of marine species are parasitic on other animals. They are usually inconspicuous. Most of the 10,000 species, which include the pill bug, the sow bug, and the gribble and are from 0.7 to 35 mm (0.28 to 1.4 inches) long; however, Bathynomus giganteus, a marine species of the Caribbean Sea, grows to 35 cm (14 inches).
The body is elongated, rather flat, and somewhat arched. The back is covered by a series of wide, armour-like plates; the thorax, or midsection, has seven segments, the abdomen six. There are usually six pairs of limbs, but sometimes as few as two or as many as eight are found.