A Phylogenetic Perspective for the Cladistically Challenged
image: Society of Australian Systematic Biologists


Author:
Sue Johnson, Monona Grove High School, Monona WI

Grades:
9-12

Suggested Time:
45-60 minutes

Students' Prior Knowledge:
The day prior to the actual activity, each student will take home drawings of each of six skulls; Alligator, Amia, dolphin, komodo dragon, mosasaur and snake. Their assignment is to decide which of the other five is more closely related to the mosasaur, based on the features of the skulls, and to provide the reasons for their choice(s). Although, on the day of the activity, each student's response may not be examined in a pre-activity class discussion, the written responses can be useful in at least two ways. First, they can facilitate a brief class discussion before the activity begins and, second, they can be collected for further consideration.

Background:
Information: When early explorers gathered organisms from around the globe, an obvious difficulty arose. How could these many types of organisms be classified so that working with them would not be such a daunting task? The first major classificatory scheme was created by Linneas and relied on classification according to similarities and differences in the key features of those organisms.

But, if we classify organisms with a scheme such as this, and ignore their evolutionary history, we miss the important aspects of the origin of those similarities and differences. Using this perspective in our classificatory scheme, organisms fall into a particular group because they share a common ancestor. Thus, all organisms that have descended from a common ancestor are grouped together, and called a clade. This type of classification is called cladistics and the visual representation of a cladistic scheme is called a cladogram.

One major difference between a cladistic approach to classification and that of a traditional Linnean approach is that a type of organism can never evolve out of their clade. Birds, for example, are thought to be descendents of dinosaurs and would be grouped with them using a cladistic approach. In a Linnean system, however, birds are in the Class Aves while dinosaurs are placed into the Class Reptilia. As one can see, a cladistic scheme is helpful in reminding the user of the ancestry of groups of organisms. Whether you would use a Linnean perspective or a cladistic one depends on what you plan to use your classification scheme to do. If you just want to group organisms with similar characteristics to make them easier to work with, a simple Linnean system is adequate. But if you want to take into account evolutionary origins, or who is related to whom a cladistic approach is required. With the obvious advantages of a cladistic approach, it is not surprising that the Linnean system may soon have a place only in the history of science.

In cladistics, the features that vary from one organism to another are called characters (much as we use the term traits in genetics). Variations in those characters can be called character states. If an organisms character state remains unchanged from its ancestors it is described as primitive. If, however, the character state has changed, it is called derived. Groups of organisms that share common ancestry also share derived characteristics.

A basic rule in cladistics is to follow the general principle known as Ockham's Razor. This principle states that when there are multiple explanations for an event, the simplest (one that is based on the fewest assumptions) is best. For example, if two groups of organisms share a specific character we assume that the character only evolved once, but by putting those two groups in closer proximity on our cladogram (to fit that character) we may disrupt the grouping that we would make based on other characters. So cladistics, like many things in our daily lives, is a matter of compromise. One way to assess a cladogram is by the number of changes that would have had to occur if we place groups in certain relative positions. The fewer changes the better!

Deciding where to place various types of organisms in this type of classificatory scheme is made more difficult when two different groups of organisms share a common characteristic not because they have a common ancestor, but rather because the characteristic evolved on several different occasions. But, without challenges such as this, science might not be as interesting!

Materials:

• diagrams of skulls
• real skulls or models of skulls

Student Activity:

1. Look carefully at the drawing of the Mosasaur skull. Next compare it to the drawings of the skulls of the other five organisms. Which one(s) of the five do you think is(are) more closely related to the Mosasaur.

mosasaur	Amia
dolphin	Komodo Dragon
snake	Alligator

full-size PDF files with skull diagrams:
page 1: Amia-dolph-mos.pdf
page 2: Snake-alli-komo.pdf

2. Ideas in science, such as your answer to the previous question, should be supported by evidence. What evidence do you see in the skulls that support your idea?

Procedure:
A mosasaur is a marine organism that lived approximately 80 million years ago. It was a fierce predator that reached lengths of 20 to 30 feet, or even longer. Although mosasaurs no longer exist, other than as fossils, they share characters with types of organisms living today and therefore can be considered to have common ancestry.

You will be given a second group of drawings of the skulls (and possibly actual skulls or reproductions of skulls) of a mosasaur, along with those of a snake, and Alligator, a dolphin, an Amia, and a Komodo dragon (or a monitor lizzard). In this second group of drawings particular bones that are similar in origin are colored with the same color. Carefully study the drawings (and also any three dimensional skulls that are available) to attempt to determine which one is the skull of a representative of a group of organisms most closely related to the mosasaurs. Do any of the other groups show similar characters in their skulls? Your task is to choose one of the groups represented here as being more closely related to the mosasaur. The most important part of that task will be to describe the reasons for your choice. Your description will be assessed based on its clarity and reasoning.

Name ________________________
Date__________________________

I have chosen the _______________________ as the representative of the group most closely related to the mosasaurs.

My reasons for doing so are:

Teacher Notes:
This activity can easily be done without actual skulls or skull replicas, but having a three dimensional representation to study could be both helpful and fascinating. There are a number of sources for skulls and/or skull replicas. Some of those sources are provided below.

Acorn Naturalists
17821 East 17th Street Suite 103
Tustin, CA 92780
Phone: 714-838-8888
Website: http://www.acornnaturalists.com/

The Bone Room
1569 Solano Avenue
Berkeley, CA 94707
Phone: 510-526-5252
Website: http://www.boneroom.com/
Email: evolve@boneroom.com

Tonys Trading Post
3 West Superior Street
Duluth, MN 55802
Website: http://www.tonystradingpost.com/squamata.htm
Email: sales@tonystradingpost.com

Skulls Unlimited
Phone: 800-659-SKULL
Website: http://www.skullsunlimited.com/
Email: info@skullsunlimited.com

Vocabulary:
cladistics, clade, Ockhams Razor, character, character state, derived character, mosasaur

Extension Activities:
This activity can be done without having the students construct a cladogram for the six types of organisms, but if a more detailed look at cladistics is preferred, having the students construct a cladogram would be a helpful way to gain a deeper understanding. Other cladograms could be constructed with a variety of different types of organisms and used to assess their understanding.

Wisconsin State Science Standards:
A.12.2
http://www.dpi.state.wi.us/standards/scia12.html
Show how conflicting assumptions about science themes lead to different opinions and decisions about evolution, health, population, longevity, education and the use of resources, and show how these opinions and decisions have diverse effects on an individual, a community, and a country, both now and in the future.

A.12.4
http://www.dpi.state.wi.us/standards/scia12.html
Construct arguments that show how conflicting models and explanations of events can start with similar evidence.

A.12.6
http://www.dpi.state.wi.us/standards/scia12.html
Identify and replace inaccurate personal models and explanations of science-related phenomena using evidence learned or discovered.

A.12.7
http://www.dpi.state.wi.us/standards/scia12.html
Re-examine the evidence and reasoning that led to conclusions drawn from investigations, using the science themes.

B.12.5
http://www.dpi.state.wi.us/standards/scib12.html
Explain how science is based on assumptions about the natural world and themes that describe the natural world.

C.12.5
http://www.dpi.state.wi.us/standards/scic12.html
Use the explanations and models found in the earth and space, life and environmental, and physical sciences to develop likely explanations for the results of their investigations.

C.12.6
http://www.dpi.state.wi.us/standards/scic12.html
Present the results of investigations to groups concerned with the issues, explaining the meaning and implications of the results, and answering questions in terms the audience can understand.

F.12.5
http://www.dpi.state.wi.us/standards/scif12.html
Understand the theory of evolution, natural selection and biological classification.

F.12.6
http://www.dpi.state.wi.us/standards/scif12.html
Using the concepts of evolution and heredity, account for changes in species and the diversity of the species, include the influence of these changes on science, e.g., breeding of plant and animals.

G.12.2
http://www.dpi.state.wi.us/standards/scig12.html
Design, build, evaluate and revise models and explanations related to earth and space, life and environmental, and physical science.

H.12.6
http://www.dpi.state.wi.us/standards/scih12.html
Evaluate data and sources of information when using scientific information to make decisions.

H.12.7
http://www.dpi.state.wi.us/standards/scih12.html
When making decisions, construct a plan that includes the use of current scientific knowledge and scientific reasoning.