Operational Taxonomic UnitsCharacter StatesBio 3A Lab: Classification and Phylogenetic Systematics Objectives 1. Understand the difference between a system of classification and a phylogeny, and how they are related. 2. Understand the basic method used in cladistic analysis. 3. Generate simple phylogenies based upon this method. 4. Compare traditional and cladistic classifications. Systems of Classification Classification of "things" seems to come naturally to the human species. You do it on a regular basis. Enter any kitchen and you'll find the pots and pans grouped together, the spices in a single cabinet, the dinnerware stacked neatly by type. Within a certain category there are sub-categories. Open the silverware drawer. The overall category: Eating Utensils. Within this category we find sub-categories, such as spoons, forks, knives, and chopsticks. Look closer and each of the sub-categories are further subdivided. Within spoons there are soupspoons, teaspoons, sugar spoons, serving spoons. This type of classification system is called a hierarchy. You may have used a similar method to organize your thoughts before writing a paper (the outline), or, if you are familiar with organizational structure of your computer, think of the root directory. In the culinary example above, we could organize our objects in the following way: Major Category: Kitchen Items 1st Sub-category: Silverware 2nd Sub-category: Spoons Final Sub-category: Soupspoons (or any of the others) A natural outcome of this kind of thinking was the sorting of plants and animals into hierarchical groups. You should know that this kind of sorting was originally begun before the concept of evolution was elaborated. Carl von Linne' (a.k.a. Carolus Linneaus) was one of the first to put forth a coherent method for organizing organisms. He was, in fact, a botanist, and his original work dealt with plants. Later, he worked on animals. In Linne's time there was a real need to organize the plants and animals in some coherent way. People were traveling extensively and bringing back or reporting hundreds (and soon thousands) of new organisms. Museums began to fill up, and you had to handle all of this new material. Linne', and those who followed, gave us a framework for sorting out the potential mess. Thus, the first plant and animal classification systems were created primarily for convenience. In other words, for the convenience of the user, just like your kitchen classification system. In using such a system, we usually work from the "top down." For 11/19/2006 Classification Lab, Page 1 of 9instance, you purchase a new type of soupspoon. In finding the proper place for it, you will a) enter the kitchen, b) go to the silverware drawer, c) look at the spoon area, d) place the new soup spoons into the soup spoon sub-category. Another example is found in the library. To find a reference to the moons of Jupiter, you might have to drop through several hierarchical categories such as science, astronomy, solar system, planets, Jupiter, and moons. In fact, learning to use the library means learning the librarian's classification system. Creating systems and classifying organisms is called taxonomy (the process of grouping, a taxon is a group). The idea as you can see is probably as old as the human brain. Following the growth of evolutionary theory, it became clear that a classification system could do much more. It could indicate the evolutionary relationships between organisms. Classifying organisms based upon relatedness is called systematics, and on the surface it may appear rather straightforward, even simple. In some cases, it may be; for instance, you probably have no trouble grouping birds together as a single related taxon. However, when the first specimens of the platypus were sent to the British museum, taxonomists were unsure whether they were birds or mammals. Today, heated debates rage on concerning the relationships of various groups of organisms. Moreover, there is considerable controversy over how we should go about quantifying our measurements of similarity. When I show you a bird, you look at it, and create a quick mental "scorecard" of characteristics. The first character that you note is probably the presence of feathers. Next, you might look at the characters of the feet or beak, or color patterns on the body. When you have assessed the suite of characters, you do a quick mental calculation, and decide that it is a cardinal. At this time, there are a few competing methods used in systematics. The major competitor is called cladistic analysis. In a nutshell, cladistic analyses attempt to group taxa based upon recently evolved (derived), shared characteristics. The attempt is to determine the common ancestry of the groups. The method is, by far, the most rigorous and objective of the systematic methods. Cladograms As we postulate relatedness, we can begin to illustrate in a diagram. In this case, the diagram is called a cladogram. C B A Figure 1. Sample cladogram with three operational taxonomic units, A, B, and C In Figure One, group A has more derived characters in common with group B than with group C. A and B are more closely related to each other, than to group C. As you might guess, you could draw cladograms for all sorts of things; and the single most important part of the analysis would be the selection of characters. In the "real systematic world" this is touchy business. The characters chosen must be homologous; that is they must represent common evolutionary origins. For instance, looking at the characteristics of the humerus in three species of higher primates would be perfectly acceptable, as the humerus is homologous in this group. However, characters of the wings of bats and flies is clearly disallowed, since those structures are clearly not related (they are analogous, similar 11/19/2006 Classification Lab, Page 2 of 9function, different origins). Next, the character must be evolutionarily new to the group (or groups). So far we have called such characters "derived", but cladists have their own nomenclature. They call such evolutionary novelties apomorphies. Primitive characters are called plesiomorphies. You will recall that the method of grouping was based upon shared, derived characters