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Arthropod Instruct Booklet 2012

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Arthropod Instruct Booklet 2012 1 Understanding Evolutionary History: An Introduction to Tree Thinking Laura R. Novick Kefyn M. Catley Emily G. Schreiber Vanderbilt University Western Carolina University Vanderbilt University Version 1 © February, 2009 Version 2 © February, 2010 Version 3.1 © August, 2010 Version 3.2 © August, 2012 Basic Concepts Introduction Imagine the evolutionary relationships among everything that has ever lived being shown in a single diagram. If life evolved only once (and there is no evidence to the contrary) and if we accept the tenet of “descent with modification” (Darwin, 1859), then such a diagram is theoretically possible. This diagram—an evolutionary tree—would be incredibly large, and it would contain an unimaginable amount of information that would be of unprecedented value to science. Such a tree would show the evolutionary history of every type of living thing. Because it is impossible to know everything that lived in the past, let alone their precise relationships, science has developed a methodology called cladistics that can be used to reconstruct the evolutionary history of taxa based on observable and testable evidence (Hennig, 1966; Thanukos, 2009). A taxon is any taxonomic category ranging from a species (e.g., blue jay) to a higher-order group (e.g., birds, amniotes, vertebrates). The plural of taxon is taxa. It is common in everyday life to group things together that are similar in some way. Of course, there are lots of ways to determine whether two things are similar. For example, one person might say that a crow is similar to a bat because both are black and can fly or that a dolphin and a shark are similar because they both live in the ocean. Another person might say that a crow is similar to an ostrich and a hummingbird because all have feathers and bird DNA even though they do not look much alike; and a dolphin is similar to a bat and a lion because all have an amniotic egg and nurse their young. Cladistics proposes to group taxa together based on a special type of similarity called a synapomorphy, which is a shared, derived character. A character is considered to be derived if (a) two or more taxa inherited it from their most recent common ancestor and (b) the ancestor of that ancestor did not possess that character (i.e., the character appeared for the first time in the most recent common ancestor of the taxa in question). For example, having a skull is a synapomorphy that defines fish, amphibians, reptiles, and mammals as belonging to a single group, labeled Craniata. This character was new to the most recent common ancestor of these taxa; the most recent common ancestor of the Craniata group plus the echinoderms (starfish, sea urchins, etc.) did not have a skull. Again, the critical difference between a synapomorphy and the more general notion of similarity is the concept of a character not only being shared, but also being derived. This simple technique for defining biologically-meaningful groups has proved to be a powerful organizing and predictive tool in modern biology (e.g., Thanukos, 2009). 2 Cladogram Structure Using the principles of cladistics, biologists depict evolutionary relationships among taxa in a type of hierarchical branching diagram called a cladogram. In a cladogram, taxa are grouped into levels based on most recent common ancestry. Figure 1 shows a very simple cladogram involving only three taxa: lizards, bears, and felines. Cladograms can be read from the top down or from the bottom up. In Figure 1, the arrow shows historical time moving from the bottom of the cladogram to the top. In the past, scientists relied on observed similarity (shared characters) as the basis for grouping taxa. “Similarity was similarity,” and there was no way to determine whether it was the result of independent evolution or a shared most recent common ancestor (MRCA). With independent evolution, multiple taxa share a character because that character evolved separately multiple times. In contrast, when the shared character is a result of a shared MRCA, the character evolved only one time in the MRCA of the taxa in question, thus providing solid evidence to support grouping those taxa. As noted earlier, the cladistic approach is to group taxa only if their shared characters are synapomorphies—that is, are a result of a MRCA. The cladogram in Figure 1 groups bears and felines together because they share a MRCA that possessed the novel (derived) character of having body hair. Thus, body hair is a synapomorphy that defines the group consisting of bears and felines (i.e., mammals). Lizards do not have body hair because that character was newly derived in the MRCA of bears and felines. Farther back in time, though, lizards, bears, and felines do share a MRCA, one that evolved the novel character of possessing an amniotic egg (i.e., an egg with inner membranes). Thus, these three taxa all have an amniotic egg because they share a most recent common ancestor at this point (see Figure 1). Amniotic egg is a synapomorphy that defines the group consisting of lizards, bears, and felines (biologists call this group Amniota). These nested levels of most recent common ancestry imply a time arrow running from earlier in historical time at the bottom of the cladogram to more recent historical times moving toward the top of the cladogram, as shown in Figure 1. It is very important to appreciate the difference between a common ancestor of two or more taxa and the most recent common ancestor (MRCA) of those taxa. Saying that two taxa share a common ancestor provides no useful information about similarities between those taxa because all life ultimately shares a common ancestor. That is, every taxon has a common ancestor with every other taxon if one goes back far enough in evolutionary history. Thus, common ancestors per se are not informative when trying to reconstruct the evolutionary history of a group of taxa. MRCAs, on the other hand, are highly informative because they contain exactly the information biologists need to piece together the tree of life. MRCAs, defined by their synapomorphies, provide the evidence needed to create the hierarchical 3 structure of the tree of life. The pattern of most recent common ancestry among a set of taxa defines the structure of the cladogram depicting the evolutionary relationships among those taxa. An important principle for understanding cladogram structure is the 3-taxon statement. This principle states that given a set of three taxa, two taxa have to be more closely related to each other than either is to the third taxon. The cladogram in Figure 1 shows that bears and felines are more closely related to each other than either of these two taxa is to lizards. Therefore, using parenthetical notation, we group bears and felines together first, writing (bears + felines). The entire 3-taxon statement, then, can be written in parenthetical format as (lizards + (bears + felines)). The cladogram in Figure 1 shows evolutionary relationships among only three taxa. As you might expect from the earlier discussion of the entire tree of life, cladograms can be constructed to depict such relationships among any number of taxa. These larger, more complicated cladograms are compiled from a series of 3-taxon statements and their supporting MRCAs. This is why the 3-taxon statement is critical for understanding cladogram structure. This is illustrated in Figure 2. Each new 3- taxon statement is colored in red in the cladograms. The cladogram in Figure 2b adds the 3-taxon statement (fish + (lizards + bears/felines)) to the original 3-taxon statement (lizards + (bears + felines)) shown in the cladogram in Figure 2a. In the cladogram in Figure 2c, felines are divided into tigers and lions, which yields the new 3-taxon statement (bears + (tigers + lions)). Figure 2d puts these two additions to the original 3-taxon statement together to create a cladogram depicting evolutionary relationships among five taxa. The numbers and corresponding black bars on the cladograms in Figure 2 represent synapomorphies. Notice that each synapomorphy is found in the same position in all four cladograms and supports a group containing the MRCA and all the taxa above it. 4 Practice What You’ve Learned #1 Answer the questions below without referring back to the previous pages. After you’ve completed the problems, you’ll have a chance to check your answers against the correct answers. If anything is still unclear at that point, you may return to the earlier pages to revisit a topic. 1. Taxon A is more closely related to Taxon C than to Taxon B. True False 2. Taxon D is more closely related to Taxon F than to Taxon C. True False 3. Taxon G is more closely related to Taxon H than to Taxon E. True False 4. Taxon C is more closely related to Taxon F than to Taxon H. True False 5. Is (D + (E + F)) a valid 3-taxon statement? If not, write the correct statement. Yes No 6. Is (C + (D/E + F)) a valid 3-taxon statement? If not, write the correct statement. Yes No 7. What numbered character was possessed by the MRCA of Taxon D and Taxon E? ____ 8. What numbered character was possessed by the MRCA of the taxa C, E, and G? ____ 9. What numbered character was possessed by the MRCA of Taxon A and Taxon G? ____ STOP! DO NOT TURN THE PAGE UNTIL YOU HAVE FINISHED “PRACTICE WHAT YOU’VE LEARNED #1” 5 Answers to Practice what you’ve learned: Check your answers on the previous page with the correct answers below. Note that an explanation for each answer is printed in blue.
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