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Constructing Evolutionary Trees – the Basics

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University of Colorado • Campus Box 470 • Boulder, CO 80309-0470(303) 492-8230 • Fax (303) 492-4916 • http://www.colorado.edu/Outreach/BSI HHMI Biological Sciences InitiativeConstructing Evolutionary Trees – the BasicsIntroduction:An evolutionary tree is a map of the evolutionary relationships between organisms. Themore closely two organisms are related, the nearer they will be to each other on the tree.Steps in constructing an evolutionary tree:1.) Select a group of organisms that you want to analyze.2.) For each member of the group, determine some observable characters (traits). Ideally,the characters should have only two possible states. For example, given the character“wings”, there are two possible states – “wings present” and “wings absent”.3.) For each character, determine which state is evolutionarily older (ancestral) and whichis evolutionarily newer (derived). This is usually done by comparison with a moredistantly related organism termed the “outgroup”. Characters shared with the outgroupare likely to be ancestral traits. Characters that differ from the outgroup are likely to havearisen since the group being considered branched from its shared common ancestor withthe outgroup. These differing characters are derived. If two or more organisms share aderived character, this is evidence that these organisms are related.4.) Construct a table showing the states of the characters exhibited by each organism inyour analysis.5.) Construct a matrix showing the number of identical characters exhibited by each pairof organisms in your analysis.6.) Use the matrix to identify the organism most closely related to the outgroup(determined by the number of shared characters with the outgroup). This organism willbe placed closest to the outgroup on the evolutionary tree.7.) Repeat the last step to determine the next organism to place on the tree.Example: Evolutionary tree of insectsCharacterOutgroupCollembola(springtails)Thysanura(silverfish)Odonata(dragonflies)Isoptera(termites)Hymenoptera(bees, ants,wasps)Jointed legsyesyesyesyesyesMouthpartsinternalexternalexternalexternalexternalWingsnonoyesyesyesWing flexionnononoyesyesCompletemetamorphosisnonononoyes2Matrix showing number of characters shared:CollembolaThysanuraOdonataIsopteraThysanura4Odonata34Isoptera234Hymenoptera1234Hypothetical Evolutionary Tree for Insects:* Typically, the outgroup is not represented on the tree, but if it were, it could berepresented on a separate branch, indicating a shared common ancestor with the otherorganisms.As you can see from the tree diagram, the most ancestral group among the insects is theThysanura, which shares the most characters with the outgroup. The most derived groupis the


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