The Great Clade Race
Based on: D. W. Goldsmith (2003) American Biology Teacher 9: 679-682 Imagine a race through the woods with eight runners (A-H). All the runners start at a single starting line but at various places the course forks, and runners are free to choose either path. To help keep track of the race, each runner carries a card that gets stamped at check -in stations distributed at various places in the course. The following rules apply: Runners are obligated to collect a stamp from each check-in station they pass Each check-in station uses only one stamp No two check-in stations use the same stamp (this rule will be dropped later) All eight runners finish the race carrying their cards, but they each cross a different finishing line. The aim is to use the information in the cards collected from the eight runners to reconstruct the course and the placement of check-in stations. Draw a map of the course. Labeling the starting point, each check-in station (with the symbol of the stamp it uses), and each finishing line (with the letter of the runner who emerges there). When you are done, copy it onto the overhead transparency provided. What principles did you used in generating your map? Compare your map to those generated by other groups. How are they similar? How do they differ? Consider the analogy between the great clade race and biological evolution. What are the biological analogs of: a) The runners b) The check-in stations c) The race course Discuss the following questions: a) Can you infer whether a particular path went right or left? b) Can you infer whether particular segments of the course (between one branch point and the next) are longer or shorter? c) When there were two check-in stations on a segment, can you figure out which came first? d) In the biological case, when is it valid to assume that each “runner” runs the same total distance? e) Is it reasonable to expect that the check in stations will be distributed evenly across the course? f) In the biological case, is it valid to assume that the “runners” receive a blank card at the start and that they always gain, but never lose, stamps?
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Clade Race Two. This is the same as the original clade race with two important exceptions: 1) The runners all the get the same card at the beginning of the race, but it is not necessarily empty – it could have some stamps already added 2) The check-in stations either add a stamp or erase a stamp of a particular shape. To make the challenge easier, only one station on the course acts on each shape: if one station erases a square then no other station either erases or adds square; if one station adds circles, no other station either adds or erases circles, etc. You are given the eight runners‟ cards (“Clade race 2a”) and are told that at the first split in the course, runners A and G went one way, whereas everybody else went the other way. Reconstruct the course. If you had not been told that A and G split at the first branch-point, would this challenge have been possible? Clade Race Three. For simplicity, this case is like the original clade race in that you are told that runners start with a blank card. However, we will now relax the assumption that only one station adds a given shape and will also allow that stations might erase particular preexisting shape. It will remain true, however, that a given station will act exactly the same on all runners that pass. For example, it could be that in the whole course two stations add squares and one station removes squares, but a given station that adds squares will do so to all runners that pass. In evolutionary biology the technical term to describe cases where characteristics are either gained in parallel (convergence) or gained and later lost (reversal) is “homoplasy.” Use the cards („clade race 3a‟) and make your best guess as to the correct course. In the previous cases there was only one possible solution. Is that true this time? Why has homoplasy confused things? Suppose several alternative courses were offered to explain these data. How would you decide which was most likely to be true? Can you think of a general criterion for picking among alternative trees? Can you use it to decide which of the three attached trees is best and which is worst? Using the method you propose, will you always identify the correct tree? Will you know if you have identified the correct tree?
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