Table 3.

Sample student written responses for part B″ of the PhAT

Part B″. Explain the reasoning that you used in your answer for part B′. In other words, how did you decide that one tree was better than the other?

“Evolution of any kind is unlikely, so by Occam's Razor the tree with the fewest events is most likely. Occam's Razor states that the solution with the fewest assumptions is usually right.”

“The total change in traits in phylogenetic tree I is 8 changes, in tree II it is 7, the principal [sic] of parsimony says the simplest explanation is most likely the correct one, seven changes is simpler than 8 so tree II was chosen.”

“The reasoning is that [in] Tree I the animals would go through 8 evolutionary events and in tree II 9 events. Because animals tend to evolve in the least complex manor [sic], or with as few changes as possible, tree I makes more sense for these phyla because of the less events.”

“Based on the rule of parsimony and Ockham's razor, which states a phenomena should ‘make the least # of assumptions possible,’ I based my decision by counting the evolutionary changes in each figure. Tree I has 8 while Tree II has 7. Since 7 has less, it is simpler and [therefore] better.”

“Tree II has fewer evolutionary events than Tree I and thus follows the rule of parsimony more closely (which calls for fewest assumptions to be made and simplest explanations to be explored first).”