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← Theoretical B

Q1 — Dichotomous Key Branching & Taxonomic Resolution

Theoretical B Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Taxonomic Keys & Identification

Dichotomous keys are essential tools in taxonomy. They consist of a series of statements, each highlighting a distinct morphological characteristic that may be true or false depending on the organism under observation. As the organism is compared with these statements, the key systematically directs the user through a decision tree, ultimately leading to a specific taxonomic classification.

A three-level dichotomous decision tree of circles connected by branches labeled T (true) and F (false), starting from one root circle and ending in four leaf circles at the bottom. Figure 1. Dichotomous tree model with three levels; T - true, F - false.

To determine the species of an unknown organism, referred to as X, you utilize a hypothetical dichotomous key with 10 levels, structurally similar to the one depicted in Figure 1. The final level of this tree provides a conclusive taxonomic identification, with no further branching points. The tree has a complete set of nodes without terminating branches before the final level.

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. Determining the species of an organism with more complex morphological structures than that of organism X using the provided key will take more than 9 steps.
B. The maximal number of species that could be identified with the key is 512.
C. An identification step at level 3 (counting from the top) has four times more possible final taxonomic outcomes than an identification step at level 5.
D. Consider three taxa A, B, and C. If A and B share a more recent common ancestor than A and C, then the paths along the dichotomous key leading to A and B will certainly diverge at an earlier step than the paths leading to A and C.
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Question reproduced from IBO 2024, Theoretical Exam Part B, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF