USA Biology Olympiad (USABO) Practice Exam

A valid taxonomic group for the described cladogram would include which of the following?

1. Wooly

2. Spider

3. Wooly and Spider

4. Mammals

The correct answer is: Wooly and Spider

In the context of cladistics, a valid taxonomic group, or clade, consists of an ancestor and all of its descendants. This characteristic means that for a group to be considered valid, it must be monophyletic, meaning it includes all parts of a lineage that descend from a common ancestor. In the case of the cladogram described in the question, if both the "Wooly" and "Spider" are derived from a common ancestor, they collectively represent a valid clade. Therefore, including both "Wooly" and "Spider" captures the entirety of their shared evolutionary lineage. This comprehension aligns with the principles of cladistics, where groups that share an evolutionary history, like "Wooly" and "Spider" if they originate from the same ancestor in the cladogram, form a legitimate taxonomic category. In comparison, the other group options, even if valid on their own, do not fulfill the requirement of encompassing the evolutionary relationship shared by both entities fully, which is essential for a valid clade.