If a method of classification is used that compares similarity in shared ancestry, this method is using ________ as the basis of classification.

The method is using Phylogeny as the basis of classification.

Step 1: Understand the question - The question is asking about a method of classification that is based on shared ancestry.

Step 2: Identify the method - The method that classifies organisms based on their evolutionary history and shared ancestry is known as Phylogenetic classification or Phylogeny.

Step 3: Answer the question - Therefore, if a method of classification is used that compares similarity in shared ancestry, this method is using Phylogeny as the basis of classification.