Benchmark 5A
The Living Environment: Diversity of Life
Grades 6-8, page 104

Similarities among organisms are found in internal anatomical features, which can be used to infer the degree of relatedness among organisms. In classifying organisms, biologists consider details of internal and external structures to be more important than behavior or general appearance.

NSES Content Standard C 
Life Science: Diversity and adaptations of organisms 
Grades 5-8, page 158 
Millions of species of animals, plants, and micro-organisms are alive today. Although different species might look dissimilar, the unity among organisms becomes apparent from an analysis of internal structures, the similarity of their chemical processes, and the evidence of common ancestry. 

NSES Content Standard C 
Life Science: Biological evolution 
Grades 9-12, page 185 
Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities which reflect their evolutionary relationships. Species is the most fundamental unit of classification.