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.
 

 
Benchmark 5A The Living Environment: Diversity of Life
Grades 3-5, page 103
A great variety of kinds of living things can be sorted into groups in many ways using various features to decide which things belong to which group.

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.

Benchmark 5A The Living Environment: Diversity of Life
Grades 6-8, page 104
For sexually reproducing organisms, a species comprises all organisms that can mate with one another to produce fertile offspring.

Benchmark 5A The Living Environment: Diversity of Life
Grades 9-12, page 105
The degree of kinship between organisms or species can be estimated from the similarity of their DNA sequences, which often closely matches their classification based on anatomical similarities.