Final answer:
To determine if a species is part of a clade, scientists use cladistics to group organisms by common ancestry, employing phylogenetic trees or cladograms that illustrate shared derived characteristics and rely on comparing traits of an ingroup to an outgroup to ascertain evolutionary relationships.
Step-by-step explanation:
To determine if a species is part of a clade, scientists use cladistics, which is a method of analysis that groups organisms by common ancestry. Monophyletic groups, or clades, are formed by a species and all its descendants that stem from a single ancestor. These groups can be identified through the study of shared characteristics. One begins by comparing homologous traits across species to infer evolutionary relationships. Traits that have arisen in the ancestor and are present in all its descendants are considered shared derived characteristics, and help establish clade memberships.
Clades are represented in diagrams known as phylogenetic trees or cladograms. To build a phylogenetic tree, scientists use morphological and molecular data to distinguish homologous from analogous traits. By comparing these traits between an ingroup—the set of organisms under study—and an outgroup—organisms that diverged before the ingroup—scientists can determine character polarity and thus evolutionary pathways. Technologies such as DNA sequencing and computer programs like BLAST assist in this process by providing molecular evidence for relationships. Maximum parsimony is often applied, which calls for the simplest evolutionary pathway with the least number of steps.