Final answer:
Protists are grouped into six supergroups based on the concept of a single common ancestor, not just on recent common ancestry or similarity like other organisms. This is because protist classification is complicated by convergent evolution, which can cause genetically unrelated organisms to appear similar.
Step-by-step explanation:
Differences in Grouping Protists
The process taxonomists use to group protists differs from the grouping of other organisms. Typically, organisms are grouped based on recent common ancestry and similarities, reflecting evolutionary relationships. However, protists are grouped into six supergroups that not only include protists but also animals, plants, and fungi. This organization is based on the evolutionary concept that all members evolved from a single common ancestor and share certain features of the eukaryotic cell structure, such as the nucleus, the cytoskeleton, and endosymbiotic organelles. Classification of protists is challenging due to the phenomenon of convergent evolution, where unrelated organisms independently develop similar traits in response to similar environmental pressures.
Despite the goal of reflecting evolutionary relationships in the taxonomic hierarchy, protist classification remains a complex task. The presence of analogous structures due to convergent evolution complicates the interpretation of these relationships. As a result, genetic data has become crucial in clarifying the relationships among protists, which has led to the current organization into the six supergroups, although some of these groups' monophyly still lacks evidence.