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Why are algae not considered one closely related evolutionary group?

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“mermaid's wine glass” algae — Algae The macroscopic genus of algae known as *Acetabularia* is commonly called “mermaid's wine glass” because of the distinctive umbrella-like shape of the tips of its stalks.

Algae are not considered a single, closely related evolutionary group because they do not form a unified lineage. Instead, the term "algae" describes a diverse collection of predominantly aquatic, photosynthetic organisms that are scattered across various evolutionary branches.12

Scientific research, including molecular studies and electron microscopy, has revealed that different groups of algae share closer evolutionary relationships with non-algal organisms—such as protozoa and fungi—than they do with other algae. For example:123

Green algae are closely related to land plants.1
Chromophyte algae show affinities with certain aquatic fungi and protozoa.1
Euglenophyceae are closely related to specific flagellates, including those that cause sleeping sickness.1

Because these organisms evolved independently or diverged significantly over time, they lack a common ancestor that would define them as a single, natural taxonomic class. Consequently, classifying algae as a single evolutionary group is considered impracticable by scientists.12