Viruses are not included in standard phylogenetic trees because they are not made up of cells. Their acellular nature makes it difficult to determine where they would fit into a tree of life that organizes evolutionary relationships based on cellular structures.

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The representation of the tree of life has continually evolved to reflect new scientific discoveries in biological diversity. It progressed from Carolus Linnaeus's $${}1758$$ two-branch model, to Ernst Haeckel's $${}1866$$ four-kingdom model, and to Robert Whittaker's $${}1969$$ five-kingdom tree, all based on observable traits. In the $${}1970$$s, the advent of molecular genetics allowed Carl Woese and George Fox to propose a genetics-based tree, leading to the creation of the three-domain system.

Evolution of the Tree of Life

https://openstax.org/details/books/microbiology/

Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

Microbiology is produced through a collaborative publishing agreement between OpenStax and the American Society for Microbiology Press. The book aligns with the curriculum guidelines of the American Society for Microbiology.

OpenStax Microbiology Textbook

In $${}1969$$, American ecologist Robert Whittaker proposed a phylogenetic tree that expanded upon previous models by adding a fifth kingdom, Fungi. Whittaker's five-kingdom tree became the standard phylogeny for many years.

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