To distinguish between organisms that have membrane-bound nuclei (eukaryotes) and those that do not (prokaryotes), Robert Whittaker's tree of life introduced a level of categorization above the kingdom level, known as the empire or superkingdom level.

Empire or Superkingdom Level

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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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

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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.

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Whittaker's Five-Kingdom Tree

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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