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.

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.

Viruses in Phylogeny

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.

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A phylogenetic tree, also known as a tree of life, is a branching diagram used to depict the evolutionary relationships among different groups of organisms. In these diagrams, all branches typically originate from a single 'trunk' representing a common ancestor, and organisms are arranged in branching structures based on how closely related they are thought to be.

Phylogenetic Tree

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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.

OpenStax Microbiology Textbook

Modern phylogenetic analyses rely heavily on molecular genetics to determine evolutionary relationships, replacing the subjective observable characteristics used in early models. By comparing nucleic acids (deoxyribonucleic acid [DNA] or ribonucleic acid [RNA]) or proteins among organisms, scientists can standardize taxonomy; the greater the similarity in these macromolecules, the more closely related the organisms are considered to be.

Methods of Phylogenetic Analysis

Ernst Haeckel was a German biologist, philosopher, and physician who expanded upon the two main branches of Linnaeus's tree of life (the animal and plant kingdoms). In $${}1866$$, he proposed the addition of a third kingdom, Protista, for unicellular organisms, and later suggested a fourth kingdom, Monera, for unicellular organisms whose cells lack nuclei, such as bacteria.

Ernst Haeckel

Evolution of the Tree of Life

Robert Whittaker ($${}1920$$–$${}1980$$) was an American ecologist who significantly advanced biological taxonomy by proposing a five-kingdom phylogenetic tree in $${}1969$$. He introduced the Fungi kingdom to the tree of life and created the empire or superkingdom level of categorization to distinguish between eukaryotes (organisms with membrane-bound nuclei) and prokaryotes (organisms without them).

Robert Whittaker

Carl Woese was an American microbiologist who, alongside his collaborator George Fox in the $${}1970$$s, revolutionized biological taxonomy by creating a genetics-based tree of life. By analyzing the gene sequences coding for small subunit ribosomal RNA (rRNA), Woese discovered that archaebacteria (archaea) were genetically distinct from other bacteria and eukaryotes.

Carl Woese

Students can enhance their understanding of phylogenetic trees by exploring the Wellcome Trust's interactive Tree of Life online. This educational website features animations, photos, and detailed information that allow users to select different organisms and observe how they are evolutionarily related to one another.

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