16: Systematics

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Biodiversity and Systematics

Biodiversity refers to the diversity of living organisms. Botany, rather than strictly plant biology, includes the study of many different groups of organisms. This section begins by discussing the organization (classification) and naming (taxonomy) of these groups based on their evolutionary history and genetic relatedness. The following chapters explore the wealth of diversity, beginning with the unicellular prokaryotes and acellular viruses. The other chapters cover eukaryotic groups that are often distinguished based on life cycles, morphology, nutritional mode, and cellular composition.

Fungi, officially classified in the same group as plants until 1969, are heterotrophic eukaryotes more closely related to animals. Other heterotrophic organisms once thought to be fungi, including the slime molds and water molds, are discussed in the Protists chapter. Protists include an unrelated assemblage of eukaryotes that simply don't fit in as plants, fungi, or animals. Some of these lineages engulfed photosynthetic organisms and gained chloroplasts. These include the brown algae and diatoms from one engulfing event, and the red algae and green aglae from another.

The red and green algae share their ancestors with land plants. These terrestrial, multicellular organisms can be divided into four major groups based on important evolutionary adaptations: bryophytes, seedless vascular plants, gymnosperms, and angiosperms. The angiosperms are the most diverse and most recent lineage, containing nearly 90% of all plant species. Unlike other plants, angiosperms make flowers and fruits.

Mushrooms growing among moss — Figure 12.1: This photo includes many of the organisms mentioned above. The central mushrooms are part of a lichen relationship, *Lichenomphalia umbellifera*, which includes a fungus and a green alga. The mushrooms are surrounded by mosses (of the bryophytes), ferns (from the seedless vascular plants), and a branch from a conifer (belonging to the gymnosperms) lays across them. Among the mosses, microscopic diatoms can likely be found living epiphytically, as well as free-living cyanobacteria. Uncharacteristically, one of the few lineages not pictured here is angiosperms. Perhaps they are all growing upon a fallen tanoak or some other hardwood tree. Photo Credit: Richard Tehan, CC BY-NC.

16.1: Introduction
All life on Earth is related. This section explores how we determine and depict those relationships.
16.2: Organizing Life on Earth
In scientific terms, the evolutionary history and relationship of an organism or group of organisms is called phylogeny. Phylogeny describes the relationships of an organism, such as from which organisms it is thought to have evolved, to which species it is most closely related, and so forth. Phylogenetic relationships provide information on shared ancestry but not necessarily on how organisms are similar or different.
16.3: Plant Taxonomy
16.4: Determining Evolutionary Relationships
Scientists must collect accurate information that allows them to make evolutionary connections among organisms. Similar to detective work, scientists must use evidence to uncover the facts. In the case of phylogeny, evolutionary investigations focus on two types of evidence: morphologic (form and function) and genetic.
16.5: Perspectives on the Phylogenetic Tree
The concepts of phylogenetic modeling are constantly changing. It is one of the most dynamic fields of study in all of biology. Over the last several decades, new research has challenged scientists’ ideas about how organisms are related. New models of these relationships have been proposed for consideration by the scientific community.
16.6: Chapter Summary
A brief summary of the concepts covered in Chapter 2.1

Contributors and Attributions

Chapter thumbnail by Pengo, Public domain, via Wikimedia Commons.