2: Biodiversity (Organismal Groups)

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Melissa Ha, Maria Morrow, & Kammy Algiers
Yuba College, College of the Redwoods, & Ventura College via ASCCC Open Educational Resources Initiative

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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 \(\PageIndex{1}\): 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.

2.1: Systematics
By following pathways of similarities and changes—both visible and genetic—scientists seek to map the evolutionary past of how life developed from single-celled organisms to the tremendous collection of creatures that have germinated, crawled, floated, swam, flown, and walked on this planet.
2.2: Prokaryotes and Viruses
Most organisms on Earth, and in fact most of the cells in your body, are prokaryotic. These unicellular organisms are ubiquitous across ecosystems and organisms, involved in every aspect of ecology. Prokaryotes can be divided into two major groups: Bacteria and Archaea. Unlike prokaryotes, viruses are acellular, and so are not considered living. However, they are distinct biological entities with important roles in evolutionary history and the life histories of organisms.
2.3: Fungi
Kingdom Fungi includes an enormous variety of living organisms. While scientists have identified about 150,000 species of fungi, this is only a fraction of the several million species of fungi likely present on Earth. Organisms in this group are heterotrophic eukaryotes that eat by external digestion, then absorption. Fungi can be unicellular (yeasts) or composed of filamentous cells called hyphae, which taken together form a thallus called the mycelium.
2.4: "Protists"
Protists are an artificial group of eukaryotes that are neither animals, fungi, nor plants. They represent the vast diversity of eukaryotic organisms, and thus span the breadth of possibilities with regard to life history traits. They can be heterotrophs or autotrophs, unicellular to massively multicellular (though rarely with any specialized tissue organization), and can be found across ecosystems worldwide. Photosynthesis within protists is the result of multiple separate endosymbiotic events.
2.5: Early Land Plants
Bryophytes were the first group of plants to evolve on land, followed by the seedless vascular plants. These early plants, accompanied by their fungal mutualists and other microbes, transformed the rocky terrestrial landscape into an ecosystem with stratified soils and complex biotic communities. Synapomorphies of bryophytes derive from the challenges of life on land, while those of seedless vascular plants relate to increases in height and opportunities for meiosis (i.e. competition).
2.6: Seed Plants
Seeds represent one of the most important innovations in plant evolution: a protected, nutrient-supplied embryo with the ability to await appropriate conditions for germination. Seeds and pollen allowed plants to limit their reliance on water for completion of their life cycle. The first plants to evolve seeds were the gymnosperms, which grew wider and taller with secondary growth. Angiosperms then improved upon seed dispersal and pollination strategies with the evolution of fruits and flowers.
2.7: Angiosperm Diversity
It is likely that most plants you see are angiosperms. Of the nearly 400,000 species of land plants described, nearly 90% are angiosperms. Angiosperms can be divided into two major groups: monocots and dicots. Dicots can be further divided into basal angiosperm lineages (magnoliids and ANA grades) and eudicots. Monocots produce one cotyledon, while dicots produce two. However, there are other characteristics that can be used to differentiate between these groups.

Attribution

Content by Maria Morrow, CC BY-NC