MODERN BOTANY — P. RAVEN — 1990

SECTION IV. DIVERSITY

CHAPTER 10. CLASSIFICATION OF LIVING ORGANISMS

Kinship Among Eukaryotes

One of the most frequently used Classification systems divides organisms into five kingdoms: prokaryotes (Monera, or Bacteria) and four eukaryotic kingdoms (Fig. 10-9). Bacteria differ sharply from all eukaryotes, making their Separation into a distinct kingdom fully justified. As for eukaryotes, their interrelationships are much less clear-cut. Most phyla and divisions (equivalent taxonomic categories with different names in zoology and botany, which incidentally causes confusion in the classification of protists) consist entirely of single-celled organisms that exhibit astonishing diversity. These groups, along with some of the related multicellular lineages as will be shown, constitute The Kingdom Protista.

Class="center">Fig. 10-9. One possible scheme of evolutionary relationships among organisms. Solid lines indicate phylogenetic connections, while dashed lines represent the establishment of symbiotic relationships. All eukaryotes originated from a single Lineage of Cells containing Cell/35.html">Mitochondria, which in turn were derived from symbiotic bacteria. This lineage gave rise to the entire diversity of unicellular protists. Protists that entered into Symbiosis with various groups of photosynthetic bacteria gave rise to several distinct lineages of modern Algae. Fungi, plants, and animals evolved from specific groups of unicellular protists.

THE ORIGIN OF Multicellularity

Throughout eukaryotic history, Multicellular Organisms have emerged from single-celled ancestors on multiple occasions. In each such instance, the ancestors—where known—belong to the kingdom Protista. Three groups of multicellular organisms are particularly large and distinctive: plants (Plantae), animals (Animalia), and fungi (Fungi), which we recognize as separate kingdoms. These comprise almost exclusively multicellular organisms, although a few single-celled fungi (Yeasts) are known, likely descended from multicellular ancestors. Plants, animals, and fungi almost certainly arose from different groups of unicellular protists. They differ markedly in their mode of Nutrition: plants manufacture their own food, animals ingest it, and fungi secrete digestive Enzymes that break down food outside the body before absorbing it.

Besides these major multicellular groups classified as separate kingdoms, there are three other divisions that include well-represented multicellular organisms. Brown algae are exclusively multicellular, red algae are almost entirely so, and green algae comprise a vast number of both unicellular and multicellular forms. Because members of these divisions are capable of Photosynthesis and many are multicellular, some scientists argue they should be placed in the plant kingdom. However, since red and brown algae share no common ancestor with green algae and plants, this view is clearly incorrect from an evolutionary standpoint. Green algae include the ancestors of plants, yet they differ sharply from any modern forms of the latter (not least in their aquatic habitat). For these reasons, we prefer to recognize plants as a separate kingdom and to classify red, brown, and green algae as protists. Although these groups feature a great Abundance of multicellular organisms, it should be noted that most protist phyla and divisions also include at least a few multicellular representatives.

Mitochondria and METABOLISM/14.html">Chloroplasts

One of The most significant events in eukaryotic evolution was the acquisition of mitochondria and chloroplasts. As discussed in Chapter 2, there is compelling evidence that these complex Organelles are of symbiotic origin, a fact supported by their Structure and The Nature of their genetic apparatus. Mitochondria are particularly similar to prokaryotes known as purple non-sulfur bacteria (see Chapter 11). In this group, The Plasma Membrane forms elaborate folds extending throughout the Cytoplasm, resembling the cristae of mitochondria. Biochemical similarities are also evident.

Chloroplasts appear to be descendants of at least three distinct groups of symbiotic, photosynthetic, aerobic bacteria. The various divisions of photosynthetic protists ("algae") are evidently not always closely related to one another and acquired their chloroplasts independently and at different times in the distant past. Some of the bacteria likely involved in the origin of chloroplasts will be examined in Chapters 11, 14, and 15.

Formal Classification of Organisms

Below is a Brief Overview of the classification system used in this book, which recognizes five kingdoms of organisms (Table 10-2).

KINGDOM MONERA

This kingdom comprises bacteria—that is, prokaryotic organisms lacking nuclear envelopes, Plastids, mitochondria, and flagella with a (9+2) structure. They exhibit unicellular or colonial-unicellular Organization (see Fig. 10-4) without protoplasmic connections between cells. Most groups are absorptive heterotrophs, though some are photosynthetic or chemosynthetic. They reproduce primarily by Cell Division, although genetic recombination occurs in a few groups. Locomotion is achieved via simple flagella, gliding, or they may be non-motile. They are discussed in detail in Chapter 11.

Table 10-2. CLASSIFICATION OF LIVING organisms traditionally grouped with plants (for brief descriptions of these groups, see Appendix A)

Kingdom Protista

In the scope adopted here, this kingdom (see Fig. 10-5) encompasses all organisms formerly treated as Protozoans (unicellular "animals") as well as all traditional "algae," with the exception of "blue-green algae," which are actually a group of bacteria. The term "algae" is informal; it is used to denote non-plant photosynthetic eukaryotes and cyanobacteria, with nearly all algae living in water. The kingdom Protista also includes several heterotrophic organisms, including water molds and related forms (division Oomycota), chytrids (division Chytridiomycota), cellular slime molds (division Acrasiomycota), and plasmodial slime molds (division Myxomycota). These four groups were traditionally classified as fungi.

Reproductive cycles among members of this kingdom vary, but typically involve both cell division and sexual reproduction. Protists move via flagella or cilia with a (9+2) structure, or by amoeboid movement; they may also be non-motile. The predominantly unicellular groups covered in this book are discussed in Chapter 14, while the three major algal groups—green, brown, and red algae—are covered in Chapter 15. Heterotrophic protists known as protozoans, traditionally classified as animals, are not discussed in this book.

Thus, protists are a highly heterogeneous assemblage of unicellular, colonial, and multicellular eukaryotes that lack the distinctive characteristics of animals, plants, or fungi.

Kingdom Animalia

This kingdom comprises multicellular organisms with wall-less Eukaryotic cells lacking plastids and Photosynthetic Pigments. Nutrition is accomplished primarily by ingestion of food and Digestion within an internal cavity, although some forms absorb food, and certain groups lack an internal digestive cavity. The level of tissue organization and differentiation in complex animals far exceeds that found in other kingdoms, a fact primarily attributed to The Development of sophisticated sensory and neuromotor systems. Locomotion of organisms (or parts thereof, in sessile forms) is driven by contractile fibrils. Reproduction is predominantly sexual. Animals are treated in this book solely in terms of certain ecological relationships they share with plants and other organisms discussed herein.

Kingdom Fungi

Fungi (see Fig. 10-6) are non-motile, filamentous eukaryotes lacking plastids and photosynthetic pigments, which absorb nutrients from dead or living organisms. Traditionally classified as plants, fungi are now universally recognized as an independent evolutionary lineage. Aside from their filamentous structure, fungi share virtually nothing in common with any algal groups. Their cell walls feature a chitinous matrix, and their spore-forming structures are often complex. The reproductive cycles of fungi can also be quite intricate, typically involving both sexual and asexual processes. This kingdom is covered in detail in Ch. 13.

Kingdom Plantae

This kingdom encompasses bryophytes (mosses, liverworts, and hornworts) along with nine phyla of vascular plants—photosynthetic organisms adapted to terrestrial life. Their ancestors were specialized green algae. All plants are multicellular, consisting of eukaryotic cells with vacuoles and cellulosic cell walls. Their primary mode of nutrition is photosynthesis, although some species have evolved heterotrophy. Over the course of terrestrial evolution, plants underwent structural differentiation, exhibiting a trend toward the development of specialized photosynthetic, anchoring, and supporting Organs (see Fig. 10-7). In more complex groups, this organization led to The Emergence of specialized assimilatory, vascular, and Dermal Tissues. Plant reproduction is predominantly sexual, featuring an alternation of Haploid and Diploid generations. In more advanced members of the kingdom, the haploid generation (gametophyte) became evolutionarily reduced. Bryophytes are discussed in Ch. 16, and vascular plants in Ch. 17, 18, and 29.