BIOLOGY OF PLANTS — PETER H. RAVEN — 1990

PART IV. DIVERSITY

CHAPTER 10. THE CLASSIFICATION OF LIVING ORGANISMS

Conclusion

Biologists have developed Methods for naming and classifying living things that allow organisms to be identified with great precision, which is a vital factor in scientific communication. Classification also makes it possible to demonstrate the evolutionary relationships between a given form and other living organisms.

Organisms have a formal two-word (binomial) scientific name. The first word is the genus name, and the second is the specific epithet; together they constitute the full species name. Species are sometimes subdivided into subspecies or varieties. Genera are grouped into families, families into orders, orders into classes, classes into divisions (phyla), and divisions into kingdoms. The kingdom is the highest category in the CLASSIFICATION OF LIVING organisms.

In this book, Living organisms are grouped into five kingdoms, the first of which comprises prokaryotes, while the rest consist of eukaryotes: (1) Monera (Bacteria or prokaryotes); (2) Protista (Protozoa, eukaryotic Algae, slime Molds, and Water molds); (3) Animalia (multicellular non-photosynthetic organisms); (4) Fungi (fungi); and (5) Plantae — bryophytes and vascular plants — terrestrial photosynthetic organisms that are more complexly organized than algae.

Cell/35.html">Mitochondria, characteristic of all eukaryotes, likely originated from aerobic (oxygen-requiring) prokaryotes resembling purple non-sulfur bacteria, whereas METABOLISM/14.html">Chloroplasts apparently evolved from at least three distinct groups of photosynthetic aerobic bacteria. Therefore, the presence of chloroplasts in various eukaryotic groups does not imply a direct phylogenetic relationship among them. Multicellularity arose multiple times and independently in various protist lineages. Some of the multicellular evolutionary lines, including red, brown, and green algae, are classified as protists in the five-kingdom system used here, whereas the three major multicellular lineages — animals, plants, and fungi — are treated as separate kingdoms.

In the evolution of organisms, diploidy emerged following the evolution of sexual reproduction. In primitive eukaryotes and all fungi, the zygote formed during syngamy immediately undergoes meiotic division. From ancient life cycles of this type, more complex cycles evolved independently in several lineages, incorporating diploid phases with mitotically dividing zygotes. When the haploid Cells produced by Meiosis function directly as Gametes, The life cycle is typical of animals and certain protist groups. Conversely, when they divide mitotically, as in algae, all plants, and two genera of chytrids, they are considered spores. The diploid generation that produces spores is called the sporophyte, while the haploid generation that develops from spores via mitosis is the gametophyte. Ultimately, through mitosis, the gametophyte gives rise to gametes. If the gametophyte and sporophyte in a life cycle are roughly equal in size and structural complexity, the cycle is termed isomorphic; if they differ significantly in these features, it is heteromorphic.