25.1 Biologists name organisms in a systematic way.
The Classification of Organisms
• Organisms were first classified by Aristotle over 2000 years ago. (p. 510)
• Similar individuals were eventually classified into units called genera (groups). (p. 510)
• Many descriptive terms were used in naming until the mid-1700s, when Linnaeus began using a binomial system. (p. 510)
• Taxonomy is the science of classifying living organisms; a group of similar organisms is put into a shared taxon. (p. 510)
• Taxonomic classifications are composed of (in ascending order): species, genus, family, order, class, phylum, kingdom, and domain. Each level may include anywhere from one to several taxa. (p. 511)
25.2 Scientists construct phylogenies to understand the evolutionary relationships among species.
Systematics
• The field of systematics constructs and studies evolutionary relationships (phylogenies). (p. 512).
• Descendant species should be relatively similar to their ancestors, and the amount of time since separation should be positively correlated with the degree of genetic divergence. But evolution is not constant in its rate, not unidirectional, and not always divergent. (p. 512)
• Cladistics distinguishes between ancestral and derived similarity, and only considers shared derived characters informative. (p. 512)
• Cladograms are constructed to show hypothesized evolutionary relationships, with species sharing derived characteristics put into the same clade. (p. 513)
• Systematists use the principle of parsimony and favor hypotheses with the fewest assumptions and the fewest evolutionary events. (p. 514)
• A monophyletic group contains the most recent common ancestor and all its descendants; a paraphyletic group does not include all the descendants; and a polyphyletic group does not include the most recent ancestor or all the members of the group. (p. 514)
25.3 Phylogenetics is the basis of all comparative biology.
Analogy Versus Homology
• Homologous structures are derived from the same body part in a common ancestor, while analogous structures are derived from different ancestral structures. (p. 516)
• Most complex characters are built up in a stepwise fashion, often with a series of evolutionary transitions. (p. 517)
25.4 All living organisms are grouped into one of a few major categories.
The Kingdoms of Life
• Most biologists now use a six-kingdom system. The kingdoms Animalia, Plantae (Virdiplantae), Fungi, and Protista contain eukaryotic organisms, while Archaebacteria and Bacteria contain prokaryotic organisms. (p. 518)
• Three different domains, a classification level higher than kingdom, are now recognized as well: Archaebacteria, Bacteria, and Eukarya. (p. 518)
Domain Archaea (Archaebacteria)
• Archaebacteria appear to have diverged from bacteria early, and are more closely related to eukaryotes than to bacteria. (p. 519)
• The archaebacteria are grouped into three general categories based on their environment: methanogens, extremophiles (e.g., thermophiles and halophiles), and nonextreme archaebacteria. (p. 519)
Domain Bacteria (Bacteria)
• Bacteria are the most abundant organisms on earth, and they carry out much of the earth's photosynthesis. (p. 520)
• Most taxonomists recognize 12-15 major groups of bacteria. (p. 520)
Domain Eukarya (Eukaryotes)
• Eukaryotes appear in the fossil record only about 2.5 billion years ago. (p. 521)
• Complex cellular organization with an extensive system of intracellular membranes is the most distinctive characteristic of eukaryotes. (p. 521)
• Compartmentalization, multicellularity (in many eukaryotes), and sexuality are all key characteristics of eukaryotes. (p. 522)
Viruses: A Special Case
• Viruses are segments of DNA or RNA wrapped in a protein coat. They can infect cells and use cellular machinery to replicate but are not considered living organisms. (p. 524)
• Viruses defy taxonomic efforts. (p. 524)
25.5 Molecular data are revolutionizing taxonomy.
The Impact of Molecular Cladistics
• Within traditional classification systems, no clear basis exists for determining the most important traits. (p. 525)
• Systematic phylogenies use cladistics to build taxonomic hierarchies. (p. 525)
Making Sense of the Protists
• Molecular systematics demonstrates that protists are paraphyletic. (p. 525)
• "Protist" is a catchall word encompassing 200,000 species. (p. 525)
Origin of Land Plants
• Although the origin of land plants from a green algal ancestor has long been recognized, new evolutionary relationships have been hypothesized. (p. 526)
Sorting Out the Animals
• New RNA investigations are raising questions about how closely annelids are related to arthropods. (p. 527)
• Arthropods are the most diverse of all the animal phyla. (p. 527)
• New DNA evidence is also bringing into question the traditional evolutionary paths of some mammals. (p. 528)
• Over 90% of mammals are placental mammals. (p. 528)