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Chapter 18: Classification. 18-1 History of Taxonomy. 18-2 Modern Phylogenetic Taxonomy. 18-3 Two Modern Systems of Classification. 18-1 History of Taxonomy. I. Early Systems of Classification (Aristotle, Greece ~2,100 years ago).

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Chapter 18: Classification

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Chapter 18: Classification

18-1 History of Taxonomy

18-2 Modern Phylogenetic Taxonomy

18-3 Two Modern Systems of Classification


18-1 History of Taxonomy

I. Early Systems of Classification (Aristotle, Greece ~2,100 years ago)

  • Classified organisms as plants or animals BASED on HABITAT  LAND, WATER, and AIR dwellers, used COMMON names.
  • 17th Century DRAWBACKS: Common names began to VARY from one locale to next; Common names did NOT describe accurately AND stated nothing about INTERRELATEDNESS between organisms.

(1) Taxonomy

  • NAMES and GROUPS organisms according to MORPHOLOGY and EVOLUTIONARY history.

II. Carolus Linnaeus’s System (Swedish botanist—1700s)

  • Grouped organisms INTO hierarchical categories, based on MORPHOLOGY. [Father of “Binomial-Nomenclature”]

Critical Thinking

(1) Linnaeus’s work was done many years before Darwin and Mendel. Explain why many of Linnaeus’s categories are still relevant in light of genetic and evolutionary relationships among organisms.


(A) Levels of Classification

  • A nested hierarchy of 7 levels, allowing species to be GROUPED with increasingly RELATED organisms.

(1) Kingdom: Animalia

(2) Phylum: Arthropoda

(3) Class: Insecta

(4) Order: Mantodea

(5) Family: Mantidae

(6) Genus: Mantus

(7) Species: religiosa


(B) Binomial Nomenclature (written in Latin and italicized)

  • Species name (scientific name) of an organism consists of 2 parts:

(1) Genus (gets capitalized)

(2) Species Identifier (usually a descriptive word, lowercase)

Exs: Homo sapiens, Pisum sativum, Drosophila melanogaster


(1) Varieties

  • Plants belong to SAME species BUT with slightly different MORPHOLOGY.
  • Ex: Peaches and nectarines are FRUITS of TWO slightly different varieties of the peach tree, Prunus persica.

(2) Subspecies

  • Variations of a species that occur in DIFFERENT geographic areas.

Ex: Terrapene carolina triungui is a SUBSPECIES of the common eastern box turtle, Terrapene carolina, and gets its NAME from having THREE (rather than four) toes on its HIND feet.


(3) Phylogeny

  • Evolutionary HISTORY of a species.

NOTE: By using morphology, Linnaeus focused on PHENOTYPES that were largely influenced by GENES and are STILL clues of common ANCESTRY.


18-2 Modern Phylogenetic Taxonomy

I. Systematics (Systematic Taxonomy)

  • Organizes the DIVERSITY of species in context of EVOLUTION (i.e., uses PHYLOGENETIC approach to construct an evolutionary TREE)

Critical Thinking

(2) Biologists believe that there are probably millions of undescribed and unclassified species on Earth. Why might so many species still be undescribed or unclassified today?


(1) Phylogenetic Tree

  • Shows the relationships (or phylogeny) among groups of organisms.
  • NOTE: Although ONLY a hypothesis, a phylogenetic tree is BASED on several lines of EVIDENCE:

(a) Fossil Record

(b) Morphology

(c) Embryology

(d) Biochemical Comparison (amino acid, DNA, and chromosomes)


(A) The Fossil Record

  • MAY provide evidence, BUT a SYSTEMATIC TAXONOMIST would require OTHER sources of evidence.

(B) Morphology

  • The GREATER the number of HOMOLOGOUS structures between 2 species, the MORE closely-related phylogeny.

(C) Embryological Patterns of Development

  • EARLY development  EVIDENCE of phylogeny BETWEEN species. (from ZYGOTE to BIRTH)

(1) Blastula (in Kingdom Animalia)

  • A ball of dividing cells arising from ZYGOTE; only HOURS after fertilization (conception) a blastula is formed.

(2) Blastopore (in the blastula)

  • Small indentation becomes POSTERIOR end of digestive system in vertebrates AND echinoderms. (BUT…in invertebrates—it becomes the ANTERIOR end of the system)

NOTE: Each cell in an embryo of a VERTEBRATE or ECHINODERM has the POTENTIAL to form an entire organism (TWINS), WHEREAS in invertebrates, if ANY cell is removed, it will DIE (pre-determined parts).


(3) Echinoderm (from embryological evidence)

  • MORE closely related to VERTEBRATES than to other INVERTEBRATE species (including mollusks).

(D) Chromosomes and Macromolecules (amino acids, DNA)

  • Biochemical comparisons  changes in SEQUENCE are GREATER in species with MORE-DISTANT ancestors.

II. Cladistics (MOST RECENT system of taxonomy)

  • System of classification using “SHARED DERIVED CHARACTERS” to show relationships BETWEEN species.

(1) Derived Characters (CRITERIA for a CLADOGRAM)

  • A feature evolving ONLY within the GROUP under consideration.
  • (Ex: With birds, FEATHERS are an example of a DERIVED character that OTHER animals do NOT possess)

Critical Thinking

(3) Legs are an example of a shared derived character in vertebrates. Arthropods, such as lobsters and crickets, also have legs, but they are NOT accepted as a character shared with vertebrates. Why might this be?


Critical Thinking

(4) Why does the cladistic approach to classification suggest that the class Reptila (reptiles) is not a phylogenetic classification?


(2) Cladogram (result of cladistic analysis)

  • Shared DERIVED characters are evidence of COMMON ANCESTRY between SPECIES that share them.

18-3 Two Modern Systems of Classification

I. Six-Kingdom System (replacing old system of 5)

  • Research warrants TWO kingdoms of bacteria (not one), updating TO a SIX-kingdom system of LIFEFORMS.

(A) Kingdom Archaebacteria (ANCIENT bacteria)

  • Unicellular PROKARYOTIC many species are CHEMOAUTOTROPHIC while others are HETEROTROPHIC.

NOTE: Adapted to living in HARSH, hostile environments including sulfurous HOT Springs in Yellowstone National Park and SALT Lakes.


(B) Kingdom Eubacteria (modern bacteria)

  • Unicellular PROKARYOTIC, most are AEROBIC, (others anaerobic), and either heterotrophic OR autotrophic (cyanobacteria).

NOTE: Due do exponential growth AND a very short generation time, bacteria have a RAPID evolutionary RATE which may support their RESISTANCE to environmental change.


(C) Kingdom Protista

  • Unicellular AND multicellular, EUKARYOTIC organisms that can be autotrophic OR heterotrophic.

NOTE: Most unicellular AND aquatic, however can be plant-like, animal-like, or fungus-like protists.


(D) Kingdom Fungi

  • Unicellular AND multicellular, EUKARYOTIC organisms that ALL are heterotrophic.

NOTE: Fungi species secrete ENZYMES to break down organic materials into nutrients rather than ingesting them. (i.e., EXTRA-cellular digestion)


(E) Kingdom Plantae

  • Multicellular EUKARYOTIC  AUTOTROPHIC (and in RARE instances, heterotrophic)  terrestrial AND aquatic species.

(F) Kingdom Animalia

  • Multicellular EUKARYOTIC heterotrophic; use LOCOMOTION to obtain nutrition, mate, AND evade predation.

II. Three-Domain System (NEW to classification, ABOVE KINGDOMS)

  • Biochemical comparison of rRNA  shows HOW LONG since different species SHARED a common ancestor.

NOTE: Since ALL organisms (even archaebacteria) have ribosomes, the rRNA is used to study the PHYLOGENY between ANY two SPECIES.


Critical Thinking

(5) In the five-kingdom system, which is still used by some scientists, all species of bacteria are grouped into Kingdom Monera. Why might there have been only one bacterial kingdom in the past?


(1) Domains (THREE DOMAINS feed into SIX KINGDOMS)

  • PHYLOGENETIC tree drawn from rRNA shows SPECIES fall into THREE broad groups or “DOMAINS.”
  • Ex: Plants, animals, and fungi are ONE SMALL TWIG of a large branch (a domain) that includes ALL eukaryotes.

(2) Domain Archaea (OLDEST, ANCIENT domain)

  • Represents species belonging to Kingdom of Archaebacteria.

(3) Domain Bacteria (RECENTLY EVOLVED domain)

  • Represents species belonging to the Kingdom of Eubacteria.

(4) Domain Eukarya

  • Represents ALL species belonging to Kingdoms of Protista, Fungi, Plantae, and Animalia.
  • NOTE: Remember ALL eukaryotes have a TRUE NUCLEUS with linear (non-plasmid) chromosomes AND membrane-bound organelles.

Extra Slides AND Answers for Critical Thinking Questions

(1) Linnaeus based his system on morphological traits of an organism. Morphological traits are heavily influenced by genes, which are clues to common ancestry.

(2) Members of the class Reptilia appear to have evolved from several different ancestral lines. Reptiles consist of several groups that arose in parallel.

(3) Answers will vary, but students might suggest that because bacteria are small and fairly simple, they have few characters on which to base taxonomic decisions. Alternatively, they might suggest that differences between eubacteria and archaebacteria are mainly molecular and the technology for identifying such differences has only recently been developed.

(4) Many of the unclassified species may be few in number or may be located in remote habitats.

(5) Arthropod legs are structurally very different from the legs of vertebrates. Arthropod legs are covered by an exoskeleton and do not have an internal skeleton of bone.


Revisiting Evolution

  • All organisms on Earth are thought to have descended from a common ancestor.
  • Evolutionary biologists recognize six major evolutionary pathways and classify organisms into six kingdoms.

Assessing Prior Knowledge

  • What molecule in organisms is passed from one generation to the next? Why would similarities in this molecule between different organisms be a good basis for classification?
  • Identify the first type of organisms that are believed to have appeared on Earth and explain how these organisms obtained nutrition.