Chapter 18 Classification

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Chapter 18 Classification

1. Binomial Nomenclature

A. Organisms were first named and classified more than 2,000 years ago by the Greek philosopher Aristotle. He classified everything as either a plant or an animal and then grouped them into land dwellers, water dwellers, and air dwellers. This system worked very well for a period of time but eventually, new species were discovered that didn’t fit into his categories. Also, using the common names of things was problematic because not everyone speaks the same language.

B. In Linnaeus’s system, the species name (also called the scientific name) of an organism has two parts

1) The first part of the name is the genus, and the second part is the species identifier, usually a descriptive word

2) Thus, we humans are known by our genus, Homo, and by our species identifier, sapiens, which means “wise”

a. This system of two-part names is known as binomial nomenclature

b. By custom, the genus name is capitalized and sometimes abbreviated using the first letter, and the species is lower case and both names are underlined when written or typed in italics

c. Binomial nomenclature uses the Latin name for organisms, but sometimes it describes the organisms, other times, the name is given in honor of a person or place

d. Because species names are Latinized (uses Latin), they are the same in every language

e. Sometimes, because of small morphological differences, organisms are further classified into varieties (ex. Peaches and nectarines) or subspecies (species that occur in different areas)

2. List Carolus Linnaeus’s levels of classification from the most general to the most specific

A, Linnaeus devised a system for classification using hierarchal methods

1) Most of his structure uses the organism’s morphology

2) His system consists of 7 different levels of organization:

Kingdom ( Phylum ( Class ( Order ( Family ( Genus ( Species

C. Linnaeus’s largest category is called a kingdom

1) There are two kingdoms - plant and animal

D. Each subset within a kingdom is known as a phylum, in the animal kingdom, or a division, in the plant kingdom

D. Within a phylum or division, each subset is called a class

E. Each subset within a class is called an order

F. Still smaller groupings are the family

G. Then genus

H. The smallest grouping of all, which contains only a single organism type, is known as

the species

3. Name the primary criterion that modern taxonomists consider when they classify an organism

A. Taxonomy – the branch of biology that names and groups organisms according to their characteristics and evolutionary history (phylogeny)

B. Today, scientists use morphology, chromosomal characteristics, nucleotide and amino acid sequences, and embryological development to classify organisms

C. Most modern taxonomists agree that the classification of organisms

should reflect the phylogeny, or evolutionary history, of the organism

E. This phylogenetic approach is a cornerstone of a branch of biology

called systematic taxonomy, or, more commonly, systematics

F. Systematics organizes the tremendous diversity of living things in the context of evolution

G. Systematic taxonomists use several lines of evidence to construct a phylogenetic tree

4. Define phylogenetic tree, and explain what information a phylogenetic tree

shows and list four types of evidence used to organize organisms in systematic taxonomy

A. A phylogenetic tree is a family tree that shows the evolutionary relationships thought to exist among groups of organisms

1) A phylogenetic tree represents a hypothesis, and it is generally

based on several lines of evidence

2) A phylogenetic tree is subject to change, as is any hypothesis, as new information arises

a. The phylogenetic tree can be constructed using morphology, embryological development, the fossil record, or chromosomes and macromolecules

i. Fossil record – look at fossils

ii. Morphology – organism’s morphology - look at structures (homologous, analogous, vestigial) with respect to the morphology of similar and possibly ancestral organisms in the fossil record and with that of living organisms

iii. Chromosomes and Macromolecules – look at DNA, RNA, amino acids, proteins, and karyotypes (chromosomes)

iv. Embryological – study early development of organisms

a) Differences in animal phyla may appear very early in the embryological development. As development begins, the zygote begins to divide by mitosis

i) Blastula – the first “ball” of cells

ii) Blastopore – a small indentation on the outside of the blastula

1. In most invertebrate organisms, this indentation becomes the anterior end of the digestive system, but in echinoderms (starfish, sand dollars, etc.), it becomes the posterior part of the digestive system, much like it does in the vertebrates

5. Explain cladistics taxonomy, and identify one conclusion that is in conflict with classical, systematic taxonomy

A. One relatively new system of phylogenetic classification is called cladistics

1) Cladistics uses certain features of organisms, called shared derived characters, to establish evolutionary relationships

a) A derived character is a feature that apparently evolved only within the group under consideration

b) For example, if the group being considered is birds, one example of a derived character is feathers

c) Most animals do not have feathers; birds are the only animals that do

d) Therefore, it is safe to assume that feathers evolved within the bird group and were not inherited from some distant ancestor of the birds

2) Cladistic taxonomists agree that organisms that share a derived character—like feathers—probably share it because they inherited it from a common ancestor

3) So shared derived characters, particularly a group of several shared derived characters, are strong evidence of common ancestry between organisms that share them

a) Ancestry diagrams made by means of cladistic analysis are called cladograms

B. The application of cladistic taxonomy leads to a number of nontraditional conclusions

1) One of the most notable is that birds, crocodiles, and dinosaurs are more closely related to each other than any one of them is to a snake or lizard

2) In the more-classical systematic scheme used in this textbook, snakes, lizards, and crocodiles are classified in the reptile class, while birds are in a class by themselves

3) A related cladistic conclusion, which differs from that of classical taxonomy, is that the reptiles did not all spring from one common ancestor

4) Rather, reptiles are a composite of several branches that have occurred during the evolution of the vertebrates

6. Describe the six-kingdom system of classification

A. A classification system based on five kingdoms of organisms was preferred by taxonomists for many years

1) But further studies of bacteria have shown that there are two important subtypes with very different morphologies and properties

2) Recognition of these two broad types of bacteria has driven the acceptance of a newer, six-kingdom system

a) Kingdom Archaebacteria

(1) Unicellular prokaryotes with distinctive cell membranes as well as biochemical and genetic properties that differ from all other kinds of life

(2) Some are autotrophic, producing food by chemosynthesis

(3) Their waste products may include flammable gases, such as methane

(4) Many live in harsh environments such as sulfurous hot springs, and very salty lakes, and in anaerobic environments, such as in the intestines of mammals

(5) The prefix archae- comes from the Greek word for “ancient”

(6) Modern archaebacteria may be directly descended from and very similar to the first organisms on Earth, which flourished before the evolution of photosynthesis

(7) These early archaebacterial ancestors evolved before the release of large amounts of oxygen gas into the environment.

b) Kingdom Eubacteria

(1) The eu part of eubacteria means “true”

(2) Unicellular prokaryotes – autotrophic and heterotrophic

(3) Most of the bacteria that affect your life—those that cause tooth decay, turn milk into yogurt, and cause food poisoning—are members of the kingdom Eubacteria

(4) Most species of eubacteria use oxygen (aerobic), but a few species cannot live in the presence of oxygen (anaerobic)

(5) Include the greatest number of living things on Earth

(6) Both archaebacteria and eubacteria reproduce by binary fission, but they do have some ways to recombine genes, allowing evolution to occur

c) Kingdom Protista

(1) The kingdom Protista is made up of a variety of eukaryotic, mostly single-celled organisms – autotrophic and heterotrophic

(2) Some species of protests exist as multicellular organisms, like the giant kelp

(a) Although they look much like plants, multicellular protists lack specialized tissues

(3) Being eukaryotes, they have a membrane-bound true nucleus with linear chromosomes, and they have membrane-bound organelles

(4) It is difficult to make generalizations about the protists because many protist species are more distantly related to each other than plants are to animals

(a) The kingdom Protista contains all eukaryotes that are not plants, animals, or fungi, more than 50,000 species in all

(b) The sexual cycles of many protists are unknown, but most are thought to have some process of genetic recombination

d) Kingdom Fungi

(1) Heterotrophic unicellular and multicellular eukaryotic organisms

(2) Fungi absorb nutrients rather than ingesting them the way some protists, such as amoebas, do

(3) While sexual cycles are not known for many fungi, it is likely that all species have some way of promoting gene recombination

(4) There are over 100,000 species of fungi, including mushrooms, puffballs, rusts, smuts, mildews, and molds

e) Kingdom Plantae

(1) Consists of multicellular plants

(2) All except for a few parasitic forms are autotrophic and use photosynthesis as a source of energy

(3) Most plants live on land, and most have a sexual cycle based on meiosis

(4) More than 350,000 species of plants have been identified

(5) They include mosses, ferns, conifers, and flowering plants

f) Kingdom Animalia

(1) Eukaryotic, multicellular heterotrophic organisms

(2) Most animals have symmetrical body organization and move about their environment

(3) Almost all animals have a standard sexual cycle that employs meiosis for the recombination of genes


7. Explain the principal difference between the six-kingdom system and the three-domain system of classification

A. By comparing sequences of ribosomal RNA in different organisms, molecular biologist Carl Woese has estimated how long ago pairs of different organisms shared a common ancestor

1) Because all organisms, even prokaryotes, have ribosomes, the ribosomal RNA molecule can be used to study the degree of relationship between any two living things

2) The phylogenetic tree drawn from these data shows that living things seem to fall naturally into three broad groups, or domains

(a) Domain Archaea is known in the six-kingdom system as kingdom Archaebacteria

(b) Domain Bacteria is known in the six-kingdom system as the kingdom Eubacteria

(c) Domain Eukarya consists of the protists, the fungi, and the plants and animals

i. All eukaryotes have true nuclei with linear chromosomes

and membrane-bound organelles

ii. Most of the variation in this domain is among the protists

iii. Surprisingly, when considered from the perspective of the complete diversity of life on Earth, the fungi, plants, and animals are quite similar to each other

Chapter 18 Classification

□ Taxonomy is the science of grouping organisms according to their morphology and evolutionary history.

□ Carolus Linnaeus originated a seven-level hierarchy system for classifying organisms according to their morphology. Moving from the most general to the most specific, the levels are called kingdom, phylum, class, order, family, genus, and species. A version of this system is still in use.

□ A species name consists of the genus name together with a species identifier. A species denotes a single organism type.

□ A modern approach to taxonomy is systematics, which analyzes the diversity of organisms in the context of their evolutionary history.

□ Scientists consider several lines of evidence when classifying organisms according to their evolutionary history.

□ An organism’s relationship to organisms in the fossil record as well as to living organisms is taken into account in the formulation of a phylogenetic tree.

□ Similarities in patterns of embryological development provide clues to the degree of relatedness of different organisms.

□ Molecular similarities, such as those found in homologous proteins of different organisms, also indicate how closely organisms are related.

□ Shared derived characters, those traits that developed within a certain group, are clues to the degree of relatedness among organisms. The system that uses shared derived characters to deduce evolutionary history is called cladistics.

□ Many modern taxonomists use the six-kingdom system of classification, which recognizes the unique nature of the archaebacteria.

□ Archaebacteria, some of which live in extremely harsh environments, have been largely ignored until recently. Scientists now think archaebacteria closely resemble the first kinds of organisms to live on Earth.

□ An alternative classification system that employs three broad domains groups all eukaryotic organisms under the domain Eukarya. Eubacteria (domain Bacteria) and archaebacteria (domain Archaea) form each of the other two domains.

Vocabulary List

Binomial Nomenclature



Carolus Linnaeus

Chromosomes and Macromolecules



Derived character

Domain Archaea

Domain Bacteria

Domain Eukarya

Embryological Patterns of Development

Kingdom Animalia



Kingdom Eubacteria

Kingdom Fungi

Kingdom Plantae

Kingdom Protista

Levels of Classification


Phylogenetic tree


Six-Kingdom System

Species identifier

Species name




The Fossil Record

Three-Domain System



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