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Chapter 17: Organizing Life's Diversity

Chapter worksheet | Darwin's Journey

Ch. 17.1 The History of Classification

Biologists use a system of classification to organize information about the diversity of living things.

Early Systems of Classification

Biologists find it easier to communicate and retain information about organisms when the organisms are organized into groups; biological classification.
Classification- the grouping of objects or organisms based on a set of criteria.

Aristotle's System

Aristotle developed the first accepted system of biological classification as either plants or animals.
Animals were grouped by presence or absence of blood, and then further grouped by habitat or morphology.
Plants were classified by average size and structure: trees, shrubs, or herbs.

Aristotle's system was useful but had limitations. It did not account for evolutionary relationships, some organisms did not fit easily.

Linnaeus's System

Swedish naturalist Carolus Linnaeus broadened Aristotle's system and formalized it into a scientific system. Like Aristotle, he based his system on observational studies of the morphology and behavior of organisms.

First system of taxonomic organization. Taxonomy- a discipline of biology primarily concerned with identifying, naming, and classifying species based on natural relationships.
Taxonomy is part of the larger branch of biology called systematics.
Systematics is the study of biological diversity with an emphasis on evolutionary history.

Binomial Nomenclature

Linnaeus's method of naming organisms is called binomial nomenclature and remains valid today. Binomial nomenclature- gives each species a scientific name that has two parts.
The first part is the genus name, the second part is the specific epithet, or name, that identifies the species.
Latin is the basis because it is an unchanging language, and historically used in science and education.

Biologists use scientific names for species because common names vary in usage. The use of scientific names avoids the confusion that can be created with common names.


  • First letter of the genus is always capitalized
  • Italicized
  • When written by hand, both parts are underlined
  • Once written completely the genus name is often abbreviated to the first letter in later appearances.

Modern Classification Systems

The study of evolution in the 1800's added a new dimension to Linnaeus's classification system. Today, modern classification systems remain rooted in the Linnaeus tradition but have been modified to reflect new knowledge about evolutionary ancestry.

Taxonomic Categories

Taxonomic categories used by scientists are part of a nested-hierarchical-system, each category contained within another, and they are arranged from broadest to most specific.

Species and genus

Taxon (taxa)- a named group of organisms. The more characteristics of a taxon, the more organisms the taxon contains.
Genus- a group of species that are closely related and share a common ancestor.


Family- consists of similar, related genera.
Taxa are arranged in hierarchical system.

Higher Taxa

Order- contains related families. Class- contains related orders. Phylum or Division- contains related classes. Division used instead of phylum when classifying bacteria and plants. Kingdom- composed of related phyla or divisions. Domain- contains one or more kingdoms, is broadest of taxa.

Systematics Applications

Provides field guides that often include dichotomous keys; a key based on a series of choices between alternate characteristics.

Systematists identify new species and relationships among known species. Incorporate: taxonomy, paleontology, molecular biology, and comparative anatomy.
Knowing relationships between species can be important for humans: penicillin and relatives.

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Ch. 17.2 Modern Classification

Classification systems have changed over time.

Determining Species

As knowledge increases, definitions of species change.

Typological Species Concept

The idea that species are distinctly different groups based on physical similarities. This is based on the idea that species are unchanging, distinct, and natural "types".
The "type" specimen was an individual that best displayed the characteristics of that species, any difference might be considered a different species.

Typological species concept has been replaced because we now know organisms change and that some species exhibit tremendous variation.

Biological Species Concept

Defines a species as a group of organisms that is able to interbreed and produce fertile offspring in a natural setting.
Has limitations: some organisms that are different species can breed (dog & wolf, many plants). Also does not account for extinct species, or asexual species.

Phylogenetic Species Concept

Phylogeny- the evolutionary history of a species.
The phylogenetic species concept defines a species as a cluster of organisms that is distinct from other clusters and shows evidence of a pattern of ancestry and descent.
When a phylogenetic species branches, it becomes two different species. ie: geographic isolation


To classify a species, scientists often construct patterns of descent, or phylogenies, by using characters.
Characters- inherited features that vary among species. Shared morphological characters suggest that species are related closely and evolved from a recent common ancestor.

Morphological Characters

Analogous characters do not indicate a close evolutionary relationship, homologous characters might perform different functions but show an anatomical similarity inherited from a common ancestor.

Biochemical Characters

Amino acids and nucleotides are used to help determine evolutionary relationships among species.
Chromosome structure and number is also used for determining species similarities. Organisms may appear very different but have similar chromosome structures.

DNA and RNA analyses are also used. The greater the number of shared DNA sequences between species, the greater the number of shared genes-and the greater the evidence that the species share a recent common ancestor.

Phylogenetic Reconstruction

Cladistics- a method that classifies organisms according to the order that they diverged from a common ancestor.

Character Types

Two main types of characters when doing cladistics analysis.
Ancestral character is found with in the entire line of descent of a group of organisms.
Derived characters are present in members of one group of the line but not in the common ancestor.


Systematists use derived characters to make a cladogram. Cladogram- a branching diagram that represents the proposed phylogeny or evolutionary history of a species or group. The groups in cladograms are called clades, and is one branch of cladogram.

Constructing a Cladogram

Outgroup is the species or group of species that has more ancestral characters than the other organisms on the cladogram.
The cladogram is constructed by sequencing the order in which derived characters evolved with respect to the outgroup. The more derived characters that are shared between two organisms, the more recent the common ancestor.
Node; point where branches originate represent a common ancestor.

A cladogram is also called a phylogenetic tree.

Tree of Life

Charles Darwin used the analogy of a tree to suggest that all of the species developed from one or a few species.


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Ch. 17.3 Domains and Kingdoms

The most widely used biological classification system has six kingdoms within three domains.

Grouping Species

Broadest category in the classification system used by most biologists is the domain.
There are three domains: Bacteria, Archea, and Eukarya.
Within these domains are six kingdoms: Bacteria, Archaea, Protista, Fungi, Plantae, and Animalia.
Organisms are classified into domains according to cell type and structure. Organisms are separated into kingdoms by cell type, structure, and nutrition.

This system has been in use less than three decades. Scientists discovered a new kind of organism in the 1970's; unicellular prokaryotes named archaea.

Domain Bacteria

Members of Domain and Kingdom Bacteria are prokaryotes whose cell walls contain peptidoglycan. This is a diverse group that can survive in many different environments.
Some are aerobic that need oxygen to survive, others are anaerobic that die in the presence of oxygen.
Some are autotrophic and produce their own food, most are heterotrophic and get their nutrition from other organisms.

Domain Archaea

These species are thought to be more ancient than bacteria and yet more closely related to eukaryote ancestors.
Their cell walls do not contain peptidoglycan, they have some of the same proteins eukaryotes do.
Some are autotrophic, but most are heterotrophic.
Also called extremeophile because they can live in extreme environments: hot springs, salty lakes, thermal vents, and mud marshes.

Domain Eukarya

Cells with a membrane-bound nucleus and other membrane-bound organelles. Contains Kingdoms Protista, Fungi, Plantae, and Animalia.

Kingdom Protista

Protists- eukaryotic, unicellular, colonial, or multicellular. Do not fit into any other kingdoms.
Three groups:

  1. Plant-like: called algae, autotrophic
  2. Animal-like: called protozoans, are heterotrophs
  3. Fungus-like: slime molds and mildews, hetertrophs

Kingdom Fungi

Fungus- unicellular or multicellular, eukaryote, heterotrophic, lacks motility, have cell walls of chitin. Consists of threadlike filaments called hyphae.

Some are parasites, some saprobes- feed on dead or decaying organic matter.
Secrete digestive enzymes into their food source and then absorb digested materials directly into their cells.

Kingdom Plantae

Multicellular, have cell walls of cellulose, most have chloroplasts, cells are organized into tissues.

Kingdom Animalia

Multicellular, heterotrophic, no cell walls, cells are organized, most are motile.


Nucleic acid surrounded by a protein coat. Do not possess cells, are not considered living.

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Page last updated January 2, 2017.