Chapter 18 – Classification
Section 18-1: Finding
Order in Diversity
To study the great diversity
found on Earth, biologists give
organisms names
Biologists use a
classification system to
name organisms and group
them in a logical manner 
this is known as taxonomy
Using scientific names makes sure that
everyone is talking about the same organism
Larger, more general
categories are needed as well
as smaller specific categories
Organisms in the same group
are more similar to one
another than organisms in a
nearby group
Scientists began giving organisms scientific
names in the 18th century because common
names were just too confusing
Latin and Greek were well
known languages and were
used for scientific naming
The first attempts at scientific naming usually
described physical characteristics and could end
up being very long names
This also gave naming some
subjectivity because a different
scientist could describe the
organisms in a different way
Carolus Linnaeus developed a two-word naming
system called binomial nomenclature
In this system, each species
is assigned a 2 part
scientific name
The name is always written
in italics
The first word (genus) is capitalized
and the second word (species) is not
Ex. The llama is Lama glama
Genus = a group of
closely related species
The species is usually a
Latin description of the
organism
A dichotomous key can be used to help identify
unfamiliar organisms - it is a series of paired
statements that describe physical characteristics of
different organisms
Linnaeus’s classification system is hierarchical – it
consists of levels
The system includes 7 levels; (largest
to smallest) kingdom, phylum, class,
order, family, genus, and species
In taxonomic
nomenclature, each
level is a taxon
Kingdom is the largest and most inclusive of the
categories – there are 6 kingdoms
Phyla are made up of several different classes that
share at least several important characteristics
The phylum chordate is made
up of the classes of Mammalia,
Aves (birds), Reptilia, Amphibia,
and all classes of fish because
they share important body plan
features and internal functions
Classes are composed of similar orders
The class of Mammalia
includes animals that are
warm-blooded, have body
hair, and produce milk
Orders are broad categories made up of similar families
Llamas belong to the order Artiodactyla,
which are even toed hoofed animals – this
order also includes pigs, hippos, giraffes,
antelopes, and sheep as well as others
Families are composed of similar genera
Llamas are part of the Camelidae
family with other similar organisms
like alpacas and camels
Section 18-2:
Modern Evolutionary Classification
Scientists have tried to group organisms according to
biologically important characteristics – however there
are some limitations to this
Classification was mainly based on
visible similarities and differences,
but which ones are important?
Darwin’s ideas about decent with modification has
given rise to the study of phylogeny = the evolutionary
relationships among organisms
Biologists now group organisms
into categories that represent
lines of evolutionary descent,
not just physical characteristics
This method of classification is
evolutionary classification
In evolutionary classification, species in a
genus share a recent common ancestor
Similarly, all genera in a family share a
common ancestor
The higher the level of taxon, the
further back in time the ancestor is
Just because two organisms look similar
doesn’t necessarily mean they share a
common ancestor – natural selection
may just have been working on each
organism in similar environments  this
is convergent evolution
Cladistic analysis identifies and considers only those
characteristics of organisms that are evolutionary
innovations – new characteristics that have evolved
Derived characteristics =
characteristics that appear
in recent parts of a lineage
but not older parts
Cladograms are diagrams that show
relationships among a group of organisms
They help scientists understand how
one lineage branched from another
(it is like a family tree for organisms)
Characteristics can appear along the
branches of the cladograms
The genes of organisms show important
similarities at the molecular level
Similarities in DNA can help
determine classification and
Genes in very dissimilar evolutionary relationships
organisms can code for proteins
with somewhat similar functions
DNA sequences can show relationships 
the closer the DNA, the closer the
evolutionary relationship
DNA can also be used as a “molecular
clock” to estimate the amount of time two
species have been evolving independently
Section 18-3:
Kingdoms and Domains
When Linnaeus first came up with his
classification system, he named only two
kingdoms – Animalia and Plantae
The view of life was simpler
and visible differences were
considered
As biologists learned more about the natural world,
they realized that the two kingdoms didn’t fit for every
living organism
Microorganisms like bacteria and
protists were quite different and
given their own kingdom, Protista
Mushrooms, yeasts and molds
were still different and given
their own kingdom, Fungi
Later, scientists realized that
bacteria were still different from
protists and were given a new
kingdom, Monera
Only recently did scientists realize
that Monera was made of two distinct
groups and they were now named
Eubacteria and Archaebacteria
In summary, the current 6 kingdoms are
Eubacteria, Archaebacteria, Protista,
Fungi, Plantae, and Animalia
Recent work with RNA lead scientists to create a new
taxonomic category – the domain – which is larger
and more inclusive that kingdoms
The three domains are Eukarya,
Bacteria, and Archaea
1. Eukarya – includes protists,
fungi, plants and animals
All eukarya have a nucleus
Protists are any organisms that
can’t be classified as animals
plants or fungi – they have a
very wide range
Fungi are heterotrophs that usually
feed on dead or decaying matter
Plants are photosynthetic
autotrophs and also nonmotile
(can’t move from place to place)
2.
Bacteria – includes eubacteria
They are unicellular and prokaryotic
They have thick cell walls
They have a wide range from soil
bacteria to deadly parasites
3.
Archaea – includes archaebacteria
Also unicellular and prokaryotic
They live in some of the most
extreme environments and usually
can only survive without oxygen
Diseases caused by
Bacteria
Lyme disease
Tetanus
Tuberculosis
Diphtheria
Bacterial meningitis
Strep throat
Tooth Decay
Diseases caused by
Viruses
Common cold
Influenza
Smallpox
AIDS
Chickenpox
Measles
Hepatitis (A,B, & C)