Biology
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18-1 Finding Order in
Diversity
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18-1 Finding Order in Diversity
18-1 Finding Order in Diversity
Natural selection and other processes have led to a
staggering diversity of organisms.
Biologists have identified and named about 1.5
million species so far.
They estimate that 2–100 million additional species
have yet to be discovered.
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18-1 Finding Order in Diversity
Why Classify?
In the discipline of taxonomy, scientists classify
organisms and assign each organism a universally
accepted name.
When taxonomists classify organisms, they organize
them into groups that have biological significance.
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18-1 Finding Order in Diversity
Assigning Scientific Names
Assigning Scientific Names
Common names of organisms vary, so scientists
assign one name for each species.
Because 18th century scientists understood Latin
and Greek, they used those languages for
scientific names.
This practice is still followed in naming new
species.
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Assigning Scientific Names
Early Efforts at Naming Organisms
The first attempts at standard scientific names
described the physical characteristics of a species
in great detail.
These names were not standardized because
different scientists described different
characteristics.
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Assigning Scientific Names
Carolus Linneaus developed a naming system
called binomial nomenclature.
In binomial nomenclature, each species is
assigned a two-part scientific name.
The scientific name is italicized.
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Assigning Scientific Names
The first part of the name is the genus to which the
organism belongs. A genus is a group of closely
related species. The genus name is capitalized.
The second part of the name is unique to each
species within the genus. This part of the name often
describes an important trait or where the organism
lives. The species name is lowercased.
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Linnaeus’s System of
Classification
Linnaeus's System of Classification
Linnaeus not only named species, he also grouped
them into categories.
What is Linneaus’s system of
classification?
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Linnaeus's System of
Classification
Linnaeus's seven levels of classification
are—from smallest to largest—
• species
• genus
• family
• order
• class
• phylum
• kingdom
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Linnaeus's System of
Classification
Each level is called a taxon, or taxonomic category.
Species and genus are the two smallest categories.
Grizzly
bear
Black
bear
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Linnaeus's System of
Classification
Genera that share many characteristics are grouped
in a larger category, the family.
Grizzly
bear
Black
bear
Giant
panda
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18-1 Finding Order in Diversity
Linnaeus's System of
Classification
An order is a broad category composed of similar
families.
Grizzly
bear
Black
bear
Giant
panda
Red
fox
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Linnaeus's System of
Classification
The next larger category, the class, is composed of
similar orders.
Grizzly
bear
Black
bear
Giant
panda
Red
fox
Abert
squirrel
Class Mammalia
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Linnaeus's System of
Classification
Several different classes make up a phylum.
Grizzly
bear
Black
bear
Giant
panda
PHYLUM
Red
fox
Abert
squirrel
Coral
snake
Chordata
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Linnaeus's System of
Classification
The kingdom is the largest and most inclusive of
Linnaeus's taxonomic categories.
Grizzly
bear
Black
bear
Giant
panda
Red
fox
Abert
squirrel
Coral
snake
Sea
star
KINGDOM Animalia
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18-1 Finding Order in Diversity
Grizzly Black Giant
bear
bear panda
Linnaeus's System of
Classification
Coral Sea
Red Abert
fox squirrel snake star
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18-2 Modern Evolutionary
Classification
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18-2 Modern Evolutionary Classification
Evolutionary
Classification
Which Similarities Are Most Important?
Linnaeus grouped species into larger taxa
mainly according to visible similarities and
differences.
How are evolutionary relationships
important in classification?
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18-2 Modern Evolutionary Classification
Evolutionary
Classification
Evolutionary Classification
Phylogeny is the study of evolutionary
relationships among organisms.
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Evolutionary
Classification
Biologists currently group organisms into
categories that represent lines of
evolutionary descent, or phylogeny, not
just physical similarities.
The strategy of grouping organisms is based
on evolutionary history and is called
evolutionary classification.
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Evolutionary
Classification
The higher the level of the taxon, the further back in
time is the common ancestor of all the organisms in
the taxon.
Organisms that appear very similar may not share a
recent common ancestor.
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Evolutionary
Classification
Different Methods of Classification
Appendages
Crab
Conical Shells
Barnacle
Limpet
Crab
Barnacle
Molted external
skeleton
Segmentation
CLASSIFICATION BASED ON
VISIBLE SIMILARITY
Mollusk
Crustaceans
Limpet
Tiny freeswimming larva
CLADOGRAM
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Evolutionary
Classification
Superficial similarities once led barnacles and limpets
to be grouped together.
Conical Shells
Appendages
Crab
Barnacle
Limpet
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Evolutionary
Classification
However, barnacles and crabs share an evolutionary
ancestor that is more recent than the ancestor that
barnacles and limpets share.
Barnacles and crabs are classified as crustaceans,
and limpets are mollusks.
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Classification Using
Cladograms
Classification Using Cladograms
Many biologists now use a method called cladistic
analysis.
Cladistic analysis identifies and considers only new
characteristics that arise as lineages evolve.
Characteristics that appear in recent parts of a
lineage but not in its older members are called
derived characters.
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Classification Using
Cladograms
Derived characters can be used to construct a
cladogram, a diagram that shows the evolutionary
relationships among a group of organisms.
Cladograms help scientists understand how one
lineage branched from another in the course of
evolution.
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Classification Using
Cladograms
A cladogram shows the evolutionary relationships
between crabs, barnacles, and limpets.
Crustaceans
Mollusk
Barnacle
Crab
Limpet
Molted external skeleton
Segmentation
Tiny free-swimming larva
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18-2 Modern Evolutionary Classification
Similarities in DNA
and RNA
Similarities in DNA and RNA
How can DNA and RNA help scientists
determine evolutionary relationships?
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18-2 Modern Evolutionary Classification
Similarities in DNA
and RNA
The genes of many organisms show
important similarities at the molecular
level.
Similarities in DNA can be used to help
determine classification and evolutionary
relationships.
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Similarities in DNA
and RNA
DNA Evidence
DNA evidence shows evolutionary relationships of
species.
The more similar the DNA of two species, the more
recently they shared a common ancestor, and the
more closely they are related in evolutionary terms.
The more two species have diverged from each
other, the less similar their DNA will be.
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Molecular Clocks
Molecular Clocks
Comparisons of DNA are used to mark the
passage of evolutionary time.
A molecular clock uses DNA comparisons to
estimate the length of time that two species have
been evolving independently.
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Molecular Clocks
A gene in an
ancestral species
Molecular Clock
2 mutations
new
mutation
Species
A
2 mutations
new
new
mutation mutation
Species
B
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Species
C
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Molecular Clocks
A molecular clock relies on mutations to mark time.
Simple mutations in DNA structure occur often.
Neutral mutations accumulate in different species at
about the same rate.
Comparing sequences in two species shows how
dissimilar the genes are, and shows when they
shared a common ancestor.
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18-3 Kingdoms and Domains
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18-3 Kingdoms and Domains
The Tree of Life Evolves
The Tree of Life Evolves
Systems of classification adapt to new discoveries.
Linnaeus classified organisms into two kingdoms—
animals and plants.
The only known differences among living things
were the fundamental traits that separated animals
from plants.
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The Tree of Life Evolves
Five Kingdoms
Scientists realized there were enough differences
among organisms to make 5 kingdoms:
• Monera
• Protista
• Fungi
• Plantae
• Animalia
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The Tree of Life Evolves
Six Kingdoms
Recently, biologists recognized that Monera were
composed of two distinct groups: Eubacteria and
Archaebacteria.
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The Tree of Life Evolves
The six-kingdom system of classification
includes:
• Eubacteria
• Archaebacteria
• Protista
• Fungi
• Plantae
• Animalia
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The Tree of Life Evolves
Changing Number of Kingdoms
Names of Kingdoms
Introduced
1700’s
Late 1800’s
1950’s
1990’s
Plantae
Plantae
Protista
Monera
Eubacteria
Archaebacteria
Animalia
Animalia
Protista
Fungi
Plantae
Animalia
Protista
Fungi
Plantae
Animalia
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The Three-Domain System
The Three-Domain System
Molecular analyses have given rise to a new
taxonomic category that is now recognized by
many scientists.
The domain is a more inclusive category than any
other—larger than a kingdom.
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The Three-Domain System
The three domains are:
• Eukarya, which is composed of protists,
fungi, plants, and animals.
• Bacteria, which corresponds to the
kingdom Eubacteria.
• Archaea, which corresponds to the
kingdom Archaebacteria.
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The Three-Domain System
Modern classification is a rapidly changing science.
As new information is gained about organisms in the
domains Bacteria and Archaea, they may be
subdivided into additional kingdoms.
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Domain Bacteria
Domain Bacteria
Members of the domain Bacteria are unicellular
prokaryotes.
Their cells have thick, rigid cell walls that surround
a cell membrane.
Their cell walls contain peptidoglycan.
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Domain Bacteria
The domain Bacteria
corresponds to the
kingdom
Eubacteria.
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Domain Archaea
Domain Archaea
Members of the domain Archaea are unicellular
prokaryotes.
They live in extreme environments.
Their cell walls lack peptidoglycan, and their cell
membranes contain unusual lipids not found in any
other organism.
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Domain Archaea
The domain
Archaea
corresponds to
the kingdom
Archaebacteria.
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Domain Eukarya
Domain Eukarya
The domain Eukarya consists of organisms that
have a nucleus.
This domain is organized into four kingdoms:
• Protista
• Fungi
• Plantae
• Animalia
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18-3 Kingdoms and Domains
Domain Eukarya
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Domain Eukarya
Protista
The kingdom Protista is composed of eukaryotic
organisms that cannot be classified as animals,
plants, or fungi.
Its members display the greatest variety.
They can be unicellular or multicellular;
photosynthetic or heterotrophic; and can share
characteristics with plants, fungi, or animals.
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Domain Eukarya
Fungi
Members of the kingdom Fungi are heterotrophs.
Most fungi feed on dead or decaying organic
matter by secreting digestive enzymes into it and
absorbing small food molecules into their bodies.
They can be either multicellular (mushrooms) or
unicellular (yeasts).
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Domain Eukarya
Plantae
Members of the kingdom Plantae are multicellular,
photosynthetic autotrophs.
Plants are nonmotile—they cannot move from
place to place.
Plants have cell walls that contain cellulose.
The plant kingdom includes cone-bearing and
flowering plants as well as mosses and ferns.
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