Classification
• Each person might divide
these shells into different
categories
• Scientists often group
and name, or classify,
organisms using
certain guidelines
• This makes it easier to
discuss the types and
characteristics of living
things
Classification
Finding Order in Diversity
• For more than 3.5 billion years, life on Earth has
been constantly changing
• Natural selection and other processes have led to a
staggering diversity of organisms
• A tropical rain forest, for example, may support
thousands of species per acre
• Recall that a species is a population of organisms
that share similar characteristics and can breed with
one another and produce fertile offspring
• Biologists have identified and named about 1.5 million
species so far
• They estimate that anywhere between 2 and 100
million additional species have yet to be discovered
Why Classify?
• To study this great diversity of organisms,
biologists must give each organism a name
• Biologists must also attempt to organize living
things into groups that have biological meaning
• To study the diversity of life, biologists use a
classification system to name organisms and
group them in a logical manner
Why Classify?
• In the discipline known as taxonomy, scientists classify
organisms and assign each organism a universally accepted
name
• By using a scientific name, biologists can be certain that
everyone is discussing the same organism
• When taxonomists classify organisms, they organize them into
groups that have biological significance
• When you hear the word “bird,” for example, you immediately form a
mental picture of the organism being discussed—a flying animal that
has feathers
• But science often requires smaller categories as well as larger, more
general categories
• In a good system of classification, organisms placed into a
particular group are more similar to each other than they are to
organisms in other groups
Why Classify?
• You use classification systems also, for
example, when you refer to “teachers”
or “mechanics,” or more specifically,
“biology teachers” or “auto
mechanics”
• Such a process, like scientific
classification, uses accepted names and
common criteria to group things
Assigning Scientific Names
• By the eighteenth century, European scientists recognized that
referring to organisms by common names was confusing
• Common names vary among languages and even among
regions within a single country
• For example, a cougar can also be called a puma, a panther, or
a mountain lion
• Furthermore, different species sometimes share a single common
name
• In the United Kingdom, the word buzzard refers to a hawk,
whereas in many parts of the United States, buzzard refers to a
vulture
– To eliminate such confusion, scientists agreed to use a
single name for each species
• Because eighteenth-century scientists understood Latin and Greek,
they used those languages for scientific names
• This practice is still followed today in naming newly discovered
species
Early Efforts at Naming Organisms
• The first attempts at standard scientific names often
described the physical characteristics of a species
in great detail
• As a result, these names could be twenty words long!
• For example, the English translation of the scientific
name of a particular tree might be “Oak with deeply
divided leaves that have no hairs on their undersides
and no teeth around their edges”
• This system of naming had another major drawback
• It was difficult to standardize the names of organisms
because different scientists described different
characteristics
Binomial Nomenclature
• A major step was taken by Carolus Linnaeus, a
Swedish botanist who lived during the eighteenth century
• He developed a two-word naming system called
binomial nomenclature
• This system is still in use today
• In binomial nomenclature, each species is assigned
a two-part scientific name
• The scientific name is always written in italics
• The first word is capitalized, and the second word is
lowercased
Binomial Nomenclature
• For example, the grizzly bear is called Ursus
arctos
• The first part of the scientific name—in this
case, Ursus—is the genus to which the
organism belongs
• A genus (plural: genera) is a group of closely
related species
• The genus Ursus contains five other kinds of
bears, including Ursus maritimus, the polar bear
Binomial Nomenclature
• The second part of a scientific name—in this
case, arctos or maritimus—is unique to each
species within the genus
• Often, this part of the name is a Latinized
description of some important trait of the
organism or an indication of where the
organism lives
• The Latin word maritimus, referring to the sea,
comes from the fact that polar bears often
live on pack ice that floats in the sea
Linnaeus's System of Classification
• Linnaeus's classification system is hierarchical;
that is, it consists of levels
• Linnaeus's hierarchical system of
classification includes seven levels
• They are—from smallest to largest—species,
genus, family, order, class, phylum, and
kingdom
• In taxonomic nomenclature, or naming system,
each of those levels is called a taxon (plural:
taxa), or taxonomic category
CLASSIFICATION
• Taxonomy: is the science of grouping organisms
according to their presumed natural relationship
– Common names add cause confusion to the classification
system
– System used today is binomial nomenclature (two names)
• Developed by Linnaeus
–
–
–
–
–
–
–
Placed structurally similar organisms into a group called a species
Similar species into a larger group called a genus
Similar genera into a family
Similar families were placed into an order
Similar orders in a class
Similar classes into phylum
Phylum into kingdom
• Rather than use all seven categories in naming organisms,
Linnaeus chose to use the genus and specie names
Linnaeus's System of Classification
• The two smallest categories, genus and
species, were discussed in the example of
the bears
• The giant panda, resembles the grizzly
bear and the polar bear
• However, it differs enough from them
and other species in the genus Ursus
that it is placed in its own genus,
Ailuropoda
Linnaeus's System of Classification
• The grizzly bear,
Ursus arctos, and
the polar bear, Ursus
maritimus, are
classified as different
species in the same
genus, Ursus
• The giant panda is
placed in a separate
genus
Linnaeus's System of Classification
Linnaeus's System of Classification
• Genera that share many characteristics, such as Ursus
and Ailuropoda, are grouped in a larger category, the
family—in this case, Ursidae
• These bears, together with six other families of animals,
such as dogs (Canidae) and cats (Felidae), are grouped
together in the order Carnivora
• An order is a broad taxonomic category composed
of similar families
• The next larger category, the class, is composed of
similar orders
• For example, order Carnivora is placed in the class
Mammalia, which includes animals that are warmblooded, have body hair, and produce milk for their
young
Linnaeus's System of Classification
• Several different classes make up a phylum (plural:
phyla)
• A phylum includes many different organisms that
nevertheless share important characteristics
• The class Mammalia is grouped with birds (class Aves),
reptiles (class Reptilia), amphibians (class Amphibia),
and all classes of fishes into the phylum Chordata
• All these organisms share important features of their
body plan and internal functions
• Finally, all animals are placed in the kingdom
Animalia
• The kingdom is the largest and most inclusive of
Linnaeus's taxonomic categories
• Linnaeus named two kingdoms, Animalia and
Plantae
CLASSIFICATION
Linnaeus's System of Classification
• Linnaeus’s hierarchical
system of classification
uses seven taxonomic
categories
• This illustration shows
how a grizzly bear, Ursus
arctos, is grouped within
each taxonomic category
• Only some
representative species
are illustrated for each
category above the
species
Linnaeus's System of Classification
CLASSIFICATION
Modern Evolutionary Classification
• In a sense, organisms determine who belongs to their
species by choosing with whom they will mate!
• Taxonomic groups above the level of species are
“invented” by researchers who decide how to distinguish
between one genus, family, or phylum, and another
• Linnaeus and other taxonomists have always tried to
group organisms according to biologically important
characteristics
• Like any taxonomic system, however, Linnaeus's
system had limitations and problems
Which Similarities Are Most Important?
• Linnaeus grouped species into larger taxa, such as genus and
family, mainly according to visible similarities and differences
• But which similarities and differences are most important?
• If you lived in Linneaus's time, for example, how would you
have classified dolphins?
– Would you have called them fishes because they live in water and
have finlike limbs?
– Or would you call them mammals because they breathe air and feed
their young with milk?
• How about the animals shown in the figure?
• Adult barnacles and limpets live attached to rocks and have similarly
shaped shells with holes in the center
• Crabs, on the other hand, have body shapes unlike those of
barnacles or limpets
• Based on these features, would you place limpets and barnacles
together, and crabs in a different group?
Which Similarities Are Most Important?
• Classifying species
based on easily
observed adult traits
can pose problems
• Observe the crab (top
left), barnacles
(bottom left), and
limpet (right)
• Which seems most
alike?
Which Similarities Are Most Important?
Evolutionary Classification
• Darwin's ideas about descent with modification
have given rise to the study of phylogeny, or
evolutionary relationships among organisms
• Biologists now group organisms into
categories that represent lines of
evolutionary descent, or phylogeny, not just
physical similarities
• The strategy of grouping organisms together
based on their evolutionary history is called
evolutionary classification
Evolutionary Classification
• Species within a genus are more closely related to
each another than to species in another genus
• According to evolutionary classification, that is because
all members of a genus share a recent common
ancestor
• Similarly, all genera in a family share a common
ancestor
– This ancestor is further in the past than the ancestor of any
genus in the family but more recent than the ancestor of the
entire order
• The higher the level of the taxon, the farther back in
time is the common ancestor of all the organisms in
the taxon
Evolutionary Classification
• Organisms that appear very similar
may not share a recent common
ancestor
• Natural selection, operating on species
in similar ecological environments, has
often caused convergent evolution
• For example, superficial similarities once
led barnacles and limpets to be grouped
together, as shown on the left of the figure
Evolutionary Classification
Traditional Classification and Cladogram
• Early systems of
classification
grouped organisms
together based on
visible similarities
• That approach
might result in
classifying limpets
and barnacles
together (left)
Evolutionary Classification
Traditional Classification and Cladogram
Evolutionary Classification
•
However, barnacles and limpets are different in important ways
– For example, their free-swimming larvae, or immature forms, are unlike one
another
•
•
•
•
•
•
•
•
•
Certain adult characteristics are different too
Adult barnacles have jointed limbs and a body divided into segments
Barnacles periodically shed, or molt, their external skeleton
These characteristics make barnacles more similar to crabs than to
limpets
Limpets, in turn, have an internal anatomy that is closer to that of snails,
which are mollusks
And like mollusks, limpets do not shed their shells
Because of such characteristics, taxonomists infer that barnacles are
more closely related to crabs than to mollusks
In other words, barnacles and crabs share an evolutionary ancestor
that is more recent than the ancestor that barnacles share with limpets
Thus, both barnacles and crabs are classified as crustaceans, and
limpets are mollusks
Classification Using Cladograms
• To refine the process of evolutionary
classification, many biologists now prefer a
method called cladistic analysis
• Cladistic analysis identifies and considers
only those characteristics of organisms that
are evolutionary innovations—new
characteristics that arise as lineages evolve
over time
• Characteristics that appear in recent parts of a
lineage but not in its older members are called
derived characters
Classification Using Cladograms
•
•
•
Derived characters can be used to construct a cladogram, a diagram
that shows the evolutionary relationships among a group of
organisms
You can see an example of a cladogram on the right-hand side of the figure
Notice how derived characters, such as “free-swimming larva” and
“segmentation,” appear at certain locations along the branches of the
cladogram
– These locations are the points at which these characteristics first arose
•
You can see that crabs and barnacles share some derived characters
that barnacles and limpets do not
– One such shared derived character is a segmented body
– Another is a molted external skeleton
•
Thus, this cladogram groups crabs and barnacles together as
crustaceans and separates them from limpets, which are classified as
a type of mollusk
Classification Using Cladograms
Traditional Classification and Cladogram
• Biologists now group
organisms into categories that
represent lines of evolutionary
descent, or phylogeny, not just
physical similarities
• Crabs and barnacles are
now grouped together (right)
because they share several
characteristics that indicate
that they are more closely
related to each other than
either is to limpets
• These characteristics include
segmented bodies, jointed
appendages, and an external
skeleton that is shed during
growth
Classification Using Cladograms
Traditional Classification and Cladogram
Classification Using Cladograms
• Cladograms are useful tools that help
scientists understand how one lineage
branched from another in the course of
evolution
• Just as a family tree shows the relationships
among different lineages within a family, a
cladogram represents a type of evolutionary
tree, showing evolutionary relationships
among a group of organisms
CLASSIFICATION
• Inferring Phylogeny
– Infer the probable evolutionary relationships
among species that have been classified
– Sometimes a Phylogenetic Tree is used
PHYLOGENETIC TREE
CLASSIFICATION
• Binomial name of a species is called its scientific name
– Describes the organism or the range of the organism, or honors
another scientist or friend
• Classification:
– Phylum used in animal classification
– Division used in plant classification
– Classification of species:
• Subspecies (races): morphological different and are often
geographically separated
• Varieties: morphologically different and are often not geographically
separated
– Some produced by humans (apples, peaches and nectarines)
• Strain: biochemically dissimilar group within a species
– Usually used in reference to microorganisms
CLASSIFICATION
• Evidence Used in Classification
– Comparative morphology
– Embryology
• Homologous structures show evolutionary relationships between organisms
(bones in the forelimb of a lizard are embryologically similar to those in a cat)
– Chromosomes
• Karyotypes: compare numbers and shapes
– Biochemistry
• Sequence of bases in DNA
• Amino acid sequence in proteins
– Physiology
• Function of systems
– Phylogeny
• Evolutionary relationships
– Biosystematics
• Using reproductive compatibility to infer evolutionary relationships
Similarities in DNA and RNA
• All of the classification methods discussed so far are
based primarily on physical similarities and
differences
• But even organisms with very different anatomies
have common traits
• For example, all organisms use DNA and RNA to pass
on information and to control growth and
development
• Hidden in the genetic code of all organisms are
remarkably similar genes
• Because DNA and RNA are so similar across all
forms of life, these molecules provide an excellent
way of comparing organisms at their most basic
level—their genes
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
• Now that scientists can sequence, or “read,” the
information coded in DNA, they can compare the
DNA of different organisms to trace the history of
genes over millions of years
Similar Genes
• Even the genes of diverse organisms such as
humans and yeasts show many surprising
similarities
• For example, humans have a gene that codes for
myosin, a protein found in our muscles
• Researchers have found a gene in yeast that codes for a
myosin protein
• As it turns out, myosin in yeast helps enable internal
cell parts to move
• Myosin is just one example of similarities at the
molecular level—an indicator that humans and yeasts
share a common ancestry
DNA Evidence
• DNA evidence can also help show the
evolutionary relationships of species and
how species have changed
• The more similar the DNA sequences of two
species, the more recently they shared a
common ancestor, and the more closely they
are related in evolutionary terms
• And the more two species have diverged from
one another, or changed in comparison to one
another during evolution, the less similar their
DNA will be
DNA Evidence
• Consider the case of the American vulture and the
African vulture, which resemble each other
– Both birds have traditionally been classified together as
“vultures”
• One group of birds inhabits Africa and Asia, and the
other, the Americas
• But American vultures have a peculiar behavior: When
they get overheated, they urinate on their legs, and
evaporative cooling removes some body heat
• The only other birds known to behave this way are
storks, which look quite different from vultures and
have always been put in a separate family
• Does this similarity in behavior indicate a close
evolutionary relationship?
DNA Evidence
• Scientists analyzed the DNA of these three birds
• The analysis showed that the DNA sequences of the
American vulture and the stork were more similar
than those of the American vulture and the African
vulture
– This similarity in DNA sequences indicates that the
American vulture and the stork share a more recent
common ancestor than do the American vulture and the
African vulture
• Therefore, the American vulture is more closely
related to storks than to other vultures
Molecular Clocks
• Comparisons of DNA can also be used to mark the
passage of evolutionary time
• A model known as a molecular clock uses DNA
comparisons to estimate the length of time that two
species have been evolving independently
• To understand molecular clocks, think about a pendulum
clock
• It marks time with a periodically swinging pendulum
• A molecular clock also relies on a repeating process
to mark time—mutation
Molecular Clocks
• Simple mutations occur all the time, causing slight changes in
the structure of DNA, as shown in the figure
– Some mutations have a major positive or negative effect on an
organism's phenotype
• These mutations are under powerful pressure from natural
selection
• Other mutations have no effects on phenotype
– These neutral mutations accumulate in the DNA of different species at
about the same rate
• A comparison of such DNA sequences in two species can
reveal how dissimilar the genes are
• The degree of dissimilarity is, in turn, an indication of how long
ago the two species shared a common ancestor
Molecular Clocks
• By comparing the DNA
sequences of two or
more species,
biologists estimate how
long the species have
been separated
• What evidence indicates
the species C is more
closely related to species
B than to species A?
Molecular Clocks
Molecular Clocks
• The use of molecular clocks is not simple, however, because there
is not just one molecular clock in a genome
• Instead, there are many, each of which “ticks” at a different rate
– This is because some genes accumulate mutations faster than
others
– These different clocks allow researchers to time different kinds of
evolutionary events
• Think of a conventional clock
• If you want to time a brief event, you pay attention to the second
hand
• To time an event that lasts longer, you use the minute hand or the
hour hand
• In the same way, researchers would use a different molecular
clock to compare modern bird species than they would to
estimate the age of the common ancestor of yeasts and
humans
Kingdoms and Domains
• As in all areas of science, systems of classification adapt to new
discoveries
• Ideas and models change as new information arises
• Some explanations have been discarded altogether, whereas
others, such as Darwin's theory of evolution by natural selection,
have been upheld and refined through years of research
• So, it should not be surprising that early attempts at drawing life's
universal tree were based on some misguided assumptions
• Some of the earliest trees of life were dominated by humans
– These models represented vertebrates as the most important and
abundant animals
• They also implied that “higher” animals evolved from “lower”
animals that were identical to modern forms
• Biologists now know these notions are incorrect
The Tree of Life Evolves
• The scientific view of life was simpler in
Linnaeus's time
• The only known differences among living
things were the fundamental traits that
separated animals from plants
• Animals were mobile organisms that used food
for energy
• Plants were green, photosynthetic organisms
that used energy from the sun
Five Kingdoms
• As biologists learned more about the natural world, they
realized that Linnaeus's two kingdoms, Animalia and Plantae,
did not adequately represent the full diversity of life
• First, microorganisms such as protists and bacteria were
recognized as being significantly different from plants and
animals
• Scientists soon agreed that microorganisms merited their own
kingdom, which was named Protista
• Then, the mushrooms, yeasts, and molds were separated from
the plants and placed in their own kingdom, Fungi
• Later still, scientists realized that bacteria lack the nuclei,
mitochondria, and chloroplasts found in other forms of life
– Therefore, they were placed in another new kingdom, Monera
• This process produced five kingdoms—Monera, Protista,
Fungi, Plantae, and Animalia
CLASSIFICATION
• Five Kingdom System
– Monera
• Prokaryotic organisms
• Bacteria and blue-green algae
– Protista
• Eukaryotic organisms that lack specialized tissue systems
• Unicellular or multicellular
• Algae and protozoa
– Fungi
• Heterotrophic unicellular and multicellular eukaryotic organisms
– Plantae
• Eukaryotic, multicellular, autotrophic organisms with tissues
– Animalia
• Eukaryotic, multicellular, heterotrophic organisms with tissues
CLASSIFICATION
CLASSIFICATION
CLASSIFICATION
CLASSIFICATION
Six Kingdoms
• In recent years, as evidence about
microorganisms continued to accumulate,
biologists came to recognize that the Monera
were composed of two distinct groups
• Some biologists consider the differences
between these two groups to be as great as
those between animals and plants
• As a result, the Monera have been separated
into two kingdoms, Eubacteria and
Archaebacteria, bringing the total number of
kingdoms to six
Six Kingdoms
• The six-kingdom system of
classification includes the kingdoms
Eubacteria, Archaebacteria, Protista,
Fungi, Plantae, and Animalia
• This system of classification is shown in
the bottom row of the table
Six Kingdoms
• This diagram shows
some of the ways
organisms have been
classified into kingdoms
over the years
• The six-kingdom
system includes the
following kingdoms:
Eubacteria,
Archaebacteria,
Protista, Fungi, Plantae,
and Animalia
Six Kingdoms
The Three-Domain System
• Some of the most recent evolutionary
trees have been produced using
comparative studies of a small subunit
of ribosomal RNA that occurs in all
living things
• Using a molecular clock model,
scientists have grouped modern
organisms according to how long they
have been evolving independently
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
• 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
• As scientists continue to accumulate new
information about organisms in the domains
Bacteria and Archaea, these domains may be
subdivided into additional kingdoms
The Three-Domain System
• Clearly, modern classification is a rapidly changing
science, and we must pick a convention to classify
life's diversity for the purposes of this Text
• In this Text, we recognize the three domains and also
refer frequently to the six kingdoms
• The relationship between the three domains and the
six kingdoms is shown in the table
• It also summarizes the key characteristics of each
kingdom
• You can see that some groups share one or more traits
with other groups
The Three-Domain System
• Organisms are grouped
in three domains
• There is a simple
relationship between the
three domains and the six
kingdoms
• This table summarizes
key evidence used in
classifying organisms into
these major taxonomic
groups
The Three-Domain System
Domain Bacteria
• The members of the domain Bacteria are unicellular
and prokaryotic
• Their cells have thick, rigid cell walls that surround a cell
membrane
• The cell walls contain a substance known as
peptidoglycan
• The domain Bacteria corresponds to the kingdom
Eubacteria
• These bacteria are ecologically diverse, ranging
from free-living soil organisms to deadly parasites
• Some photosynthesize, while others do not
• Some need oxygen to survive, while others are killed
by oxygen
Domain Archaea
• Also unicellular and prokaryotic, members of
the domain Archaea live in some of the most
extreme environments you can imagine—
volcanic hot springs, brine pools, and black
organic mud totally devoid of oxygen
• Indeed, many of these bacteria can survive
only in the absence of oxygen
• Their cell walls lack peptidoglycan, and their
cell membranes contain unusual lipids that
are not found in any other organism
• The domain Archaea corresponds to the
kingdom Archaebacteria.
Domain Eukarya
• The domain Eukarya consists of all
organisms that have a nucleus
• It is organized into the four remaining
kingdoms of the six-kingdom system:
–
–
–
–
Protista
Fungi
Plantae
Animalia
• Organisms in these kingdoms are diverse and
varied
Domain Eukarya
• The domains Bacteria and
Archaea include the same
organisms that are in the
kingdoms Eubacteria and
Archaebacteria
• The domain Eukarya includes
the protists, fungi, plants, and
animals
• Biologists continue to
investigate how these three
large groups originated
• Which domain includes
organisms from more than one
kingdom?
Domain Eukarya
Protista
• The kingdom Protista is composed of eukaryotic
organisms that cannot be classified as animals,
plants, or fungi
• Of the six kingdoms, Protista is the least satisfying
classification, because its members display the
greatest variety
• Most protists are unicellular organisms, but some,
such as the multicellular algae, are not
• Some protists are photosynthetic, while others are
heterotrophic
• Some share characteristics with plants, others with fungi,
and still others with animals
Fungi
• Members of the kingdom Fungi are
heterotrophs
• Most feed on dead or decaying organic
matter
• Unlike other heterotrophs, these fungi secrete
digestive enzymes into their food source
• They then absorb the smaller food molecules
into their bodies
• The most recognizable fungi, including
mushrooms, are multicellular
• Some fungi, such as yeasts, are unicellular
Plantae
• Members of the kingdom Plantae are multicellular
organisms that are photosynthetic autotrophs
– In other words, they carry out photosynthesis
• Plants are nonmotile—they cannot move from place to
place
• They also have cell walls that contain cellulose
• The plant kingdom includes cone-bearing and flowering
plants as well as mosses and ferns
• Although older classification systems regard multicellular
algae as plants, in this book we group algae with the
protists
Animalia
• Members of the kingdom Animalia are
multicellular and heterotrophic
• The cells of animals do not have cell walls
• Most animals can move about, at least for some
part of their life cycle
• As you will see in later chapters, there is
incredible diversity within the animal kingdom,
and many species of animals exist in nearly
every part of the planet
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