THE TREE OF LIFE

WHY CLASSIFY?

• We classify to make organisms easier to study.

• To study the diversity of life, biologists use a classification system to name organisms and group them in a logical manner.• The field of taxonomy allows scientists

to classify organisms and assign each organism a universally accepted name in order to study the diversity of life.

LINNAEUS’S SYSTEM OF CLASSIFICATION

• During the eighteenth century, Swedish botanist Carolus Linnaeus developed a two-word naming system called binomial nomenclature• Linnaeus often classified organisms based on similarities in structures and details of anatomy

BINOMIAL NOMENCLATURE

• In binomial nomenclature, each species is assigned a 2-part Latin name• The name is always written in italics

with the 1st word is always capitilized• Example: Homo sapiens (humans)

• The 1st part of the name is the genus and the second part of the name is unique to each species within that genus

LINNAEUS’S SYSTEM OF CLASSIFICATION

• Linnaeus’s system of classification uses 7 taxonomic categories (from largest to smallest):• Kingdom – largest & most inclusive• Phylum – includes many different organisms that

share important characteristics• Class – composed of similar orders• Order – composed of similar families• Family – genera that share many characteristics• Genus – a group of closely related species• Species – a group of individuals so similar that

they can breed and produce fit offspring

HUMAN CLASSIFICATION

• Kingdom: Anamalia• Phylum: Chordata

• Subphylum: Vertebrata

• Class: Mammalia• Order: Primates• Family: Hominidae• Genus: Homo• Species: Homo sapiens

KINGDOM Animalia

PHYLUM Chordata

CLASS Mammalia

ORDER Carnivora

FAMILY Ursidae

GENUS Ursus

SPECIES Ursus arctos

PROBLEMS WITH TRADITIONAL CLASSIFICATION

• During Linnaeus’s time, scientists classified organisms based on their physical appearance…but today…we know that doesn’t always work.

• Ex: Dolphins Fish or Mammals?

• Remember Convergent Evolution sometimes organisms that are different from each other evolve similar body structures, due to the change in the environment.• This does not mean they need to be classified in the

same group…maybe they only evolved similar body structures!

• These situations make it very difficult for scientists to classify.

EVOLUTIONARY CLASSIFICATION

• Biologists now group organisms into categories that represent lines of evolutionary descent, not just physical features• Evolutionary classification (phylogeny)

is the strategy of grouping organisms together based on their evolutionary history

TRADITIONAL V/S EVOLUTIONARY CLASSIFICATION

CLASSIFICATION USING CLADOGRAMS

• To refine evolutionary classification, biologists now prefer a method called cladistics• Cladistics considers only those characteristics that

are 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

• Characteristics that are shared by all members of a lineage are ancestral/primitive characters and should not be used when building cladograms.

• Cladogram – a diagram that shows the evolutionary relationships among a group of organisms; includes new characteristics that arise as lineages evolve.

DERIVED CHARACTERISTICS

ANATOMY OF THE CLADOGRAM

A B

C D E F G H

I

J

Paraphyletic group

Monophyletic group = clade

Synapomorphy (shared derived character)

Node (represents most recent common ancestor of two or more taxa)

Lineage (represents a sequence of ancestor-descendent populations)

Taxon

MONOPHYLETICGROUPS:

include ancestor & all descendents

PARAPHYLETIC GROUPS:

includes ancestor and some, but not all descendent

MONOPHYLETIC CLADES

• A valid clade is monophyletic signifying that it consists of the ancestor species and all its descendants.

OUTGROUPS

• Systematists use a method called outgroup comparison• To differentiate between shared derived and shared

primitive characteristics

• As a basis of comparison we need to designate an outgroup• which is a species or group of species that is closely

related to the ingroup, the various species we are studying

• Outgroup comparison• Is based on the assumption that homologies

present in both the outgroup and ingroup must be primitive characters that predate the divergence of both groups from a common ancestor

PERFORMING OUTGROUP COMPARISON

OUTGROUP COMPARISONS

• First, look at the animals we are studying and establish which characteristics that they share & which are unique to each individual species.

• Then use these derived characters to build your cladogram.

CELLS BACKBONE LEGS HAIR OPPOSABLE THUMB

Slug

Catfish

Frog

Tiger

Human

Cells

Backbone

Legs

Hair

Opposable Thumbs

THE THREE-DOMAIN SYSTEM

• Molecular analyses have given rise to the most current classification system – the Three Domain System• The 3 Domain System is the most recent classification

system and includes: • Bacteria• Archaea• Eukarya

DOMAINS ARE BIGGER/MORE INCLUSIVE THAN KINGDOMS

Domains Kingdoms

Bacteria Eubacteria

Archaea Archaebacteria

Eukarya

Protista

Fungi

Plantae

Animalia

These 2 kingdoms used to be combined into one called “Monera”

REMEMBER - THERE ARE 3 METHODS OF CLASSIFYING

ORGANISMS

• The 3 Domain System (most current)• Bacteria, Archae, & Eukarya

• The 5 Kingdom System (Whittaker)• Monera, Protista, Fungi, Plantae, Animalia

• The 6 Kingdom System – still used in conjunction with the 3 domain system

• Archaebacteria, Eubacteria, Protista, Fungi, Plantae, Animalia

Classification of Living Things

Section 18-3

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DOMAIN BACTERIA

• Kingdom: Eubacteria• Unicellular• Prokaryotic• Cell wall of peptidoglycan• Autotrophic or heterotrophic• Examples: Streptococcus, E. Coli

DOMAIN ARCHAEA

• Kingdom: Archaebacteria• Unicellular• Prokaryotic• Cell walls w/out peptidoglycan• Autotrophic or heterotrophic• Methanogens & halophiles

DOMAIN EUKARYA

WHITTAKER’S FIVE-KINGDOM SYSTEM

OUR CHANGING VIEW OF BIOLOGICAL DIVERSITY

THE THREE DOMAIN SYSTEM

• Describes classification as:• Not all prokaryotes are closely related (not

monophyletic)• Prokaryotes split early in the history of living

things (not all in one lineage)• Archaea are more closely related to Eukarya

than to Bacteria• Eukarya are not directly related to Eubacteria• There was a common ancestor for all extant

organisms (monophyletic)• Eukaryotes are more closely related to each

other (than prokaryotes are to each other)