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 Monophyletic group = clade J Taxon I F G H C D E Paraphyletic group Lineage (represents a sequence of ancestor-descendent populations) A B MONOPHYLETICGROUPS: include ancestor & all descendents PARAPHYLETIC GROUPS: includes ancestor and some, but not all descendent Node (represents most recent common ancestor of two or more taxa) Synapomorphy (shared derived character)

Monophyletic Clades A valid clade is monophyletic signifying that it consists of the ancestor species and all its descendants. 14

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 15

Performing Outgroup Comparison Who is the outgroup? 16 16

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

Opposable Thumbs Hair Legs Backbone Cells

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 Bacteria Eubacteria Archaea Archaebacteria Eukarya Protista Fungi Plantae Animalia These 2 kingdoms used to be combined into one called “Monera” 20

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 21

Classification of Living Things Section 18-3 Classification of Living Things Go to Section:

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 26

Our changing view of biological diversity 27

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) 28