Taxonomy
• To sort organisms into species
• To classify species into higher taxonomic
levels
• A taxon is a taxonomic unit at any level;
for example “Mammalia” is a taxon at the
Class level
(taxa = plural)
Taxonomy
• Species that appear to be closely related
are grouped into the same genus (e.g., the
leopard Panthera pardus; African lion
Panthera leo; Tiger Panthera tigris)
• Similar genus are grouped into the same
family, and so on…
Domain
Kingdom
Phylum
Subphylum
Class
Order
Family
Diversification
Genus
Species
Taxonomy
• Classification system
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Domain
Kingdom
Phylum
Class
Order
Family
Genus
species
Eukarya
Animalia
Chordata
Mammalia
Primates
Hominidae
Homo
sapiens
Taxonomy
• The taxonomic system was developed by
Linnaeus in the 1750’s
• Binomial (two part) system; Genus species
• The scientific name includes the species
name
• Example: Homo sapiens or H. sapiens
• No 2 organisms can have the same
scientific name and this name is the same
everywhere in the world!
Taxonomy
• Scientific names are descriptive
• May describe unique characteristic, region
where species is found, etc.
• Example: Humpback whale Megaptera
novaeangliae
– Magas = large
– Pteron = wing
– Novas = new
– Angaliae = England
Taxonomy
• Common names can be deceiving…
“bear”
“dolphin”
Taxonomy
• Scientific names may change over time
• Many hierarchies are being re-examined
based on results of molecular analysis
• Linnaean taxonomy does not take into
account evolutionary relationships
• Enter phylogeny and systematics…
Systematics
• The reconstruction and study of
evolutionary relationships
• Phylogeny – an evolutionary tree; species
grouped by how long they’ve shared a
common ancestor
• Evidence for determining these
relationships must come from shared
homologous characteristics
Phylogeny
• The evolutionary history of a group is
presented graphically in phylogenetic
trees
• In a phylogenetic tree:
– Time goes from the bottom up
– Read from bottom up, not left to right
– Branch length = number of changes (not time)
– Depicts evolutionary relationships
Time
Time
Remember: Read from the bottom up; not
left to right!
Node 1: common
ancestor of human and
chimp
Node 2: c.a. of human,
chimp, and gorilla
These three versions convey the same
information (phylogenies depict evolutionary
relationships)
Phylogenetic trees reflect hierarchical
classification of taxonomic groups
Phylogeny
• Systematists prefer
monophyletic taxa
• Monophyletic – a single
ancestor gave rise to all species
in that taxon (and to no species
in any other taxa); “single tribe”
• A monophyletic group consists
of a common ancestor and all
its descendants
Monophyletic group
Common ancestor:
“Archosaur”
Monophyletic group (indicated in pink) consists of
most recent common ancestor and all of its
descendents
Paraphyletic group
1 descendent, the
hawk, is excluded
Paraphyletic group (indicated by pink) consists
of the most recent common ancestor and some
(not all) of the descendents
Polyphyletic group
Birds and bats can be
included because of
morphological similarities
(convergent evolution)
Polyphyletic group (indicated in pink) does NOT
contain the most recent common ancestor
Monophyletic versus Paraphyletic
and Polyphyletic groups
Old classification
system for plants
New classification
system for plants
Phylogeny
• What evidence is used to reconstruct
phylogeny?
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Comparative anatomy
Morphology
Embryology
Protein comparisons
DNA comparisons
• Problems of homology versus analogy
Phylogeny
• Homology (homologous structures) –
likeness due to common ancestor
• Analogy (analogous structures) – also
known as “homoplasy”; likeness due to
convergent evolution; functional constraint
• Analogous structures are the result of
separate evolution
Comparative Biology
Elongated canine teeth (saber teeth) evolved
separately in different groups of carnivores
Cladistics
• Cladistics – only shared derived
characters (syanpomorphies) are used in
determining evolutionary relationships
• Homoplasy (analogy) complicates cladistic
analysis
• Species that share a common ancestor
constitute a clade (a monophyletic group is
a clade)
• Clades are hypotheses, just as
phylogenetic trees are
clade
Time
Say what???
• Derived characters: those inherited from
the most recent common ancestor
• Ancestral characters: similarities that
arose prior to the common ancestor of the
group
• Cladistics uses shared derived characters,
termed synapomorphies
Say what??? (continued)
• Presence of hair in mammals is a
synapomorphy (derived character)
• Presence of lungs in mammals is a
symplesiomorphy (ancestral character);
lungs do not define mammals, they are
found in birds, and reptiles, amphibians.
Cladistics
• Principle of parsimony – favors the
hypothesis that requires the fewest
assumptions (the phylogeny that requires
the fewest evolutionary events is the best
hypothesis)
• Simplest is best (Occam’s Razor)
Parsimony and Homoplasy
Placement of frogs as closely related to salamanders requires
that tail loss (in adults) evolved twice (homoplasy); if frogs are
closely related to gorillas, then tail loss evolved once, but would
also have to lose amniotic membrane and hair
Building Cladograms
Recommended Reading
• Dictionary of Word Roots and Combining
Forms; Donald J. Borror
• Compiled from Greek, Latin and other
languages, with special reference to
biological terms and scientific names