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Classifying Organisms
 Systematics / Taxonomy: the study of classifying
organisms into groups
 The best classification systems categorize organisms
based on natural relationships due to common
ancestry (homologous features) rather than incidental
similarities (analogous features)
Homologous vs. Analogous
 Homologous Structures: features that are common in
different organisms because their common ancestry
also possessed this feature.
 Ex: pentadactyl limb of humans, cats, bats, whales…..
 Analogous Structures: features in different organisms
that serve a similar function, however they are
structurally different since they evolved independently
(their common ancestor does not posses this feature)
 Ex: wings of birds, bats, and insects
Vestigial Structures
 These are structures that do not serve a function in an
organism but are believed to have had a purpose in earlier
ancestors.
 “Evolutionary baggage”
 Shows homology
 Ex: appendix: While in modern humans there is no
purpose, it is believed that it used to aid in the digestion of
cellulose when our ancestor’s diets were more plant based.
 Ex: Gills in embryos
Linnaean Taxonomy
 The traditional Linnaean system of taxonomy
attempted to classify organisms based on their
morphological characteristics
 It did not account for analogous structures and
convergent evolution
 As a result, it does not show true evolutionary
relationships
Biochemical Evidence
 How does biochemical evidence support the idea that
all living organisms on Earth share a common
ancestor?
 All organisms use DNA or RNA as their molecule of
heredity.
 These nucleic acids code for proteins. All organisms
use virtually the same 20 amino acids (and in the same
conformation – left handed!)
 The genetic code is universal – all living organisms use
the same “codon dictionary” to interpret DNA and
code for amino acids
Ex: Alpha-Hemoglobin
 Analysis of the sequences for the alpha-hemoglobin
chain show that:
 The sequences or amino acids are identical in humans
and in chimps
 There are only 2 amino acids that differ in the sequences
of orangutans.
 This suggests that humans are more closely related to
chimps than they are orangutans
Cladistics / Phylogenetics
 Attempt to illustrate the evolutionary relationship
between organism with phylogenetic trees/ cladograms
 Uses morphological characteristics such as homologous
structures but not analogous structures
 Usually uses biochemical evidence such as DNA, RNA,
mDNA (mitochondrial DNA) and protein sequences
Cladistics and Phylogeny
 Cladistics uses the presence or absence of derived
traits (recently evolved traits) to determine how
closely related two groups or organisms are.
 If they share a derived trait, they are thought to be
more closely related to each other.
Cladograms and Phylogenetic Trees
 These are diagrams used to show the evolutionary
relationship between organisms.
 The terms may be used interchangeably however:
 Cladograms use morphological evidence and
Phylogenetic Trees you genetic sequences (DNA, RNA,
mDNA)
 Species A is the most recent common ancestor shared by all
groups
 Species B is a common ancestor to all groups except the rayfinned fish
 The most common ancestor of a mammal and a dinosaur is
Species C
Morphological Traits vs Genes?
 Modern evolutionary biologists use biochemical
evidence to create phylogenetic trees.
 They often use mDNA.
 Mitochondrial DNA in all organisms is quite similar.
However, the differences can provide insight as to the
evolutionary relationships between organisms.
Phylogenetic Tree
The point where the tree
branches is called a “node”. This
is represents a common
ancestor.
This node represents the
common ancestor of a
human and a mouse.
The alpha-hemoglobin in these 2
ancestral species is different.
There are a minimum of 51
substitution mutations between
them
 Phylogenetic trees are usually confirmed by studying
other conserved proteins such as cytochrome c (an
enzyme essential for cellular respiration), as well as the
fossil record and other taxonomic data such as
evolutionary clocks
Evolutionary Clock
 A technique in molecular genetics to date when two
species diverged.
 It calculates the time since their common ancestry by
examining the number of differences between their
DNA or protein sequences.
 Assumes that the rate of evolutionary change of any
protein or DNA sequences is constant over time.
Analysis of Cladograms
 Note: Although cladograms provide strong evidence for the
evolutionary history of a group, they cannot be regarded as
infallible proof
 Cladograms are constructed on the assumption that the
smallest possible number of mutations occurred to account
for current base or amino acid sequence
 Sometimes the assumptions made by cladograms are
incorrect.
 It’s best to compare several versions produced by using
different genes/traits
Reclassification
 Evidence from cladistics has led to the renaming and
the regrouping of organisms.
 Occasionally organisms long thought to be related
because of shared physical traits are reclassified based
on phylogenetic trees constructed from biochemical
evidence.
Evidence from traditional cladistics has shown that
classification of some groups based on morphology does
not correspond with the evolutionary origins of a group
of species.
Cladograms and falsification
 Falsification of theories with one theory being
superseded by another has been observed from the
reclassification of plants as a result of evidence from
cladistics.
 The classification of angiosperms into families based on
their morphology was begun by French botanist Antoine
Laurent de Jussieu in 1789, and was revised repeatedly
during the 19th century.
Classification of the figwort family
 Until recently, Figworts
(Scrophulariaceae family) were the
8th largest family of angiosperms
(flowering plants).
 It was one of the original families
proposed by de Jussieu in 1789. As
more plants were discovered, the
family grew from 16 genera in 1789
(based on similarities in their
morphology) to 275 genera, with
more than 5,000 species.
Classification of the figwort family
 Taxonomists recently examined the base sequences of
three chloroplast genes and found that the 5000
figwort species were not a true clade and that 5 clades
had incorrectly been combined into one family.
 Less than half of the original species remain in the
Figwort family; now only the 36th largest among
angiosperms.
 Reclassification was helpful since old Figwort family
was too large and dissimilar to be a helpful grouping.
Antirrhinum majus has been
transferred from the figwort
family to the plantain family.
Scrophulara peregrina has
remained in the figwort
family.
Definitions
 Cladistics: a method of taxonomy based on
constructing groups, or clades comprising organisms
which share unique homologous characteristics.
 Clade: a group of organisms with a single common
ancestor and ALL the descendants of that ancestor.
 The group shares common characteristics, or derived
characteristics.
 All 3 of these cladograms
show the exact same
information
 4 present day species are
indicated (A, B,C,D)
 Node 1: shows a common
ancestor to all 4 species
 Node 3: is only common
to C and D
 Since species B and C
share a more recent
common ancestor (node
2) they are more closely
related that A and B.
 Cladistics only
recognizes monophyletic
groups
 MONOPHYLETIC –
groups
that include a single
ancestor and ALL of its
descendants.
 Polyphyletic: groups that do not include a common
ancestor
 Paraphyletic: groups that include the common
ancestor but not all of its descendants
How to
make a
cladogram
Constructing a Cladogram
Exit Card
 Draw a cladogram based on the following information:
Answer: