Phylogenies & Classifying species (AKA Cladistics &
Taxonomy)What are phylogenies?How do we read them?
How do we estimate them?
Carolus Linneaus:Systema Naturae (1735)
• Swedish botanist & natural theologist
• Hierarchical classification based on “ideal of unchanging types”.
• Binomial nomenclature - replaced polynomials
• Categories:• K, P,C, O, F, G, S• Assumes a static,
unchanging Universe
Charles Darwin and Fig. 1
• Origin of Species (1859)
• Species diverge; they do so gradually
• Articulated idea of shared ancestry, branching of lineages (cladogenesis) and change along branches (anagenesis)
• Only figure in OS: a dendrogram
Problems with Linnean system
• It does NOT represent evolutionary history.• Hierarchical categories do not imply
degree of evolutionary change or divergence.– Are Rabbits as different from Rodents as they
are from Elephants?
• When and how should we split groups into separate categories? - splitting vs. lumping– How different is different enough to warrant
separate groupings?– Ex: Dogs & wolves
Dendrogram ~ Phylogenies
• Phylogeny: Interpretation of the evolutionary history and relationships between a group of organisms
• Phylogenetic tree: Hypothesis of ancestor-descendent relationships among populations, species, or larger groups*
Why construct phylogenies?
1. Understand evolutionary history2. System of classifying organisms &
organizing diversity– Most biologists agree: most efficient
way to classify organisms is based on phylogenetic relatedness
All species coalesce to a common ancestor:
• Recall our lab:• Just as all genes coalesce
to a single ancestral gene, all individuals coalesce to a single ancestor.
• Like tracing a family tree
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•Tips•Branches•Nodes•Root•Most Recent Common Ancestor (MRCA)
Phylogeny of Vertebrates
Name another tipName another node
Monophyletic groupings
Who is the MRCA of Mammals and Crocodiles?
Monophyletic groupings
Nodes name monophyletic groups:An ancestor and ALL of its descendents
Reading phylogenies• 5 of these are identical• Which one is not the same?*
Paraphyletic groupings
Tree of Life
What data should we use to generate trees?
• What data do you use?• Often do it without understanding why
What data should we use to determine evolutionary relationships?
• Characters: distinguishable variations of an organism
1.Morphologic2.Developmental3.Genetic• Quantifiable• Independent of each other• Homologous: due to common ancestry
ACGTTGAAACCTTGTA
Phenetic approach• Sokal (1950) Made classification rigorous.
Based groupings on many characters rather than few “important” ones.– Calculated overall SIMILARITY. Generated
phenograms (Basically, phylogenetic trees)
• Characters: Vary independently of other features. Homologous among ingroups.
• Character states: alternative versions of character.– Presence/absence– 1/2/3/4…etc.
Characters• Are the bones of the upper
limb homologous structures?• Are wings of bats, birds and
pterosaurs homologous structures?
PitfallsWhen Similarity ≠ Close
Relationship!• 2 taxa may share a derived character state• OR may share an ancestral character state• OR may share a character state due to
convergence to same phenotype (MRCA did not have the character) These are analogous traits– Bird and bat wings– Dermopterans, Flying squirrels, Sugar gliders
• Only if first case is true will you ALWAYS infer correct phylogenetic relationship.
Anura Caudata Caecilians Mammals Turtles Crocs Birds Tuatara Lizards Snakes
Diapsida
Where is the MRCA of Tuatara and Crocs?
Anura Caudata Caecilians Mammals Turtles Crocs Birds Tuatara Lizards Snakes
B
C
A
D
The Monophyletic group that includes Birds, Crocs & Turtles begins at node:
Willi Hennig and Cladistics
• Grouping scheme based ONLY on phylogenetic relationships– Not on degree of similarity
and adaptive divergence
• Used presence of shared derived characters (synapomorphies) to infer evolutionary relationships
• Based trees upon total weight of ALL synapomorphies
Phylogenetic terms*• Plesiomorphy: Ancestral character state• Symplesiomorphy: Ancestral character
state shared by many taxa - phylogenetically uninformative
• Apomorphy: Derived (novel) character - phylogenetically uninformative
• Synapomorphy: Shared derived character - GOLD
• ”Hennig has indeed emphasized and defined some procedures of phylogenetic analysis that have long been used by systematists. He might even have clarified them if he had not unnecessarily replaced the usual plain-language terms by a bizarre and idiosyncratic new terminology." (1978) G. G. Simpson
Similarity ≠ Relationship
• 2 taxa may share a derived character state• OR may share an ancestral character state• OR may share a state due to convergence
– Bird and bat wings– Dermopterans, Flying squirrels, Sugar gliders
• Only if first case is true will you infer the phylogenetic relationship correctly.
synapomorphysymplesiomor
phyanalogy
Synapomorphies identify monophyletic groups
• Unite groups with shared, (only among each other) derived (from some ancestral group) characters
Problems• What characters (traits) are “best” for
assessing evolutionary relationships?– Skull length, body size, pelage color, limb
modification and specialization, chromosome #?• When and how should we split groups into
separate categories? - splitting vs. lumping– How different is different enough to warrant
separate groupings?• Can/should hierarchical categories imply
degree of evolutionary change or divergence?– Are Hyraxes as different from Manatees as they
are from Bats?– Are dogs as different from cats as they are from
humans?
Problems with characters
• Which characters are phylogenetically informative?
• How do we quantify # of possible states & frequency of change among them?
• How do we establish polarity?• How do we deal with continuous
traits: Quantify or discard them?
Ideal character• States are discrete• # of states are knowable (across taxa)• Transition frequencies (rates of change)
are estimable
DNAONLY 4 states; A C G TNo polarityCan measure transition
frequencies
Method of inferring a tree• Choose the most parsimonious one• Parsimony: Methodological reductionism;
Explanation which requires the fewest undocumented assumptions is probably correct.– Occam’s Razor: The easiest explanation is
probably the correct one.– William of Occam: 14th century Franciscan monk
• In cladistics/phylogeny: The tree which requires us to postulate the least evolutionary change & fewest homoplasies is probably correct
Parsimony• Find all possible tree topologies; calculate
total number of changes required to produce each topology.
• Topology with fewest changes = the most parsimonious tree*
A B C D C A B D
Taxa: A, B, C, DCharacters: › › ›
Placement of Cetaceans• Morphology of astragalus unites Artiodactyls• Most parsimonious placement = one gain of
pulley-shaped astragalus
Placement of Cetaceans• Mounting genetic evidence suggests Hippos
have shared genes with Cetaceans more recently than they have with other Artiodactyls (e.g. deer)
Placement of Cetaceans• Appearance of SINE’s (mutations)
throughout the genome suggests that Cetaceans & Hippos are close relatives– Rare; Selectively neutral; Once established, lost
only via drift