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Overview: Investigating the Tree of Life
• Phylogeny is the evolutionary history of a species or group of related species
• The discipline of systematics classifies organisms and determines their evolutionary relationships
• Systematists use fossil, molecular, and genetic data to infer evolutionary relationships
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Concept 26.1: Phylogenies show evolutionary relationships
• Taxonomy is the ordered division and naming of organisms
Binomial Nomenclature
• 18th century Carolus Linnaeus published a system of taxonomy based on resemblances
• Two key features of his system remain useful today: 1) twopart names for species2) hierarchical classification
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Figure 26.3 Hierarchical classification
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Figure 26.4 The connection between classification and phylogeny
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Figure 26.5 How to read a phylogenetic tree
• Each branch point represents the divergence of two species
• Sister taxa are groups that share an immediate common ancestor
• A rooted tree includes a branch to represent the last common ancestor of all taxa in the tree
• A polytomy is a branch from which more than two groups emerge
• Application Example Phylogenies of anthrax bacteria helped researchers identify the source of a particular strain of anthrax
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http://www.indiana.edu/~ensiweb/famtree_v_phylog.html
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http://www.indiana.edu/~ensiweb/famtree_v_phylog.html
Fish
Salamander
You
Common ancestor of all vertebrates
Common ancestor of fourlegged vertebrates
Common ancestor of mammals
Cat
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Concept 26.3: Shared characters are used to construct phylogenetic trees
• Once homologous characters have been identified, they can be used to infer a phylogeny
Cladistics
• Cladistics groups organisms by common descent
• A clade is a group of species that includes an ancestral species and all its descendants
• Clades can be nested in larger clades, but not all groupings of organisms qualify as clades
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Figure 26.10 Monophyletic, paraphyletic, and polyphyletic groups
• A valid clade is monophyletic, signifying that it consists of the ancestor species and all its descendants
• A paraphyletic grouping consists of an ancestral species and some, but not all, of the descendants
• A polyphyletic grouping consists of various species that lack a common ancestor
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(A) Cladists classify birds and reptiles together because they have a recent common ancestor and are a monophyletic group.
(B) The evolutionary systematics approach considers both common ancestry and extent of divergence that has occurred since two taxa split. Reptiles are a paraphyletic group. Lizards, snakes, and crocodiles are phenotypically most similar, but crocodiles, dinosaurs, and birds are most closely related because they evolved most recently from a common ancestor.
The cladistic approach, shown in (A) is preferred becaused it recognizes reptiles and birds as a monophyletic group.
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Shared Ancestral and Shared Derived Characters
• A shared ancestral character is a character that originated in an ancestor of the taxon.
Example ancestor of mammals have a backbone.
• A shared derived character is an evolutionary novelty unique to a particular clade.
Example all mammals have hair which is not found in its ancestor
• A character can be both ancestral and derived, depending on the context.
Example backbone can be a shared derived character at a deeper branch point
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Figure 26.11 Constructing a phylogenetic tree
0 = character absent, 1 = character present
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Figure 5 : The information on patterns of evolutionary descent is the same regardless of the lengths of branches.
http://www.nature.com/scitable/topicpage/readingaphylogenetictreethemeaningof41956Source of Figure 5
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Figure 6 : These trees depict equivalent relationships despite being different in style.
http://www.nature.com/scitable/topicpage/readingaphylogenetictreethemeaningof41956Source of Figure 6
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Concept 26.6: New information continues to revise our understanding of the tree of life
Now there are 3 domains even broader than a kingdom!
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Figure 26.21 The three domains of life
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Is the Tree of Life Really a Ring?
• Some researchers suggest that eukaryotes arose as an endosymbiosis between a bacterium and archaean.
• If so, early evolutionary relationships might be better depicted by a ring of life instead of a tree of life.