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Phylogeny and the Tree of Life Chapter 26 Phylogeny What is phylogeny? The evolutionary history of a group Systematics attempts to reconstruct phylogeny, by analyzing evolutionary relatedness ◦ Use morphological and biochemical similarities Molecular systematics uses DNA RNA and proteins to infer evolutionary relatedness Different tools are used to reconstruct phylogenies Systematics Uses evidence from fossil record and existing organisms to reconstruct phylogeny Use branching taxonomic categories such that they reflect phylogeny Binomial nomenclature Genus species keeps identity of organism universal Other taxa: Domain, Kingdom, Phylum, Class, Order, Family Genus, species Taxonomy Cladistics Cladogram is a tree with two way branch points Each branch point represents divergence from common ancestor Each branch is called a clade Clades are monophyletic Phenetics Phenetics = ◦ compares many anatomical characteristics to (overall phenotype) to construct phylogeny ◦ Does not sort homologous from analogous structures. phylogenetic trees Monophyletic vs. Polyphyletic Taxa Monophyletic = single ancestor gave rise to all species in the taxon Polyphyletic = taxon whose members derive from 2 or more different ancestors Paraphyletic = excludes species that share the common ancestor Creating the ideal monophyletic taxa is not always easy Not all Similarities Represent Common Ancestry Homologous structures indicate shared common ancestry Homologous structures are therefore evidence of divergent evolution Analogous structures are similar in function but not in evolutionary history Analogous structures are evidence of convergent evolution It is not always easy to sort homologous from analogous structures How do we differentiate between homologous and analogous structures? Compare embryonic development of the structures in question Look for structures that are complex The more complex the structures are the more likely that they are homologous Compare macromolecules along with anatomical features How would you compare the fins in these 2 organisms? In what way are these organisms displaying examples of convergent evolution? Cladistics Cladistics ◦ sorts homologous from analogous structures ◦ sorts primitive and shared derived characteristics Shared Derives Characteristics Need to differentiate between shared primitive characters and shared derived characters All similar characters Analogies Primitive (ancestral) Homologies Derived (unique to a clade) Outgroups Distinguishes between shared primitive and shared derived characteristics Closely related to ingroup Performing Outgroup Comparison What is the shared ancestral characteristic? Notochord Does not mean that turtles evolved more recently than salamander Maximum Parsimony and Maximum Likelihood Maximum parsimony ◦ Simplest explanation consistent with facts Maximum likelihood ◦ Use most likely sequence of events to explain evolutionary changes ◦ If we assume DNA changes occur at equal rates among all organisms then…. Four Species Problem After analyzing DNA sequences and mutations, this tree is the simplest explanation of the changes And sometimes the simplest explanation is not the best… Which is more correct? Parsimony does not always work, nature does not always take the simplest course Modern Systematics Hominidae Pongidae Classical Taxonomy Hominidae Cladistic Taxonomy Pongidae Molecular Clocks Proteins and mitochondrial genomes have constant rate of change over time Use these rates to determine relative evolutionary relatedness. Accuracy of these clocks is still debated Recently used to date origin of HIV The End!
