Presentation on theme: "Estimating and using phylogenies"— Presentation transcript:
1 Estimating and using phylogenies Ozzie VilhelmssonZoology Building, Room 213Tel.: ( ) 2867Estimating and using phylogenies
2 Taxonomy and Phylogeny What fossils tell usWhat living organisms tell usCladisticsConstructing phylogeniesClassification & Evolutionary RelationshipsMolecular Analyses - the way ahead!
3 Fossils Incompleteness of fossil record Fossilization an unlikely eventOnly found in sedimentary rocksHabitat biasAge knownIntermediates observedCan access extinct lines
4 PHYLOGENETIC TREES Pedigree of a lineage Evidence of dates of separation(trees)TimeTime
5 Tree construction Similarity matrix Tree Simple, right? Gather data: MorphologyDevelopmentMetabolicBiochemicalGeneticAnything, reallySimilaritymatrix(numerical taxonomy)TreeSimple, right?
6 But, .... Turtle/birds/crocodile picture ... Different data can yield different trees!
7 CLADISTICS Aims to distinguish reliable from unreliable characters: Homologies vs. HomoplasiesDerived vs. ancestral homologies
8 CLADISTICSMethod of determining evolutionary histories - displayed as treesClade: entire portion of phylogeny from a common ancestor = Monophyletic groupCladogram: unrooted evolutionary tree (no ancestors but points where lineages diverged)
9 HOMOLOGIESA trait shared between species and inherited from their common ancestor = homologousAncestral (general) homologies: shared by all species in lineage - eg. vertebrae in vertebratesDerived (special) homologies: shared by few species in lineage - eg. indeterminate incisors in vertebrates
14 Identifying non-useful traits Divergence = traits unrecognizableeg. plant leavesHomoplasy = trait evolves more than oncedifferent structures resemble each other by convergent evolution eg. bat/bird/insect wingsBoth cases = analogous traits
15 Hennig’s MethodSame trait in 2 species = provisionally homologous ie. innocent, until proven guiltyAncestral homology = found in group and outside in species = outgroupOutgroup = branched off from below base of lineage
16 What about wings? Homoplasy/homology depends on reference/outgroup Homoplaseous? Ancestral? Derived?Fig in textbookHomoplasy/homology depends on reference/outgroup
17 Rooting the treeHaving figured out which traits are important, we can draw acladogram. But, where does it root?(Possible roots picture)DistanceParsimonyMaximum likelihoodThree methods:
18 DistanceSimple principle: How similar are the species? (similarity matrix/measurement)Works well for simple molecular methods, such as DNA:DNA hybridization data“Molecular clock” assumption
20 PARSIMONY Simple distance rooting assumes: trait evolution irreversible, ie. ancestral to derivedtrait can change only once per lineage UNREALISTICBut, cladogram requiring fewest reversals/changes most likely to be correctPARSIMONY = simplest is correct!
25 A phylogenetic tree Relative evolutionary time Ancient events HagfishPerchSalamanderLizardCrocodilePigeonMouseChimpanzeeLungsJawsClaws ornailsFour-chamberedheartFeathersFur, mammaryglandsRelative evolutionary timeAncient eventsRecent events
26 Properties of cladogams Temporal order of splitsHorizontal axis NOT correlated with similarity8 vertibrates cladogram = perfect because traits arose & not lost - BUT SNAKES???
27 Classification & Evolutionary Relationships Linnaeus - predated evolution as central concept of biologybut what features natural? important?Modern taxonomists - classification reflects evolutionary relationshipsBUT should classification reflect time or rate of evolution??
28 Defining clades Monophyletic - share common ancestor Polyphyletic - NO common ancestorParaphyletic - some, but not all, from common ancestor
29 (mon/para/polyphyletic picture; similar to 23.12 in textbook)
30 The problem of paraphyly Birds and crocodiles - more recent ancestor than crocs. and snakes/lizardsCrocs. evolved more slowly than birds since lineages separatedBirds as separate class recognizes their rapid evolution = major unique derived traits
31 Systematicists Still many polyphylectic groups Detect convergent evol. ==> change classificationBUT favour retaining paraphyletic groups to underscore rapid evolutionSTABILITY of taxonomic system
32 Future of Systematics Molecular genetics & powerful computers Fossil history - dating and derived vs ancestral traitsMolecular = more traits than ever beforeCombining two lines of evidence produces accurate dated phylogenies