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Ozzie Vilhelmsson Zoology Building, Room 213 Tel.: (01224 27) 2867 Estimating and using phylogenies.

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Presentation on theme: "Ozzie Vilhelmsson Zoology Building, Room 213 Tel.: (01224 27) 2867 Estimating and using phylogenies."— Presentation transcript:

1 Ozzie Vilhelmsson Zoology Building, Room 213 Tel.: ( ) Estimating and using phylogenies

2 What fossils tell us What living organisms tell us Cladistics Constructing phylogenies Classification & Evolutionary Relationships Molecular Analyses - the way ahead! Taxonomy and Phylogeny

3 Fossils Incompleteness of fossil record Fossilization an unlikely event Only found in sedimentary rocks Habitat bias Age known Intermediates observed Can access extinct lines

4 PHYLOGENETIC TREES Pedigree of a lineage Evidence of dates of separation Time (trees)

5 Morphology Development Metabolic Biochemical Genetic Anything, really Tree construction Similarity matrix (numerical taxonomy) Tree Gather data: Simple, right?

6 But, Different data can yield different trees! Turtle/birds/crocodile picture

7 CLADISTICS ? Aims to distinguish reliable from unreliable characters: ? Homologies vs. Homoplasies ? Derived vs. ancestral homologies

8 ? Method of determining evolutionary histories - displayed as trees ? Clade: entire portion of phylogeny from a common ancestor = Monophyletic group ? Cladogram: unrooted evolutionary tree (no ancestors but points where lineages diverged) CLADISTICS

9 HOMOLOGIES A trait shared between species and inherited from their common ancestor = homologous Ancestral (general) homologies: shared by all species in lineage - eg. vertebrae in vertebrates Derived (special) homologies: shared by few species in lineage - eg. indeterminate incisors in vertebrates

10 Why the fuss? Only this one is useful!

11 To reiterate: Derived homologous traits order TIME of separation Ancestral homologous traits no use for this -all members of lineage have them

12 Divergence = traits unrecognizable –eg. plant leaves Identifying non-useful traits

13 (fig in textbook)

14 Identifying non-useful traits Divergence = traits unrecognizable –eg. plant leaves Homoplasy = trait evolves more than once –different structures resemble each other by convergent evolution eg. bat/bird/insect wings Both cases = analogous traits

15 Hennigs Method Same trait in 2 species = provisionally homologous ie. innocent, until proven guilty Ancestral homology = found in group and outside in species = outgroup Outgroup = branched off from below base of lineage

16 What about wings? Homoplaseous?Ancestral?Derived? Homoplasy/homology depends on reference/outgroup Fig in textbook

17 Rooting the tree Having figured out which traits are important, we can draw a cladogram. But, where does it root? Distance Parsimony Maximum likelihood Three methods: (Possible roots picture)

18 Distance Simple principle: How similar are the species? (similarity matrix/measurement) Works well for simple molecular methods, such as DNA:DNA hybridization data Molecular clock assumption

19 (Panda example)

20 PARSIMONY Simple distance rooting assumes: – trait evolution irreversible, ie. ancestral to derived – trait can change only once per lineage UNREALISTIC But, cladogram requiring fewest reversals/changes most likely to be correct PARSIMONY = simplest is correct!

21 PARSIMONY (counting changes picture)

22 Maximum likelihood Requires a lot of data, massive computing power Need model of evolutionary change to calculate probabilities Probably the most widely used method today (sequence homologies, etc.)

23 Drawing a cladogram 8 vertebrates traits +/- hagfish = outgroup derived traits = acquired since hagfish cladistics minimizes branching - ie. assumes minimal homoplasy

24 Drawing a cladogram

25 Hagfish Perch Salamander Lizard Crocodile Pigeon Mouse Chimpanzee Jaws Lungs Claws or nails Four-chambered heart Fur, mammary glands Feathers A phylogenetic tree Relative evolutionary time Ancient eventsRecent events

26 Properties of cladogams Temporal order of splits Horizontal axis NOT correlated with similarity 8 vertibrates cladogram = perfect because traits arose & not lost - BUT SNAKES???

27 Classification & Evolutionary Relationships Linnaeus - predated evolution as central concept of biology – but what features natural? important? Modern taxonomists - classification reflects evolutionary relationships – BUT should classification reflect time or rate of evolution??

28 Defining clades Monophyletic - share common ancestor Polyphyletic - NO common ancestor Paraphyletic - some, but not all, from common ancestor

29 (mon/para/polyphyletic picture; similar to in textbook)

30 The problem of paraphyly Birds and crocodiles - more recent ancestor than crocs. and snakes/lizards Crocs. evolved more slowly than birds since lineages separated Birds as separate class recognizes their rapid evolution = major unique derived traits

31 Systematicists Still many polyphylectic groups Detect convergent evol. ==> change classification BUT favour retaining paraphyletic groups to underscore rapid evolution STABILITY of taxonomic system

32 Future of Systematics Molecular genetics & powerful computers Fossil history - dating and derived vs ancestral traits Molecular = more traits than ever before Combining two lines of evidence produces accurate dated phylogenies

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