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1 Systematics and the Phylogenetic Revolution Chapter 23.

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1 1 Systematics and the Phylogenetic Revolution Chapter 23

2 2 Introduction All organisms: –Are composed of one or more cells –Carry out metabolism –Transfer energy with ATP –Encode hereditary information in DNA Tremendous diversity of life –Bacteria-----whales----sequoia trees Biologists group organisms based on shared characteristics

3 3 Systematics Since fossil records are not complete, scientists rely on other types of evidence to establish the best hypothesis of evolutionary relationships Systematics: the study of evolutionary relationships Phylogeny: a hypothesis about patterns of relationship among species

4 4 Darwin envisioned that all species were descended from a single common ancestor He depicted this history of life as a branching tree. Now called a cladogram Systematics

5 5 Twigs of a tree represent existing species Joining of twigs and branches reflects the pattern of common ancestry back in time to a single common ancestor Darwin called this process “descent with modification” Systematics

6 6 Phylogenies depict evolutionary relationships Systematics

7 7 Key to interpreting a phylogeny: look at how recently species share a common ancestor Similarity may not accurately predict evolutionary relationships –Early systematists relied on the expectation that the greater the time since two species diverged from a common ancestor, more different would be Systematics

8 8 Evolution can occur rapidly at one time and slowly at another (punctuated and gradual evolution) Systematics

9 9 Oscillating selection: Traits can evolve in one direction, then back the other way Evolution is not always divergent: convergent evolution –Use similar habitats –Similar environmental pressures Evolutionary reversal: process in which a species re-evolves the characteristics of an ancestral species Systematics

10 10 Cladistics Derived characteristic: similarity that is inherited from the most recent common ancestor of an entire group Ancestral: similarity that arose prior to the common ancestor of the group In cladistics, only shared derived characters are considered informative about evolutionary relationships To use the cladistic method character variation must be identified as ancestral or derived

11 11 Characters can be any aspect of the phenotype –Morphology- Physiology –Behavior- DNA Characters should exist in recognizable character states –Example: Teeth in amniote vertebrates has two states, present in most mammals and reptiles and absence in birds and turtles Cladistics

12 12 Examples of ancestral versus derived characters Presence of hair is a shared derived feature of mammals Presence of lungs in mammals is an ancestral feature; also present in amphibians and reptiles Cladistics

13 13 Determination of ancestral versus derived –First step in a manual cladistic analysis is to polarize the characters (are they ancestral or derived) Example: polarize “teeth” means to determine presence or absence in the most recent common ancestor Cladistics

14 14 –Outgroup comparison is used to assign character polarity A species or group of species not a member of the group under study is designated as the outgroup –Outgroup species do not always exhibit the ancestral condition Cladistics

15 15 When the group under study exhibits multiple character states, and one of those states is exhibited by the outgroup, then that state is ancestral and other states are derived Most reliable if character state is exhibited by several different outgroups Cladistics

16 16 Following the character state-outgroup method –Presence of teeth in mammals and reptiles is ancestral –Absence of teeth in birds and turtles is derived Cladistics

17 17 Construction of a cladogram Polarize characteristics Clade: species that share a common ancestor as indicated by the possession of shared derived characters Clades are evolutionary units and refer to a common ancestor and all descendants Synapomorphy: a derived character shared by clade members Cladistics

18 18 A simple cladogram is a nested set of clades Plesiomorphies: ancestral states Symplesiomorphies: shared ancestral states, not informative about phylogenetics. Cladistics

19 19

20 20 Cladistics

21 21 Homoplasy: a shared character state that has not been inherited from a common ancestor –Results from convergent evolution –Results from evolutionary reversal If there are conflicts among characters, use the principle of parsimony which favors the hypothesis that requires the fewest assumptions Cladistics

22 22 Parsimony and Homoplasy Cladistics

23 23 A Cladogram; DNA Cladistics

24 24 A Cladogram: DNA Cladistics

25 25 Other Phylogenetic Methods Some characters evolve rapidly and principle of parsimony may be misleading Rate at which some parts of the DNA genome evolve –Mutations in repetition sequences, not deleted by natural selection Statistical approaches Molecular clock: rate of evolution of a molecule is constant through time

26 26 Systematics and Classification Classification: how we place species and higher groups into the taxonomic hierarchy –Genus, family, class.. Monophyletic group: includes the most recent common ancestor of the group and all of its descendants (clade) Paraphyletic group: includes the most recent common ancestor of the group, but not all its descendants

27 27 Polyphyletic group: does not include the most recent common ancestor of all members of the group Taxonomic hierarchies are based on shared traits, should reflect evolutionary relationships Why should you refer to birds as a type of dinosaur? Systematics and Classification

28 28 Monophyletic Group Systematics and Classification

29 29 Paraphyletic Group Systematics and Classification

30 30 Polyphyletic Group Systematics and Classification

31 31 Old plant classification system Systematics and Classification

32 32 New plant classification system Systematics and Classification

33 33 Phylogenetic species concept (PSC) –Focuses on shared derived characters Biological species concept (BSC) –Defines species as groups of interbreeding population that are reproductively isolated Phylogenetic species concept: species should be applied to groups of populations that have been evolving independently of other groups Systematics and Classification

34 34 Comparative Biology Phylogenetics is the basis of all comparative biology Homologous structures are derived from the same ancestral source (e.g. dolphin flipper and horse leg) Homoplastic structures are not (e.g. wings of birds and dragonflies): -Parental care Dinosaurs, birds, crocodiles Homologous behavior

35 35 Parental care in dinosaurs and crocodiles Comparative Biology

36 36 Homoplastic convergence: saber teeth –Occurred in different groups of extinct carnivores –Similar body proportions (cat) –Similar predatory lifestyle –Most likely evolved independently at least 3 times Comparative Biology

37 37 Distribution of saber-toothed mammals Comparative Biology

38 38 Homoplastic convergence: plant conducting tubes –Sieve tubes facilitate long-distance transport of food that is essential for the survival of tall plants –Brown algae also have sieve elements –Closest ancestor a single-celled organism Comparative Biology

39 39 Convergent evolution of conducting tubes Comparative Biology

40 40

41 41 Most complex characters evolve through a sequence of evolutionary changes Modern-day birds –wings, feathers, light bones, breastbone Initial stages of a character evolved as an adaptation to some environmental selective pressure First featherlike structure evolved in theropod phylogeny –Insulation or perhaps decoration Comparative Biology

42 42 Comparative Biology

43 43 Disease Evolution HIV evolved from a simian (monkey) viral counterpart SIV –First recognized in 1980’s –Current estimate: >39 million people infected; > 3 million die each year –SIV found in 36 species of primates –Does not usually cause illness in monkeys –Around for more than a million years as SIV

44 44 Disease Evolution

45 45 Phylogenetic analysis of HIV and SIV First: HIV descended from SIV –All strains of HIV are nested within clades of SIV Second: a number of different strains of HIV exits –Independent transfers from different primate species –Each human strain is more closely related to a strain of SIV than to other HIV strains Disease Evolution

46 46 Third: humans have acquired HIV from different host species Disease Evolution


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