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Systematics and the phylogenetic revolution

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Title: Systematics and the phylogenetic revolution


1
Chapter 23
  • Systematics and the phylogenetic revolution

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
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
Systematics
  • 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

5
Systematics
  • Phylogenies depict evolutionary relatedness
  • 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

6
Systematics
  • Punctuated evolution- occurs rapidly
  • Gradual evolution- occurs slowly (Gradualism)

7
Systematics
  • 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

8
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

9
Cladistics
  • 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

10
Cladistics
  • 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

11
Cladistics
  • 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

12
Cladistics
  • 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
  • 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

13
Cladistics
  • Following the character state-outgroup method
  • Presence of teeth in mammals and reptiles is
    ancestral
  • Absence of teeth in birds and turtles is derived

14
Cladistics
  • ?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

15
Cladistics
  • A simple cladogram is a nested set of clades
  • Plesiomorphies ancestral states
  • Symplesiomorphies shared ancestral states

16
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17
Cladistics
18
Cladistics
  • 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

19
Cladistics
  • Parsimony and Homoplasy

20
Cladistics
  • A Cladogram DNA

21
Cladistics
A Cladogram DNA
22
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

23
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

24
Systematics and Classification
  • 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?

25
Systematics and Classification
  • Monophyletic Group

26
Systematics and Classification
  • Paraphyletic Group

27
Systematics and Classification
  • Polyphyletic Group

28
Systematics and Classification
  • Old plant classification system

29
Systematics and Classification
  • New plant classification system

30
Systematics and Classification
  • 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

31
Systematics and Classification
  • BSC cannot be applied to allopatric populations
  • PSC can be applied to allopatric populations
  • PSC can be applied to both sexual and asexual
    species
  • BSC can be applied only to sexual species
  • PSC still controversial

32
Systematics and Classification
  • Paraphyly and phylogenetic species concept

33
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

34
Comparative Biology
  • Parental care in dinosaurs and crocodiles

35
Comparative Biology
  • 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

36
Comparative Biology
  • Distribution of saber-toothed mammals

37
Comparative Biology
  • 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

38
Comparative Biology
  • Convergent evolution of conducting tubes

39
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40
Comparative Biology
  • 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

41
Comparative Biology
42
Comparative Biology
  • Phylogenetic methods can be used to distinguish
    between competing hypotheses
  • Larval dispersal in marine snails
  • Some snails produce microscopic larvae that drift
    in the ocean currents
  • Some species have larvae that settle to the ocean
    bottom and do not disperse
  • Fossils show increase in nondispersing snails

43
Comparative Biology
  • Increase through time in proportion of species
    whose larvae do not disperse

44
Comparative Biology
  • Two processes could produce an increase in
    nondispersing larvae
  • Evolutionary change from dispersing to
    nondispersing occurs more often than change in
    the opposite direction
  • Species that are nondispersing speciate more
    frequently, or become extinct less frequently
    than dispersing species
  • The two processes would result in different
    phylogenetic patterns

45
Comparative Biology
46
Comparative Biology
  • Analysis indicates
  • Evolutionary increase in nondispersing larvae
    through time may be a result of both a bias in
    the evolutionary direction and an increase in
    rate of diversification
  • Lack of evolutionary reversal

47
Comparative Biology
  • Loss of larval stage in marine invertebrates
  • Nonreversible evolutionary change
  • Marine limpets show direct development has
    evolved many times
  • 3 cases where evolution reversed and larval stage
    re-evolved

48
Comparative Biology
49
Comparative Biology
  • Phylogenetics helps explain species
    diversification
  • Use phylogenetic analysis to suggest and test
    hypotheses
  • Species richness in beetles
  • Coleoptera 60 of all animals are insects and
    80 of all insects are beetles
  • Phytophaga clade with most herbivorous beetles
  • Family Nemonychidae specialized on conifers
    since Jurassic

50
Comparative Biology
  • Evolutionary diversification of the Phytophaga

51
Comparative Biology
  • Phylogenetic explanations for beetle
    diversification
  • Not the evolution of herbivory
  • Specialization on angiosperms a prerequisite for
    diversification
  • Risen 5 times independently within herbivorous
    beetles
  • Angiosperm specializing clade is more
    species-rich than the clade most closely related

52
Disease Evolution
  • HIV evolved from a simian (monkey) viral
    counterpart SIV
  • First recognized in 1980s
  • Current estimate gt39 million people infected gt
    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

53
Disease Evolution
54
Disease Evolution
  • 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

55
Disease Evolution
  • Third humans have acquired HIV from different
    host species
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