Transcript Chapter 23

Systematics and the Phylogenetic
Revolution
Chapter 23
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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
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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
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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
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Systematics
• 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”
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Systematics
Phylogenies depict evolutionary
relationships
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Systematics
• 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
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Systematics
• Evolution can occur rapidly at one time
and slowly at another (punctuated and
gradual evolution)
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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
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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
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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
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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
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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
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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
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Cladistics
• 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
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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
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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
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Cladistics
• A simple cladogram is a nested set of
clades
• Plesiomorphies: ancestral states
• Symplesiomorphies: shared ancestral
states, not informative about phylogenetics.
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Cladistics
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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
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Cladistics
Parsimony and Homoplasy
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Cladistics
A Cladogram; DNA
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Cladistics
A Cladogram: DNA
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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
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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
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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?
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Systematics and Classification
Monophyletic Group
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Systematics and Classification
Paraphyletic Group
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Systematics and Classification
Polyphyletic Group
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Systematics and Classification
Old plant classification system
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Systematics and Classification
New plant classification system
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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
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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
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Comparative Biology
Parental care in dinosaurs and crocodiles
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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
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Comparative Biology
Distribution of saber-toothed mammals
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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
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Comparative Biology
Convergent evolution of conducting tubes
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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
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Comparative Biology
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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
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Disease Evolution
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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
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Disease Evolution
• Third: humans have acquired HIV from
different host species
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