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Systematics and Phylogeny
Chapter 25
BCOR 012
March 19 and 21, 2007
I. The Fossil Record and Geologic Time
The fossil record is written in sedimentary rocks
Geologic timetable
Radiometric dating permits the absolute dating of fossils.
Plate tectonics
When the fossil and molecular evidence do not agree
II. Systematics: Connecting Classification to Phylogeny
Systematics, Taxonomy, and Phylogeny
Constructing cladograms
Phylogenetic Classification
Molecular Systematics
Parsimony
Two kinds of knowledge inform our
study of evolutionary relationship:
• Direct inference: the fossil record
• Indirect inference: phylogeny reconstruction
The Fossil Record and Geologic Time
Sedimentary rocks contain a sequence of fossils that reveal much of the
history of life on earth.
SEA CLIFF WITH STRATA AT JOGGINS, NOVA SCOTIA
WILLIAM SMITH (1769-1839)
and his map.
Jurassic beds near Lyme Regis on the south coast of Dorset, England, and ammonites from
one of the beds. While studying these beds and others about 1800, William Smith developed
the concept of "guide fossils."
William Smith’s map - inferring a vertical sequence upward from older to younger rocks.
Georges Cuvier (1769-1832) and Alexandre Brongniart (17701847) early recognized the phenomena of restriction of distinctive fossils
to particular zones, formations or series — guide fossils — and applied
this tool in their stratigraphical studies. Moreover, they observed a pattern
or trend in the change from level to level. Of the shells found in the
upper, more recent levels, he states that the "eye of the most expert
naturalist cannot distinguish from those which at present inhabit the
ocean." Forms of life recovered from successively more ancient
strata were observed to become progressively more strange and
"peculiar" (Cuvier 1817:13, 108-109).
The Geological Time Scale
The fossil record chronicles
the relative age of fossil
groups.
Radiometric dating permits the absolute dating of fossils.
Systematics and Phylogeny
Systematics is the study of biological diversity
in an evolutionary context. It includes (among
other aspects) taxonomy, classification, and
phylogeny.
Taxonomy is the branch of systematics concerned
with naming and classification.
•Scientific names are binomials
•Example: Acer saccharum
- Acer is the genus name - it is a Latin noun
- saccharum is the specific epithet - it is a Latin
adjective
Biological classifications
are hierarchical: each
taxonomic group is nested
within a more inclusive
higher order group.
(Note that only the genus name
and specific epithet are italicized.)
“Our classifications
will come to be, as
far as they can be
so made, genealogies.”
- Charles Darwin, 1859
Phylogeny is the study of evolutionary relationships
among organisms.
Willi Hennig, 1913 - 1976
The Father of Phylogenetic
Systematics
A cladogram summarizes
information about
ancestor-descendent
relationships.
Branch points represent inferred
common ancestor. on the diagram,
the red star denotes the common
ancestor of leopard and turtle, while
the yellow star indicates the common
ancestor of tuna, salamander, turtle,
and leopard.
A monophyletic group (also called a clade) includes an ancestor and all of its
descendents. (Note that clades nest within larger clades, as, say, species nest within
genera.)
How to construct a cladogram:
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Choose a study group
Choose an appropriate outgroup
Compile data matrix
Polarize characters using the outgroup
Use shared, derived characters to associate
study group taxa and construct the
cladogram
The outgroup is the group used to polarize character
states in the study group. It should be the group
most closely related (on the basis of other lines of
evidence) to the study group that is not actually part
of the study group.
The nested relationship of clades is reflected in the nested
relationship of taxa in the resultant classification.
It it critical to distinguish homology from analogy in phylogeny
reconstruction
Incorrect interpretation of the fourchambered heart as a homology would
lead to incorrect assessment of the
relationship of birds and mammals.
Convergent evolution of
Stem succulence in Euphorbiaceae and Cactaceae
The four-chambered heart evolved at least twice in
vertebrate history.
Molecular Phylogenetics
G
In molecular phylogenetic studies, individual nucleotide
positions are the characters, while the particular
nucleotide occurring at that position is the character
state.
Using parsimony analysis to choose among competing cladograms.
5
5
5
5
5
A
B
C
Under the principle of parsimony, tree A would be
preferred over B and C as it is one or two steps
shorter.
A cladogram is an evolutionary hypothesis.
It can be revised as new evidence becomes
available.
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A molecular clock can
be used to estimate
the time that HIV first
jumped from non- human
primates to man.
It’s rather remarkable,
wouldn’t you say?
Molecular systematics
is shedding new light
on the evolution of life on
earth.
Quic kTime™ and a
TIFF (Unc ompres sed) dec ompres sor
are needed to see this pic ture.
EXTRAS
The Situation in the Late Precambrian
Precambrian origin of integrated organisms
and basic body plans
Parazoa (Porifera) sponges
Radiata (Cnidaria) sea anemones
and sea pens
Bilateralia
Protostomes
primitive molluscs
unidentified worms
Charnia masoni - a sea pen
Dickinsonia costata annelid?
Glossopteris flora
The geographic distribution of
particular fossils is explainable
in terms of continental drift.
The landmasses of
earth drift about the
surface of the globe,
floating on the hot,
underlying mantle.
Glossopteris leaf fossil
Permian period
ca. 260 mybp
At present, Glossopteris fossils are found in :
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South America
Antarctica
Africa
India
Australia
How do we account for this disjunct distribution pattern?
Distribution of Late Paleozoic Fossils in the Southern Landmasses
Lystrosaurus, a mammal-like reptile
Pangaea, the late Paleozoic supercontinent
The distinctive biota of
Australia is understandable
in terms of the long isolation
of that continent from other
southern landmasses.
The marsupial wolf,
extinct in Australia since the 1950s
Sometimes fossil
and molecular
evidence do not
agree.