Transcript Ch 25 Taxonomy

Chapter 25:
Phylogeny and
Systematics
“Taxonomy is the division of organisms into
categories based on… similarities and
differences.” p. 495, Campbell & Reece (2005)
Shown is a phylogenetic tree
Fossil Record
• Fossils are preserved remnants of organisms
that lived in the past
• Fossils form in sedimentary rock, the oldest
fossils in the lower strata and younger on the
upper strata
• Fossil record is incomplete and favors larger
organisms, those with hard bodies or parts,
those that were abundant and widespread.
Fossils
Morphological Evidence
• Homology- similarity due to common
ancestry (same structure different
function): wing of bat, arm of human,
flipper of whale
• Analogy- similarity due to convergent
evolution ( same function different
structure) :two kinds of moles
Marsupial
Analogies
These
“moles”
only
look
similar
Placental
Molecular Evidence
Computers are used to analyze DNA
sequences for homologous nucleic bases.
Taxonomic Categories
Did (Domain)
Karen’s
(Kingdom)
Pups (Phylum)
Chew (Class)
On (Order)
Fuzzy (Family)

Grey (Genus)
Squirrels
(Species)
Binomial Nomenclature
 Example: Escherichia coli, E. coli, Escherichia spp.,
 The genus name (Escherichia) is always capitalized
and comes first
 The species name (coli) is never capitalized
 The species name is never used without the genus
name
 The genus name may be used without the specific
epithet
 Both genus and species names are always
underlined or italicized
 Genera may be abbreviated, e.g., the E. in E. coli,
Cladistics
 Cladistics is a technique by which organisms are
assigned to different (monophyletic) taxa
 Cladistics works by grouping together organisms
such that within taxa individuals share more
homologies than they do with individuals found in
different taxa
 Cladistics also rejects the inclusion of similarities...
• …that result from convergent evolution (i.e.,
analogies)
• …that are homologies that are shared with other
taxa (i.e., shared primitive characteristics)
Classifying Clades
Each deeper
branch point
represents
greater
divergence
Classifying Proper Clades: Passing the
Test
A clade consists of an
ancestral species plus all
descendant species
Here is a missed
ancestor species
Monophyletic
All descendants
A correct
grouping
A clade
Ancestor
Paraphyletic
This is missing
descendant
species
Polyphyletic
Convergent evolution?
Polyphylies happen when
species are included in “clades”
that they don’t belong in
Convergent Evolution
 Polyphyletic taxa occur as a consequence of
mistaking analogies for homologies
 Analogies result from convergent evolution: the
two species do similar things in similar
environments so evolve similarly
 The key difference between an analogy and a
homology is:
• The common ancestor between the two species
will lack the common structure
Analogies
Shared Derived Characters SDC
Cladogram based on Shared Derived
Characteristics.
An outgroup is a
species that is related to
the species being
studied (ingroup), but
less closely related
Molecular Clocks
The molecular clock is a yardstick for
measuring absolute time of evolutionary
change based on the observation that some
genes and other regions of genomes seem to
evolve at constant rates
Neutral theory states that much evolutionary
change in genes and proteins has no effect
on fitness
It states that the rate of molecular change in
these genes and proteins should be regular
like a clock
Applying a Molecular
Clock- The Origin of HIV
• Phylogenetic analysis shows that HIV is
descended from viruses that infect
chimpanzees and other primates
• Comparison of HIV samples throughout
the epidemic shows that the virus
evolved in a very clocklike way
Universal Tree of Life
• The tree of life is
divided into three great
clades called domains:
Bacteria, Archaea, and
Eukarya
• 1- last common
ancestor of all living
things
• 2. ancestor of
eukaryotic cells
• 3. symbiosis of
mitochondrial ancestor
• 4. symbiosis of
chloroplast ancestor