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Using Phylogeny to Establish
Evolutionary Relationships
A phylogeny is the evolutionary history of a species or group
of species.
Example-legless lizards look like snakes yet they are more
closely related to legged lizards than they are to snakes. The
phylogeny of legless lizards demonstrates this.
Short tail
post. anus
Legged Lizards
Leggless Lizards
Systematics is the process used to classify organisms and
establish evolutionary relationships.
Systematics uses DNA,
biochemistry, fossils,
embryology to establish
phylogenies. The classification
of organisms is based on
evolutionary relationships.
Organisms are given two
names-Based on Latin. The
first is the genus name, the
second is the species name.
Related species are grouped
together in the same genera,
related genera are grouped
together in the same family and
so on. There are three Domains
based on ribosomal
sequencing- Eukarya, Archae,
and Bacteria
Which of the following would indicate that snakes and
legless lizards are not closely related
A) Snakes have scales and legless lizards do not.
B) Snakes have tails and legless lizards do not.
C) Snakes have a highly mobile jaw and the legless lizards
do not.
D) Snakes lay eggs and legless lizards do not
Snakes and legless lizards are both reptiles. They have
the shared derived characteristic of scales and are
ectothermic. Both have tails and lay eggs but snakes
have a highly mobile jaw and the legless lizard does not.
A taxon is a taxonmic
unit at any of these
levels. Higher units of
taxons are usually
defined by morphological differences
rather than biochemical differences.
Phylogenic tree- is a
branching diagram
that represents
evolutionary history. Often it mirrors hierarchal classification
of an organism. Node #2 represents the most recent
common ancestor of the weasel and dog family, whereas
node # 1 represents the most recent common ancestor of the
canine genus.
If two organisms are in the same class, then
which of the following is true?
A) They are in the same phylum
B) They are in the same genus
C) They are in the same order
D) They are in the same family
Phylogenetic trees cannot
1. Does not indicate absolute age of a species. It
only indicates the most recent ancestor.
2. It does not assume that one taxon on a
phylogenetic tree evolved from the taxon next to
it or on the same level.
There are five reconized
super groups in the
domain Eukarya
1. Excavata
2. Chromalveolata
3. Rhizaria
4. Archaeplastida
(plant kingdom
found in this group)
5. Unikonta
(Animal and Fungi
Kingdom found in
this group)
All the tan boxes
represent groups
that used to be in the
Protista Kingdom.
Analogous structures result from two different
species in two different environments with similar
selection pressures. The two plants below are not
closely related but are very similar due to similar
selection pressures.
Homologies can be extended to DNA sequences.
The more closely related two species are, the closely
their DNA sequences will be alike.
Cladistics is a systematic approach in classifying organisms.
Organisms are placed in groups called clades which includes
common ancestor with a particular unique trait. This unique
trait is passed on to all of its descendents. Therefore a clade
contains the ancestor and all of its descendents.
Clades, like taxonomic ranks are nested within larger clades.
A clade is only considered a taxon IF it contains an ancestral
all of its descendents. Taxon 1 is considered to be a clade
whereas taxon 2 and taxon 3 are not considered to be clades.
Monophyletic relationship in grouping is a true clade whereas
paraphyletic grouping (b) will leave out one or more
descendant groups in this case J. Polyphyletic grouping (c)
has two or groups without a common ancestor. B is the
common ancestor but it is left out.
backbones but that is not unique to mammals. All
vertebrates have backbones and therefore the backbone is a
shared ancestral character that originated in the ancestor of
the taxon. Hair is shared by all mammals but not the
ancestors of mammals thus hair is considered to be a shared
derived character that separates them from all the other taxa.
Clades, like
ranks are
nested within
larger clades.
This shows
how a
cladogram is
reflective of
and it
Example hair is considered to be a shared derived
characteristic of all mammals but not turtles and retractable
claws is a shared derived characteristic of members of the
family Felidae (cat family) but not found in wolves, horses or
Below is a chart containing SHARED DERIVED
CHARACTERS for a variety of vertebrates and a cladogram
based on the chart. Lancets are considered to be an
outgroup in that they diverged from the group before
vertebrates evolved and do not have a vertebral column.
Lancets are closely related to vertebrates but are not in the
Cladograms and phylogentic trees are a ways to diagram
evolutionary history. They are very similar and quite often
used interchangeably. A phylogenetic tree measures the
degree of similarity and its evolutionary history whereas a
cladogram measures the degree of shared derived characters
from some common ancestor.
Both can be based on either morphological characteristics or
molecular characteristics.
For example, if the ancestral state has an A at some
nucleotide position and some of your samples have a G at
this position, it is assumed that those sharing the G have
inherited this change from the same source and so are
related to each other by descent.
Of course you need to look at many characters to determine
what is related to what. In phenetics, you do not need to
determine what is ancestral and what is a shared derived
character. You only care about who appears most similar to
whom at one point in time. Cladistics is generally considered
to be more reliable for the determination of evolutionary
The first classification system created by Linnaeus
considered only two kingdoms, Plant and Animals.
Prokaryotes were considered plants because of their stiff cell
wall, and fungi were considered plants because the grew in
the ground like plants. Later it was recognized that
prokaryotes, protists, and fungi were so different from plants,
that there should be five kingdoms.
eukaryotes that are autotrophs
Fungi- multicellular,
eukaryotes that are absorptive
Animalia- multicellular,
eukaryotes that are ingestive
One challenge has been
evidence that there are two
distinct lineages of
prokaryotes. It is based on
the sequencing of the rRNA
found in the small subunit.
One lineage of prokaryotes
is more closely related to
eukaryotes than it is to the
other lineage of
prokaryotes. Most
taxonomists now recognize
the domain as the largest
taxon. The three taxa at the
domain level are the
Bacteria, Archae, and
Within those domains are
super groups. The
kingdoms are found within
the super groups. Of the
five kingdoms only three
have survived-Animalia,
Plantae, and Fungi. The
Monerean and Protista
kingdoms are no longer