Modern Phylogeny
and the three domain system
Linneaus and taxonomy
• He relied on morphology
• He relied on the fossil
record .
Modern
taxonomists rely on morphology,
fossils, and other tools:
MORPHOLOGY – truly homologous structures are
separated from analogous ones. The greater the
number of homologous
features shared, the more
closely related the organisms
are thought to be.
• nucleotide sequences
• chromosomal characteristics
DNA, RNA & PROTEINS comparisons –
Rates of change in Amino acid sequences,
and Molecular Clocks
The molecular clock uses DNA comparisons to
estimate the length of time that two species have
been evolving independently as separate species.
The degree of dissimilarity indicates how long
ago the two shared a common ancestor.
EMBRYOLOGICAL PATTERNS OF
DEVELOPMENT
Because of embryological developmental
characteristics, echinoderms, such as starfish
were seen as more closely related to vertebrates than
invertebrates such as mollusks.
(The blastopore is an indentation in the embryologic
hollow sphere of cells called the blastula. The
blastopore becomes the mouth in
echinoderms and
vertebrates, while the
blastopore
becomes the anus
in other animals.)
Systemic Taxonomy
• Also called systemics, it is the
organization of living things in the context
of evolution.
• They use phylogeny and phylogenic trees
to show the evolutionary relationships
among groups of organisms.
phylogenic
tree
The branches of
this phylogenic
tree show that
chordates and
echinoderms
shared a
common
ancestor more
recently than
did echinoderms
and other animals.
Cladistics
• Cladistics uses certain features of
organisms called shared derived
characters, to establish evolutionary
relationships.
• A derived character is a feature that
apparently evolved only within the group
under consideration.
shared derived character- example
• When considering the group “birds” the
derived character is “feathers.”
Most animals don’t have feathers; birds are the
only animals that do. THEREFORE, it is safe to
assume that feathers evolved within the bird
group and were not inherited from some
different ancestor of birds.
(Strong evidence of common ancestry)
Organisms were
grouped according
to similarities in
appearance.
Until about 150
years ago,
limpets (snail-like
mollusks) were
grouped together
with barnacles due
to their conical shells
(cone-shaped).
Biologists now group
organisms on
evolutionary descent,
not just physical
similarities. In the
cladogram, crabs
and barnacles are
grouped together
because they share
important evolutionary
characteristics, such
as a segmented body,
and an exoskeleton that
the organism molts.
Vertebrate Cladogram
The Three Domain System
The Three Domain System, proposed by Carl Woese
and others, is an evolutionary model of classification
based on differences in the sequences of
nucleotides in the cell's ribosomal RNAs (rRNA),
as well as the cell's membrane lipid structure and its
sensitivity to antibiotics.
(Comparing rRNA structure is especially useful. Because
rRNA molecules throughout nature carry out the same
function, their structure changes very little over time.
Therefore similarities and dissimilarities in rRNA
nucleotide sequence are a good indication
of how related or unrelated different cells
and organisms are.)
This system proposes that a common ancestor cell
gave rise to three different cell types, each
representing a domain.
The three domains are the Archaea (archaebacteria), the
Bacteria (eubacteria), and the Eukarya (eukaryotes).
The Eukarya are then divided into 4 kingdoms: Protists,
Fungi, Anamalia, and Plantae. A description of the three
domains follows: