The Classification of Living Things (Taxonomy)
Nature of Classification
Classification of living things is hierarchical
There is a specific hierarchy by which all living things are classified
example of the hierarchy - classification of the house cat:
Species: Felis domesticus
Why is the classification
Do any of these levels mean
anything? That is, is there something special about a family as opposed
to an order?
What is a taxon (pl. Taxa)?
Classification as Scientific Theory
A Classification represents the evolutionary (geneological) degree of relatedness
between living things.
Is classification "card collecting"?
Details and methodology
What is a "type specimen"
What is a species: special problems for paleontology
- a group of individuals
- if two groups hybridize
at their boundary, are they the same species?
- but you cannot tell the
breeding habits of fossils?
- a group of individuals
that shares a common set of genetic characteristics
A Classification of living things
(note: this scheme, taken in part from Lane's "Life of the Past", is just
one of several classifications that you will find; There is disagreement
about the relationships between organisms at several levels)
Prokaryotes (cells without true
- Phylum Bacteria
- Phylum Cyanobacteria
(cells with a true nucleus)
- Phylum Sarcodina
- Phylum Flagellata:
- Phylum Ciliata:
- Phylum Algae:
Photosynthetic aquatic plants in fresh or salt water, single-celled
- Subphylum Chlorophyta:
The green algae, with same photosynthetic pigments as the higher
- Subphylum Chrysophyta:
The yellow-green algae. Includes several groups, one of which,
diatoms, is important as fossils. Mostly unicellular; many with
a silica skeleton.
- Subphylum Phaeophyta:
The brown algae. Mostly all marine; rarely with preservable hard
parts. Not important as fossils.
- Subphylum Rhodophyta:
The red algae. Exclusively marine, usually multicellular, and
with a complex calcareous skeleton that is commonly preserved
as a fossil.
- Phylum Mycophyta:
The fungi. Nonphotosynthetic land plants that derive food from the
soil and decaying organic matter. Includes mushrooms, molds, rusts,
and mildew. Not important as fossils.
- Phylum Bryophyta:
The mosses and liverworts. Photosynthetic land plants that lack
vascular tissues and readily preservable hard parts. Not important
- Phylum Tracheophyta:
The vascular land plants. Fossil record extends from the Silurian
to the Recent. Includes a host of different varieties that are today
dominated by the flowering plants.
Parazoa: Cellular grade
of organization. Symmetry radial, spherical, or absent.
Eumetazoa: Tissue, or
organ, or organ system grade of organization.
- Phylum Porifera:
The sponges. Skeleton of spicules.
radial symmetry. Tissue grade of organization. Mesoderm incipient, basically
two germ layers.
Bilateria: Symmetry bilateral
and some with radial symmetry (the echinoderms). Mostly with mesoderm, and
a coelom is developed in most.
- Phylum Coelenterata
or Cnidaria: The corals, jellyfish, sea anemones.
Acoelomata: No body
cavity. Includes the flatworms and ribbon worms that are not important
in the fossil record.
cavities partly lined with mesoderm. Includes several phyla of worms and
rotifers not important in the fossil record.
Eucoelomata: Body cavity
a true coelom.
Lophophorata: Two phlya
characterized by a special circular structure around the mouth with dliated
tentacles for food gathering, called a lophophore.
- Phylum Brachiopoda:
Skeleton of two valves. Very important fossils.
- Phylum Bryozoa:
The moss animals. All colonial, most with a skeleton. Important fossils.
originates from a split in the mesoderm.
- Phylum Mollusca:
Unsegmented except for one small primitive group (Monoplacophora). Commonly
bilaterally symmetrical. Mostly with calcareous exoskeleton secreted
by a mantle that surrounds vital organs. Includes the clams (Class Pelecypoda
or Bivalvia); the snails (Class Gastropoda); and the cephalopods, as
well as other, less important, classes.
- Phylum Annelida: The
annelid worms. Segmented, with a round, elongate body; lack jointed
appendages. Tiny chitinous jaws (scolecodonts) Found as fossils as well
as rare, carbon-film impressions of body.
- Phylum Arthropoda:
The jointed-leg animals; conspicuously segmented. Includes crabs, shrimp,
barnacles, spiders, and insects. The extinct trilobites are the most
conspicuous fossil group.
coelom formed from outpocketings of the embryonic gut.
- Phylum Echinodermata:
The spiny-skinned animals. Conspicuous five sided radial symmetry; unique
water vascular system. Includes sand dollars and sea urchins (echinoids),
starfishes, sea cucumbers (holothurians), and crinoids. In addition
to the five living classes, there are as many as sixteen extinct fossil
- Phylum Chordata:
Elongate, rodlike cartilaginous notochord runs along the back for support.
Main nerve cord dorsal (top); heart ventral (bottom). Most adults with
gill slits, a bony vertebral column (backbone), or both. The most important
groups are the Hemichordata, which include the extinct graptolites,
and the Vertebrata, which include all fishes, amphibians, reptiles,
birds, and mammals.
Using morphological traits to determine how closely something is related:
- Ancestor-descendant lineages are untestable hypotheses
- we use similarity instead.
- "Primitive" (synplesiomorphies) characteristics are those shared
by a "larger" group of organisms because they belonged to a distant
- When comparing humans to all tetrapods, having five fingers is
primitive-the first tetrapod to venture on land had 5 fingers and
5 toes. Just because carnivores, rodents, anteaters, turtles and
lizards have 5 fingers and 5 toes doesn't make any of these groups
any more related to people than are horses.
- The primitive covering for tetrapods is scales.
- "Derived" characteristics (synapomorphies) are those shared by
a smaller group of organisms because they were "innovated" by a recent
- humans and apes lack tails, monkeys and most other tetrapods
still have tails.
- horses have one toe on each foot, birds and some dinosaurs share
the trait of four toes on the hind feet.
- Without knowing the ancestors from the descendants, we determine
how recently one group diverged from another by counting the number
of shared characteristics, and sorting out the synapomorphies from
the synplesiomorphies by comparison with an outgroup.
- If I were trying to determine how closely different species of
parrots were related to each other, I would compare them to some
other bird (a crow, for example). Characteristics shared with the
crow are probably primitive.
- Convergent evolution may make it difficult to sort out synapomorphies,
because if the same trait is derived independently, then it is not
the "same" trait.
- does the lack of a tail in a frog make it more closely related
to a human than a salamander is?
- the more synapomorphies are shared by two taxa, the more closely
these two taxa are related, hence the more recently their lineages
- We use the cladogram to represent these relationships
Impact of the study of evolution on taxonomy
- if a taxon includes all of the descendants of a single species, then
it is said to be "monophyletic" it may also be called a "clade"
- if a taxon does not include all of the descendants of a single species,
it is said to be "paraphyletic"