BIOLOGICAL CLASSIFICATION & EVOLUTION
(Know the following bold terms and how to apply them to various groups of invertebrates. You will not
be asked to define them formally, but you must know how they are used)
We will be using the Linnean Hierarchy as our system of biological classification for this course. The
hierarchy contains 7 basic “nested sets” of taxa (taxonomic categories). Know their order.
Kingdom
Phylum
Class
Know the order of this hierarchy. A good mnemonic device is: King Phillip Calls
Order
Out For Good Sex.
Family
Genus
Species
Each level of the hierarchy is considered a taxon; Kingdom is the most inclusive taxon and the species
is least inclusive taxon. Multiple taxons are called taxa.
Species names are always binomens (Latin noun (genus) + Latin adjective) – e.g. Homo sapiens
Species names are always italicized or underlined. The generic (genus = Homo) name is also italicized
or underlined. Kingdom through family names are never italicized or underlined.
There are different ways of obtaining data to build a classification system:
1) Comparative anatomy (organ and tissue structure)
2) Cellular ultrastructure (aspects of cell structure)
3) Comparative embryology (e.g., methods of gastrulation, larvae)
4) Comparative biochemistry(e.g.,
5) Molecular sequencing (e.g., amino acid and DNA sequences)
6) Comparative cytology (e.g. chromosome structure)
7) Paleontology (fossil forms)
Characteristics (sequences, structures, etc.) that come from these studies are called characters.
Characters form the basis of all classifications. For example, mammals all share some characters in
common (e.g., hair and mammary glands). However, only homologous characters (defined below)
should be used for classifications and to infer evolutionary relationships.
Character states are the various conditions of a homologous character. For example, the character
wings might come in multiple character states (large wings, small wings, etc.).
Homologous characters (or just homologues) are characters derived through evolutionary descent.
For example, hair is a homologous character among mammals because it has a common evolutionary
origin (ascertained by examination of development, anatomy, genetics, etc). However, homologues
need not look alike. For example, whale pectoral fins and human arms are homologous because they
share similar developmental origins. Bird wings and butterfly wings may look similar, but they share
different evolutionary origins are so are not homologous. Instead, they are considered convergent
characters. Convergent characters evolved independently and therefore have different genetic origins.
Systematistis (taxonomists) use only homologous characters to build classifications and infer
evolutionary history. One method used to infer evolutionary history using only homologues is called
Cladistics. Cladistics is a process of generating phylogenetic hypotheses (evolutionary trees) based
on homologous characters.
Some Cladistic Terminology
Synapomorphy = shared derived character. A homologous character that is shared between 2 or
more species (or taxa) (e.g., hair and mammary glands are synapomorphies of mammals).
Sympleisomorphy = shared primitive character. A homologous character that is present in a taxon
but does not necessarily define the group (e.g., four legs are present in all mammals, but do not define
mammals only. Instead, four legs define all tetrapods, and so evolved prior to the appearance of
mammals). Four legs is therefore a symplesiomorphy within mammals.
chordates
vertebrates
tetrapods
mammals
Bass
Primitive
chordate
lizard
horse
UML students
D
E
B C
A
A.
B.
C.
D.
E.
Vertebrae
Four Legs
Amniotic egg
Hair
Mammary glands
The evolutionary tree presented above is a simplified example of a cladogram generated using
cladistic methods. A cladogram is made up of multiple clades. A clade represents a monophyletic
(defined below) branch on a tree. Above there are four clades: chordates, vertebrates, tetrapods, and
mammals. Each clade consists of the ancestor (the branch point defined by an arrow) and the terminal
branches (e.g., horse and UML students). Note that a clade is defined by synapomorphies.
By convention, synapomorphies are placed on the evolutionary tree just prior to the branch point. For
example, there are 5 synapomorphies shown on this tree, represented by the letters A-E. A is a
synapomorphy of Vertebrates; B & C are synapomorphies of the tetrapod clade; D & E are
synapomorphies of the mammalian clade. Curiously, synapomorphies can also be symplesiomorphies.
For example, A is a symplesiomorphy within the tetrapods because it is a shared primitive character
that shows up earlier on the tree (also, B & C are symplesiomorphies within mammals). In other
words, whether or not a character is a synapomorphy or a symplesiomorphy depends on the character
and group you are talking about.
Other terms we can apply to this cladogram define ancestor-descendant relationships.
Monophyletic – a clade that includes the ancestor (branching point of the tree) and all of its
descendants (e.g., tetrapods and mammals are monophyletic clades, each with well-defined
synapomorphies). Only monophyletic clades have synapomorphies and should be used in biological
classifications.
Paraphyletic – a clade that includes an ancestor and only some of its descendants. For example, the
red circle includes the ancestor to lizards and horses, but leaves out monkeys, which are descendants
of the same lineage. Such a clade is considered “unnatural” and not used in classification.
Consequently, there are no synapomorphies that just define lizards and horses.
Polyphyletic – a clade that contains species arising from two or more immediate ancestors. For
example, the blue circle contains species of bass and lizards, but neglects their immediate ancestors.
The words ancestral or primitive refer to characters (or character states) that have been around a long
time and yet are still retained within a taxon. By definition, symplesiomorphies are a form of ancestral
or character. For example, four legs are primitive characters of horses – they evolved prior to horses,
but of course horses still retain four legs.
The words advanced or derived refer to characters (or character states) that are relatively new to a
taxon. For example, within the vertebrates, mammary glands are derived characters that mark the
origin of mammals