Sept. 1, 2005
Systematics - the study of the evolutionary relationships among organisms
Outline
Characters: features of a species
homologous - similarity the result of common evolutionary heritage
analogous - similarity the result of similar function
Cladistics, or phylogenetic systematics
Clade - an ancestor and all of its descendants
Cladogram - a diagram illustrating evolutionary relationships
Analyzing the distribution of characters among species: character analysis
Characters and character states
Primitive characters - those present in the ancestral group
Derived characters - those absent in the ancestral group but present in one or
more of the descendant groups
Primitive and derived are relative terms
A hypothetical cladogram
A real cladogram
Monophyletic, polyphyletic, paraphyletic
Systematics
Characters:
-Determining evolutionary relationships involves an examination of one or more characters and
their distribution among species
-Characters are features of an organism - most commonly morphological, but may also be
behavioral, biochemical, molecular biogeographic.
Examples: presence or absence of hair, ability to photosynthesize; composition of hard parts;
number of spines on shell. Note character states; Presence:Absence, number, discrete states
For the most part such features are morphological, and I'll focus on these.
In dealing with morphological characters and evolutionary relationships, one is always
comparing species and their characters. In such comparisons we need to be careful to distinguish
between pairs of characters than can be either:
Homologous characters - those whose similarity is the result of common evolutionary heritage
(common ancestry)
example: backbone in humans, lizards and birds
-these three groups share this character because they share a common ancestor.
Analogous characters - those whose similarity results from similar functions.
example: wing in bird, bat and insect
-these three groups share this character because all of them fly. In fact, the details of the
wing structure in each is quite different.
While you might be willing to place humans, lizards and birds together in an
evolutionary group; you should be less willing to do so with bats, birds, and insects.
Lucas, 2000
So for the purposes of systematics, it's best to focus on homologous structures. But you might
not always know ahead of time.
Sometimes the distinctions between analogous and homologous structures are easy,
sometimes not so easy, especially when you get down to some of the detailed morphological
characteristics of a couple of species.
Phylogenetic Systematics = cladistics
The modern approach to determining evolutionary relationships is commonly called
phylogenetic systematics, or even more commonly cladistics.
The term cladistics refers to an evolutionary unit known as a clade
--a clade is an ancestor and all of its descendants.
Phylogenetic systematics is a method by which cladograms can be produced.
--A cladogram is a diagram that illustrates the evolutionary relationships among species
Phylogenetic systematics analyzes the distribution of characters among species.
Character Analysis
Suppose that we want to know about the evolutionary relationships among three species. What
do we do?
Character analysis is the first step: we examine the distribution of characters among the species
in question
Each of the three species has a number of characters to consider. Some of the characters are
presence/absence, others may have more than two character states:
1.
2.
3.
4.
Number of spines: 2,3,4,5
color: r(red), w(white), or b(blue
teeth: y(yes), n(no)
hair: y(yes), n(no)
OK, now, at last we can get down to business:
we've focused on particular characters
we've looked at the distribution of characters and character states among the species in
question. (see character matrix)
Character
1 2 3 4
A 2 w n n
B 2 w y n
C 2 w y y
Species are Species A, Species B and Species C
We can now construct a cladogram.
A cladogram - a diagram that illustrates evolutionary relationships
Cladograms are constructed according to a certain style:
-All have a base line which is inclined.
-The tips of the branches are never higher than the tip of the base line
-if a character is placed on a line, all the species above that point on the line possess that
character.
-time is not shown on the vertical axis, only the order or sequence of appearance is inferred
A
B
C
teeth
hair
Two spines
White
Species C has one character that A and B doesn't have: teeth
Species B shares a Character with C that A does not have: hair
Species A shares only primitive characters (2 spines and white color) with Species C and B.
--According to this scheme then, Species B and C are more closely related to each other than
either is to A.
Species B and C are said to be sister groups.
-teeth are the shared, derived character that unites B and C in a clade
-spines and a white color are shared, primitive characters among A, B, and C.
Primitive and derived are always relative: relative to the groups under consideration.
-Among mammals, for example, the presence of hair is a primitive character.
-Among vertebrates as a whole, hair is a derived character, absent in our ancestors but
exclusively present in mammals.
Lucas, 2000
Phylogenetic systematics concentrates on using shared, derived characters in establishing
evolutionary relationships.
The more shared, derived characters between B and C, the more strongly supported is the
hypothesis of evolutionary relationship.
A real example:
Lamprey
Perch
Lizard
Mouse
Cat
3 ear bones, hair, mammary glands
amniotic egg
vertebral column, paired appendages, jaws
chambered heart, dorsal nerve chord, notochord
Problems with convergence or loss of characters:
Mother Nature Is Not Perfect: What if A and B share a character not possessed by C?
1. convergence independent acquisition of the character
2. loss of the character in C
The distinction among monophyletic, polyphyletic and paraphyletic groups.
Monophyletic - a ancestor plus all its descendants (a clade)
example: vertebrates are thought to all share one common ancestor
Polyphyletic - more than one ancestor
example: algae consist of several unrelated (or very distantly) related groups - some are
even bacteria
Paraphyletic - ancestor but with only some of the descendants example: reptiles gave rise to both
birds and mammals, but birds and mammals are not included within the traditional
classification of reptiles.
Lucas, 2000
Lucas, 2000