Systematics lecture Sept. 17, 2002

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.

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 modenr 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. Number of spines: 2,3,4,5

2. color: r(red), w(white), or b(blue

3. teeth: y(yes), n(no)

4. 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

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.

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 your hypothesis of evolutionary relationship.

A real example (on handout)

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