Evolution Biol 4802

Lecture 3 – Chapter 2


Objectives for today

1)     How can we build phylogenetic trees?

a)     What traits can you use?

b)     How do they evolve?

c)      Mechanics of tree building


What is a systematics?

1)     Study of biodiversity (past and present) and relationships among organisms through time.

a)     Two major subdisciplines:

i)        Taxonomy

(1)   Naming and classifying organisms

ii)      Phylogenetics

(1)   Building phylogenetic trees

Figure 2.2 (old and new)

Figure 2.1 (old and new)

How can we build a phylogeny?

1)  What characters can you use?

a)                 External

b)                 Internal

c)                  Behavior

d)                 Cell structure

e)                 Biochemistry

f)                    Chromosome structure

g)                 DNA sequence

2)     What is a character state?

Do these characters evolve differently?

1) Does the change in character state influence fitness?

a)     Yes

i)        Natural selection

b)     No

i)        Genetic drift chance (random)

What is genetic drift?

1)     Human reproduction

a)     80,000 ovules

b)     Millions of sperm

i)        Are they all the same?

2)     How many of these gametes are sampled?

When does genetic drift happen?

1)     Traits are neutral with respect to fitness

2)     Individuals that survive and reproduce are selected by chance

How can we use these characters to build phylogenetic trees?

1)     Have some characters

2)     Evolving according to drift and selection

3)     If species are becoming more different over time, then… use the degree of similarity/difference (not really)

Similar to Figure 2.5 (Fig. 2.4 in older version of the book)

Similarity by itself is too simple

1)     Why?

a)     Doesn’t compare changes relative to an ancestor

b)     Doesn’t take into account the sequence of trait evolution

c)      Lineages evolve at different rates

d)     The same character may evolve in different lineages

a.      Convergent evolution

How can we deal with all this?

1)     Modern phylogenetic inference (Hennig 1966)

                                 I.      Taxa may be similar because of:

a.      Ancestral characters that haven’t changed

b.      Derived characters that unique and shared by all descendents

                                                                                      i.      Only reliable evidence of nested monophyletic groups

c.      Homoplasious characters

                                                                                      i.      Convergence

                                                                                    ii.      Reversals

2)   How can you tell if a trait is:

                               II.      Ancestral or derived?

                              III.      Unique or homoplasious?

Figure 2.6 (Figure 2.5 in older version of the book)

Why is parsimony a useful concept?

  1. Ancestral states are not knowable with certainty
  2. All you can know is current state
  3. Patterns of branching inferred from current states suggest what the ancestral state probably was
  4. You need a rule

Figure 2.9 (Figure 2.8 in older version of the book)

Maximum Parsimony

1)     Modern phylogenetic inference (Hennig 1966)

a.      The simplest explanation requiring the fewest assumptions is preferred over a complex explanation that requires more assumptions.

b.      In phylogenetics, fewest evolutionary changes or “steps.”

2)     The Process

a.      Chose “outgroup” to compare “ingroup

1)     Known to be ancestral (i.e. fossil record)

b.      Build all possible trees

c.      Map traits on to the tree

d.      Choose the tree with the fewest steps

Figure 2.8 (Figure 2.7 in older version of the book)