25.3

Concept 25.3: Phylogenetic systematics informs the construction of phylogenetic

trees based on shared characteristics

Maggie, Will, Namroo, Austin, Rex

Cladistics

-Cladogram: shows patterns of shared characteristics

-Clade: a group of species including ancestral and descendants in the tree

Groupings in Phylogenic Trees

I. Monophyletic: regular clade II. Paraphyletic: ancestor is present, but

not all descendants III. Polyphyletic: descendants are

present, but no ancestor

D

C

E G

F

B

A

J

I

KH

D

C

E

B

G H

F

J

I

K

A

D

C

B

E G

F

H

A

J

I

K

Shared primitave and shared derived characters

“Character” refers to any feature that a particular taxon process

Shared primative character – A character that is shared beyond the taxon we are trying to define

Shared derived character – An evolutionary novelty unique to a particular clade

Outgroups

Outgroup comparison is used to differentiate between shared derived characters and shared primitive characters

Ingroup – The various species we are studying

Outgroup – A species or group of species that is closely related to the ingroup.

Phylogenetic Trees and Timing

Phylograms – present sequences of events relative to each other.

Ultrametric Trees – Present sequences based on the actual times they occurred.

Phylograms

Length of branch corresponds to the amount of changes that occurred.

A long branch means more changes in DNA.

Drosophila

Lance

let

Amph

ibia

n

Fish

Bird

Human

Rat

Mou

se

Ultrametric Trees

The lengths of the branches are the same lengths for each lineages.

The tree draws information from the fossil record.

Droso

phila

Lanc

elet

Amph

ibia

n

Fish

Bird

Hum

an

Rat

Mou

se

Cenozo

ic

Meso

zoic

Pa

leozo

ic

Pro

tero

zoic

54

2

25

1

65

.5

Mill

ions

of

years

ago

Maximum Parsimony and Maximum Likelihood

Systematists:› Can never be sure of finding the single best tree in a

large data set.› Narrow the possibilities by applying the principles of

maximum parsimony and maximum likelihood.

According to the principle of maximum parsimony, we should first investigate the simplest explanation that is consistent with the facts.

The principle of maximum likelihood states that, given certain rules about how DNA changes over time, a tree can be found that reflects the most likely sequence of evolutionary events.

Among phylogenetic hypotheses› The most parsimonious tree is the one that requires the

fewest evolutionary events to have occurred in the form of shared derived characters.

Human Mushroom Tulip

40%

40%

0

30% 0 Human

Mushroom

Tulip

(a) Percentage differences between sequences

0

Figure 25.14 (a)

Tree 1: More likely

(b) Comparison of possible trees

Tree 2: Less likely

15%

5%

15% 20%

5%

10%

15%

25%

Figure 25.14 (b)

Phylogenetic Trees as Hypotheses

Phylogenetic trees represent a possible way of how the species in it are related.

Phylogenetic hypotheses can change with new evidence.

Usually the most parsimonious tree is most likely

Analogy-Homology issue The more matching base pairs the less

probability they evolved independently

Parsimony more reliable if with longer segments.

Accidentally mistaking an analogy for a homology is less likely to affect the tree if the clades are defined by several defined characters.

Strongest hypotheses are supported by lots of morphological and molecular evidence and fossil evidence.

Pg.502-503

APPLICATION In considering possible phylogenies for a group of species, systematists compare molecular data for the species. The most efficient way to study the various phylogenetic hypotheses is to begin by first considering the most parsimonious—that is, which hypothesis requires the fewest total evolutionary events (molecular changes) to have occurred.

TECHNIQUE Follow the numbered steps as we apply the principle of parsimony to a hypothetical phylogenetic problem involving four closely related bird species.

SpeciesI

SpeciesII

SpeciesIII

SpeciesIV

I II III IV I III II IV I IV II III

Sites in DNA sequence

Three possible phylogenetic hypothese

1 2 3 4 5 6 7

A G G G G G T

G G G A G G G

G A G G A A T

G G A G A A G

I

II

III

IV

I II III IV

A G G G

GG

G

Bases at site 1 for each species

Base-changeevent

1 First, draw the possible phylogenies for the species (only 3 of the 15 possible trees relating these four species are shown here).

2 Tabulate the molecular data for the species (in this simplified example, the data represent a DNA sequence consisting of just seven nucleotide bases).

3 Now focus on site 1 in the DNA sequence. A single base-change event, marked by the crossbar in the branch leading to species I, is sufficient to account for the site 1 data.

Species

THE END.