Phylogeny and the Tree of Life

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A paraphyletic grouping consists of an ancestral species and some of the descendants.

A polyphyletic grouping consists of various species that lack a common ancestor.

Slide 14

Cladistics - Groups Organisms using Evolutionary Relationships

A

A

A

B

B

B

C

C

C

D

D

D

E

E

E

F

F

F

G

G

G

Group III

Group II

Group I

Monophyletic group / clade

Paraphyletic group

Polyphyletic group

Slide 15

Shared Ancestral and Shared Derived Characters

In comparison with its ancestor, an organism has both shared and different characteristics.

A shared ancestral character is a character that originated in an ancestor of the taxon.

A shared derived character is an evolutionary novelty unique to a particular clade.

A character can be both ancestral and derived, depending on the context.

Slide 16

Inferring Phylogeny from Shared Characters

TAXA

Lancelet

(outgroup)

Lamprey

Salamander

Leopard

Turtle

Tuna

Vertebral column

(backbone)

Hinged jaws

Four walking legs

Amniotic (shelled) egg

CHARACTERS

Hair

(a) Character table

Hair

Hinged jaws

Vertebral

column

Four walking legs

Amniotic egg

(b) Phylogenetic tree

Salamander

Leopard

Turtle

Lamprey

Tuna

Lancelet

(outgroup)

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

Slide 17

Maximum parsimony assumes that the tree that requires the fewest evolutionary events (appearances of shared derived characters) is the most likely.

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.

The best hypotheses for phylogenetic trees fit the most data: morphological, molecular, and fossil.

Phylogenetic bracketing predicts features of an ancestor from features of its descendents.

Slide 18

Maximum Parsimony

Human

15%

Tree 1: More likely

Tree 2: Less likely

(b) Comparison of possible trees

15%

15%

5%

5%

10%

25%

20%

40%

40%

30%

0

0

0

(a) Percentage differences between sequences

Human

Mushroom

Mushroom

Tulip

Tulip

Slide 19