Phylogeny and the Tree of Life

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5

Difficulties with Molecular Clocks

Irregularities result from natural selection in which some DNA changes are favored over others.

Estimates of evolutionary divergences older than the fossil record have a high degree of uncertainty.

The use of multiple genes may improve estimates.

Slide 27

Applying a Molecular Clock: The Origin of HIV

Phylogenetic analysis shows that HIV is descended from viruses that infect chimpanzees and other primates.

Comparison of HIV samples throughout the epidemic shows that the virus evolved in a very clocklike way.

Application of a molecular clock to one strain of HIV suggests that that strain spread to humans during the 1930s.

Slide 28

HIV Virus

Year

Index of base changes between HIV sequences

1960

0.20

1940

1920

1900

0

1980

2000

0.15

0.10

0.05

Range

Computer model

of HIV

Slide 29

Three Domain System

Fungi

EUKARYA

Trypanosomes

Green algae

Land plants

Red algae

Forams

Ciliates

Dinoflagellates

Diatoms

Animals

Amoebas

Cellular slime molds

Leishmania

Euglena

Green nonsulfur bacteria

Thermophiles

Halophiles

Methanobacterium

Sulfolobus

ARCHAEA

COMMON

ANCESTOR

OF ALL

LIFE

BACTERIA

(Plastids, including

chloroplasts)

Green

sulfur bacteria

(Mitochondrion)

Cyanobacteria

Chlamydia

Spirochetes

Slide 30

There have been substantial interchanges of genes between organisms in different domains.

Horizontal gene transfer is the movement of genes from one genome to another.

Horizontal gene transfer complicates efforts to build a tree of life.

Some researchers suggest that eukaryotes arose as an endosymbiosis between a bacterium and archaean.

Slide 31

Review

F

Polyphyletic group

Monophyletic group

Paraphyletic group

E

D

C

B

A

G

A

A

B

B

C

C

D

D

E

E

F

F

G

G

Slide 32

Clades - Characters

Slide 33

You should now be able to:

Explain the justification for taxonomy based on a PhyloCode.

Explain the importance of distinguishing between homology and analogy.

Distinguish between the following terms: monophyletic, paraphyletic, and polyphyletic groups; shared ancestral and shared derived characters; orthologous and paralogous genes.