PHYLOGENY AND THE TREE OF LIFECH 26

I. Phylogenies show evolutionary relationships

A. Binomial nomenclature: – Genus + species name • Homo sapiens

B. Hierarchical classification

Specific similarities             Broad similarities

II. Overview of phylogeny

• The evolutionary history of a species or related species• Results in the creation of evolutionary trees• Uses fossil evidence, physical characteristics, and molecular

similarities to create phylogenetic trees that show evolutionary relationships

• The more similar the physical characteristics or sequences are the more closely related the organism are

• Phylogenetic trees represent hypotheses about the evolutionary relationships between species

• Each branch point represents the divergence of species from the common ancestor

A. Linking classification to phylogeny

B. Linking time to Phylogeny

• How do we know age?– Radioactive dating of rocks and fossils based on

radioisotope half life– Location in the Earth’s layers– DNA sequence analysis: using genetic divergence

to estimate age

C. Phylogenies are Inferred from morphological and molecular data

• homologous structures show shared ancestry• gene sequences shared by a common

ancestor are also homologous

III. Shared characters are used to construct phylogenetic trees

• Cladistics groups organisms by common descent

•

• A clade: includes all of a species and their common ancestor

• Monophyletic group : includes ancestral species and all of its descendants

• Paraphyletic group: includes ancestral species and some but not all descendants

• Polyphyletic group: includes various species with different ancestors

• Constructing a phylogenetic tree based on homology

jaws lungs amnioticegg

fur binocular vision

bipedal

lamprey 0 0 0 0 0 0

shark 1 0 0 0 0 0

salamander 1 1 0 0 0 0

lizard 1 1 1 0 0 0

tiger 1 1 1 1 0 0

gorilla 1 1 1 1 1 0

human 1 1 1 1 1 1

• The branch points represent the common ancestor• The characteristics are derived characteristics:

homology• Can use physical characteristics for a broad cladogram• Can use DNA comparisons for a more narrow

cladogram (primate evolution)– The more similar the sequence, the more recent the

common ancestor– The more distant the common ancestor, the more time

there was for mutations to take place

III. Problems reconstructing phylogenies

• Not all similar traits are homologous, the result of divergent evolution (synaptomorphy)

• Synaptomorphy = a trait that is similar among species because the common ancestor of those species had that trait

• Some traits evolved by convergent evolution = homoplasy

http://evolution.berkeley.edu/evolibrary/search/imagedetail.php?id=267&topic_id=&keywords=

Then how can we accurately create phylogenetic trees?

• Must use many traits to construct phylogenetic tree

• If don’t distinguish between homoplasy and homology you will with wrong conclusions

• homology is more reliable than homoplasy BUT must be able to distinguish between them

Parsimony and constructing phylogenetic trees

• Parsimony: criteria used to construct phylogenetic trees based on minimizing the number of changes