Phylogeny and the Tree of Life

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2

Common ancestor of

taxa A–F

Branch point

(node)

Taxon B

Taxon C

Taxon D

Taxon E

Taxon F

Slide 8

What We Can and Cannot Learn from Phylogenetic Trees

Phylogenetic trees do show patterns of descent.

Phylogenetic trees do not indicate when species evolved or how much genetic change occurred in a lineage.

It shouldn’t be assumed that a taxon evolved from the taxon next to it.

Phylogeny provides important information about similar characteristics in closely related species.

Slide 9

Possible Phylogenetic Trees: Provide important information about similar characteristics in closely related species.

A

B

A

A

B

B

C

C

C

D

D

D

(a)

(b)

(c)

Slide 10

Concept 26.2: Phylogenies are inferred from morphological and molecular data

Organisms with similar morphologies or DNA sequences are likely to be more closely related than organisms with different structures or sequences.

When constructing a phylogeny, systematists need to distinguish whether a similarity is the result of homology or analogy.

Homology is similarity due to shared ancestry.

Analogy is similarity due to convergent evolution.

Slide 11

Convergent evolution occurs when similar environmental pressures and natural selection produce similar /analogous adaptations in organisms from different evolutionary lineages.

Bat and bird wings are homologous as forelimbs, but analogous as functional wings.

Analogous structures or molecular sequences that evolved independently are also called homoplasies.

Convergent Evolution - Similar Environmental Selecting Agents

Slide 12

Homology can be distinguished from analogy by comparing fossil evidence and the degree of complexity. The more complex two similar structures are, the more likely it is that they are homologous.

Molecular systematics uses DNA and other molecular data to determine evolutionary relationships.

Once homologous characters have been identified, they can be used to infer a phylogeny.

Slide 13

Cladistics groups organisms by common descent

A clade is a group of species that includes an ancestral species and all its descendants.

Clades can be nested in larger clades, but not all groupings of organisms qualify as clades.

A valid clade is monophyletic, signifying that it consists of the ancestor species and all its descendants.