Phylogeny and the Tree of Life

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Phylogenetic bracketing - predicts features of an ancestor from features of its descendents:

Common

ancestor of

crocodilians,

dinosaurs,

and birds

Birds

Lizards

and snakes

Crocodilians

Ornithischian

dinosaurs

Saurischian

dinosaurs

Slide 20

Eggs

Front limb

Hind limb

(a) Fossil remains of Oviraptor

and eggs

(b) Artist’s reconstruction of the dinosaur’s posture

Slide 21

Concept 26.4: An organism’s evolutionary history is documented in its genome

Comparing nucleic acids or other molecules to infer relatedness is a valuable tool for tracing organisms’ evolutionary history.

DNA that codes for rRNA changes relatively slowly and is useful for investigating branching points hundreds of millions of years ago.

mtDNA evolves rapidly and can be used to explore recent evolutionary events.

Gene duplication increases the number of genes in the genome, providing more opportunities for evolutionary changes.

Slide 22

Like homologous genes, duplicated genes can be traced to a common ancestor.

Orthologous genes are found in a single copy in the genome and are homologous between species.

They can diverge only after speciation occurs.

Paralogous genes result from gene duplication, so are found in more than one copy in the genome.

They can diverge within the clade that carries them and often evolve new functions.

Slide 23

Orthologous genes

Paralogous genes

Ancestral gene

Ancestral species

Speciation with

divergence of gene

Gene duplication and divergence

Species A after many generations

Species A

Species B

Species A

Orthologous genes

Paralogous genes

Slide 24

Molecular Clocks

A molecular clock uses constant rates of evolution in some genes to estimate the absolute time of evolutionary change.

Molecular clocks are calibrated against branches whose dates are known from the fossil record.

Neutral theory states that much evolutionary change in genes and proteins has no effect on fitness and therefore is not influenced by Darwinian selection.

It states that the rate of molecular change in these genes and proteins should be regular like a clock.

Slide 25

Molecular Clocks

Divergence time (millions of years)

Number of mutations

120

90

90

60

60

30

30

0

0

Slide 26