Molecular Clocks

[Pages:54]Molecular Clocks

Rose Hoberman

The Holy Grail

Fossil evidence is sparse and imprecise

(or nonexistent)

Predict divergence times by comparing molecular data

? Given

? a phylogenetic tree

? branch lengths (rt)

? a time estimate for one (or more) node

C

110 MYA

DR

M

H

? Can we date other nodes in the tree?

? Yes... if the rate of molecular change is constant across all branches

Rate Constancy?

Page & Holmes p240

Protein Variability

? Protein structures & functions differ

? Proportion of neutral sites differ

? Rate constancy does not hold across different protein types

? However...

? Each protein does appear to have a characteristic rate of evolution

Evidence for Rate Constancy

in Hemoglobin

Large carniverous marsupial

Page and Holmes p229

The Molecular Clock Hypothesis

? Amount of genetic difference between sequences is a function of time since separation.

? Rate of molecular change is constant (enough) to predict times of divergence

Outline

? Methods for estimating time under a molecular clock

? Estimating genetic distance ? Determining and using calibration points ? Sources of error

? Rate heterogeneity

? reasons for variation ? how its taken into account when estimating times

? Reliability of time estimates ? Estimating gene duplication times

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