Ooo

Kangaroo

Echidna fci

Chicken

Newt

Carp

Shark

600 500 440 400 350 270 225 180 135 70 Present

Millions of years ago

23.22 The molecular clock is based on the assumption of a constant rate of change in protein or DNA sequence. (a) Relation between the rate of amino acid substitution and time since divergence, based on amino acid sequences of a hemoglobin from the eight species shown in part b. The constant rate of evolution in protein and DNA sequences has been used as a molecular clock to date past evolutionary events. (b) Phylogeny of eight species and their approximate times of divergence, based on the fossil record.

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