Homework: Pigs, Whales and Hippos!

Chapter 20 Molecular Clocks and the Three Domains of Life

Molecular Clocks A molecular clock uses constant rates of evolution in some genes to estimate the absolute time of evolutionary change. The number of nucleotide substitutions in related genes is assumed to be proportional to the time since they last shared a common ancestor.

Differences in Clock Speed Some mutations are selectively neutral and have little or no effect on fitness. Neutral mutations should be regular like a clock The neutral mutation rate is dependent on how critical a gene’s amino acid sequence is to survival. – If the exact sequence of amino acids is essential to survival, the gene will change more slowly.

Potential Problems with Molecular Clocks Estimates of evolutionary divergences older than the fossil record have a high degree of uncertainty. The use of multiple genes may improve estimates

Forams Ciliates Euglenozoans Diatoms COMMON ANCESTOR OF ALL LIFE Land plants Animals Amoebas Fungi Red algae Chlamydias Green algae (Mitochondria)* Methanogens Proteobacteria Nanoarchaeotes Thermophiles Domain Eukarya Gram-positive bacteria (Chloroplasts)* Spirochetes Cyanobacteria Domain Bacteria Domain Archaea Three Domains (This is NOT based on molecular clocks) Carl Woese

Figure Cyanobacteria Proteobacteria Thermophiles Domain Eukarya Domain Bacteria Domain Archaea Fungi Plantae Chloroplasts Mitochondria Methanogens Ancestral cell populations