Problem box Estimating divergence times with the molecular clock

In the present day, dicotyledonous plants represent the majority of land plants. The divergence of ancestral seed plants into monocotyledonous and dicotyledonous plants was therefore a major evolutionary transition. Based on DNA divergence data for synonymous sites at nine mitochondrial genes in a range of plants (Laroche et al. 1995), a molecular clock can be used to date this event.

Table 8.2 gives DNA divergence data for comparisons of maize and wheat, both monocotyledons, with an estimated divergence time of approximately 60 million years ago. First, use the maize-wheat DNA divergence data to calibrate the absolute rate of substitution per million years for each locus. Then use this rate of change to estimate the time when monocots and dicots split given their degree of DNA sequence divergence. In terms of Fig. 8.14, the maize-wheat split is T2 and the monocot-dicot split is T1.

Table 8.2 DNA divergence data for comparisons of maize and wheat, both monocotyledons. See Problem box 8.1.

Locus

Nucleotide sites

Synonymous sites

Substitutions per site Wheat-maize Monocot-dicot

coxi

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