Chapter 10, Problem 3P

Figure 10.2a has numbers indicating the approximate number of millions of years ago that species on separate branches of the tree last shared a common ancestor.

a.	About how many millions of years ago did humans last share a common ancestor with chimpanzees, mice, dogs, chickens, and frogs? These estimates for evolutionary events were obtained in part by comparing the genomic sequences of various current-day species. The basic supposition behind these estimates is that of a molecular clock: Differences in particular types of genomic sequences accumulate at a relatively linear rate during evolutionary time. Consider the three following kinds of nucleotide changes: (i) missense mutations in coding regions that alter amino acid identity; (ii) silent (synonymous) mutations that change a codon for a particular amino acid into a different codon for the same amino acid; and (iii) mutations in introns. Which of the three types of mutations would . . .
b.	. . . represent the slowest-ticking clock? (That is, which type of mutation would accumulate the least rapidly in genomes? Hint: See Fig. 10.3.)
c.	. . . you most likely use to estimate the divergence times of species that last shared a common ancestor more than 400 million years ago?
d.	. . . be most likely to vary in the rate at which they would accumulate in different genes?