42 CHAPTER 2we consider two possible evolutionary histories. The common ancestors are labeled
A 1 , A 2 , and A 3 , from older to younger. If we assume that A 1 and A 2 had opposable
toes and that A 3 , the immediate common ancestor of chimpanzees and humans, had
nonopposable toes, we have to postulate two changes, with the chimpanzee revert-
ing to the ancestral state (see left figure). If, however, we assume that A 3 , like A 1 and
A 2 , had opposable toes, we need to infer only one evolutionary change, namely the
shift to nonopposable toes in the human lineage that is shown in the right figure. If
we assume that changes between these states are very rare, the tree with the fewest
changes is the most likely. This leads to the conclusion that the common ancestor of
humans and chimpanzees had opposable toes.Estimating time of divergence
In the 1970s, when DNA sequencing was very difficult, researchers compared
the amino acid sequences of proteins in pairs of species that were known to have
diverged from their common ancestors at various times in the past. For example, pigs
and cows belong to groups that are first recorded as fossils in the Eocene, about 50
million years ago (Mya), so they diverged from their common ancestor at least that
long ago. When a few proteins were sequenced from different species, and the corre-
sponding DNA sequence differences were plotted against such estimated divergence
times, a close relationship was found (FIGURE 2.17). That is, the proportion of base
pairs that differ between homologous DNA sequences in two species increases with
the amount of time that has elapsed since the species originated from their common
ancestor. As long as the increase is linear with time (as shown in Figure 2.17), the
difference in sequence can serve as molecular clock. Figure 2.17 shows that if you
were to sequence the same genes for two species of mammals and find 45 nucleotide
differences, you could read horizontally across to the best-fit line, and then down to
the time axis. Even if the mammal species belong to lineages that lack a fossil record,
you might estimate that they diverged about 74 Mya—as long as you assume that the
genes in these lineages have evolved at the same rate as in the mammals with fossil
records that were used to determine the original correlation of sequence difference
with time. The fossils have been used to calibrate the rate, r, at which these genes
have evolved in mammals. If r is roughly constant within a clade of organisms, the
expected difference D between two species, each evolving at rate r, is D = 2rt, where
t is time since they split from their common ancestor; hence t is estimated as t = D/2r.
(The factor 2 appears because the genes have evolved along each of the two lineages
that descended from the most recent common ancestor.)
Rates of evolution differ among the different positions in codons and among dif-
ferent genes in the genome (see Chapter 7). Rates of sequence evolution also differFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_02.17.ai Date 11-02-2016Q: There were three locations marked with an X on the graph
I just made these dots like the rest.25 50 75 100 1252550750
Mya (millions of years ago)Nucleotide differencesFIGURE 2.17 This plot of base pair differences against time since
divergence was some of the earliest evidence that the rate of
sequence evolution might be approximately constant. Each point
represents a pair of living mammal species whose most recent
common ancestor, based on fossil evidence, occurred at the time
indicated on the x-axis. (The fossil would indicate the minimal age of
the lineage to which a living species belongs.) The y-axis shows the
number of base pair differences between the species, inferred from
the amino acid sequences of seven proteins. The three green circles
represent pairs of primate species, which have diverged more
slowly than other mammal groups. The arrows show how we would
estimate that a pair of species with 45 base pair differences shared a
common ancestor about 74 Mya. (After [21].)02_EVOL4E_CH02.indd 42 3/23/17 8:59 AM