gENETIC dRIfT: EvolUTIoN AT RANdoM 181
kinds of beneficial mutations occur repeatedly, and in those cases one of the copies
will eventually spread. But other kinds of mutations appear in a population only
rarely, or perhaps just once. Their loss by drift can put an evolutionary speed limit
on how fast a species can adapt to changing conditions.The Evolution of differences among Species
Like other vertebrates, humans, sharks, and carps use hemoglobin to transport
oxygen in their blood. Humans extract the oxygen from the air, while sharks and
carps extract it from water. You might therefore expect the hemoglobins of sharks
and carps to be more similar to each other than they are to that of a human.
Surprisingly, the data tell another story. FIGURE 7.17 shows the percentage of
amino acids that differ among the α-hemoglobins of four species of vertebrates. The
hemoglobin in a carp is more similar to the hemoglobin in a human than it is to thatFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_07.16.ai Date 11-14-2016 01-18-170
100,00080,0000.9991.060,00040,00020,00000.2 0.4 0.6 0.8 1.0Ne = 10^4Generationw0 0.2 0.4 0.6 0.8 1.0Ne = 10^3Allele frequency, p Allele frequency, pFIGURE 7.16 A peak shift caused by drift is
only likely if the population size is extremely
small and the adaptive valley is shallow.
Top: The adaptive landscape for an un-
derdominant mutation when the fitness of
the heterozygote is reduced by s = 0.001
relative to the homozygotes. The popula-
tion’s mean fitness, w–, reaches its lowest
value of 0.9995 at p = 0.5. Arrows show the
direction of change in allele frequency, p,
that is favored by selection when p is above
and below 0.5. Both alleles mutate at a rate
of μ = 10–4. Bottom: Simulations of allele
frequency trajectories over 100,000 gen-
erations. The vertical dashed lines show the
bottom of the adaptive valley. Bottom left:
Ne = 10^4. Here 1/Ne << s, and selection is
stronger than drift. Among the three popu-
lations, only one crosses the adaptive valley
and evolves to an allele frequency greater
than p = 0.5. In this case, peak shifts are
very rare, even though the fitness valley is
very shallow. Bottom right: Ne = 10^3. Now
s = 1/Ne, so selection and drift are about
equally strong. The allele frequency in
this one simulation tends to stay near the
adaptive peaks at p = 0 and p = 1, but also
frequently crosses the adaptive valley.Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_07.17.ai Date 11-14-2016 01-18-17HumanShark Carp Dog Human
53% 49% 16% —57% 48% —59% ——DogCarpShark440 400 70
MyaFIGURE 7.17 Differences in the amino acid
sequence of α-hemoglobin are predicted
well by phylogenetic relationship but not
by ecological similarity. Right: The table
shows the percentage of amino acids in
α-hemoglobin that differ between pairs of
vertebrate species. The shark and the carp
are less similar to each other than either is to
terrestrial mammals (dog and human). At the
left of the table is a phylogeny showing the
dates for the most recent common ances-
tors of the species, which are known from
fossils. The percentage of amino acid dif-
ferences between a pair of species is highly
correlated with the age of their most recent
common ancestor. (After [17].)07_EVOL4E_CH07.indd 181 3/23/17 9:09 AM