gENETIC dRIfT: EvolUTIoN AT RANdoM 187
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_07.23.ai Date 11-14-2016 01-18-17Allele frequency in Han ChineseNumber of SNPs10
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Allele frequency in TibetansEPAS1110,0001000100101FIGURE 7.23 Tibetans have adapted to low
oxygen pressure. These people live at high
elevations (over 4000 m), where oxygen
pressure is less than two thirds of its value at
sea level. The plot compares allele frequen-
cies at SNPs from about 20,000 coding genes
in Tibetans and Han Chinese. The numbers
of SNPs with the corresponding frequencies
are color-coded according to the scale at
right. At most SNPs, allele frequencies in the
two populations are highly correlated. Alleles
at two SNPs in the EPAS1 gene are outliers:
they have increased by positive selection in
Tibetans (blue arrow). This rapid evolution is
thought to be an adaptation to low oxygen
pressure. (After [37].)FIGURE 7.23 shows a plot comparing frequencies of SNPs (single nucleotide
polymorphisms; see Chapter 4) across the genome in Tibetans with those of
Han Chinese, who live near sea level [37]. Overall, there is a very strong correla-
tion: alleles that are common in one population are common in the other. Two
exceptions stand out at the bottom right of the plot. These are SNPs with alleles
that are at about 90 percent frequency in Tibetans but only 10 percent frequency
in Han. Those alleles are also at low frequency among Europeans, which shows
that the difference between the Tibetans and the Han resulted from evolution in
the Tibetan population, not the Han. These data strongly suggest there has been
local adaptation in Tibetans at a site on the chromosome near to these SNPs.
The local adaptation hypothesis becomes even stronger when we learn where
the two SNPs are in the genome. Both lie in a locus called E PA S1, which is
expressed in lungs and placenta and appears to be involved in the regulation of
red blood cells. Those features suggest that E PA S1 adapted to the low oxygen pres-
sures at the high elevations of the Tibetan Plateau. Selection must have been very
intense. Tibetans colonized the plateau only 2750 years ago, and so the allele at
EPA S1 that is now common there must have swept almost to fixation in less than
150 generations. We’ll see in Chapter 21 that the E PA S1 allele carried by Tibetans
has an even more surprising evolutionary story to tell.Go to the
Evolution Companion Website
EvolUTIoN 4 E.SINAUER.CoM
for data analysis and simulation exercises, quizzes, and more.07_EVOL4E_CH07.indd 187 3/23/17 9:09 AM