Expert answer:Evolving altitude aptitude, Science Question!

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Evolving altitude aptitude
If you live in the lowlands, you
may have experienced the huffing
and puffing that typically
accompanies a trip to higher
altitudes. That’s because oxygen
levels go down as one goes up.
Travelling to Denver from sea
level means a 17% decrease in
available oxygen. Our bodies
compensate for even this small
change with faster breathing and
a higher heart rate — at least until
we acclimate to the thinner
atmosphere. And a loftier
vacation spot (for example, La
Paz, Bolivia at 11,942 feet) could
bring on serious altitude sickness
with insomnia, nausea, and swelling — but not for everyone. Tibetan highlanders have no trouble living at
13,000 feet year in year out, and many Nepalese Sherpas (who are ethnically Tibetan) climb parts of
Mount Everest without the supplementary oxygen most people require. New research makes it clear that
Tibetan highlanders have evolved unique physiological mechanisms for dealing with low oxygen levels.
When lowlanders visit high altitudes, their bodies produce more red blood cells — the purveyors of oxygen
in the body. These extra cells to help transport available oxygen around the body — and eventually
compensate for decreased oxygen levels. This is an example of phenotypic plasticity, shifts in an
organism’s body, physiology, or behavior in response to its environment, not upon a genetic change or
mutation. Because they don’t reflect a shift in the genetic makeup of a population, such changes are not
adaptations in an evolutionary sense.
The Tibetan highlander population has evolved adaptations that allow individuals to thrive in a low oxygen
environment. One of these adaptations is almost exactly the opposite of a lowlander’s response to high
altitude: Tibetans have alleles that cause them to produce fewer red blood cells. Extra red blood cells make
blood thicker. This cell-laden blood can actually get so thick that it doesn’t pass through capillaries
efficiently to oxygenate cells. Having blood with too many red blood cells can be particularly problematic
during pregnancy since it is linked to slow fetal growth and high rates of fetal mortality. One study found
that Han Chinese people (lowland relatives of Tibetans whose bodies respond to high altitudes by
producing more red blood cells) living in the Tibetan highlands were three times as likely to suffer pre- or
post-natal infant death than were ethnic Tibetans in the long run, producing extra red blood cells may do
more harm than good.
The basis for the Tibetans’ adaptation is a change in a gene called EPAS1, which codes for a regulatory
protein. This protein senses oxygen and helps control the process of producing red blood cells. The change
in EPAS1 seems to make Tibetans less likely to overproduce red blood cells at extreme altitudes and
probably helps them avoid altitude sickness and deliver oxygen more effectively to developing fetuses.
Other unique traits of Tibetans (a higher breathing rate and blood vessels which expand to allow better
oxygen transport) likely also contribute to their altitude aptitude.
Some genetic studies estimate that the Tibetans split from the Han Chinese population and began
migrating to the highlands less than 3000 years ago — and that all this adaptation at high elevation has
occurred in just a hundred or so generations. If that estimate is accurate, this would represent the fastest
example of human evolution yet documented.
Adapted from Evolving Altitude Aptitude: http://evolution.berkeley.edu/evolibrary/news/101001_altitude
1
Imagine that the Tibetan highland population migrates to the lowlands and stays there. What would you
expect to happen to the frequency of the altitude adaptations in the EPAS1 gene in that population? Use
the Hardy Weinberg Equilibrium simulator to model what you might expect to happen. Keep in mind that
the mutations in EPAS1 that help Tibetans handle high altitudes are found at low frequencies in Han
Chinese populations.
References
Beall, C. M. Cavalleri, G. L., Deng, L., Elston, R. C., Gao, Y., Knight, J., . . . Zhang, Y. T. (2010). Natural selection on EPAS1 (HIF2a)
associated with low hemoglobin concentration in Tibetan highlanders. Proceedings of the National Academy of Sciences
USA 107: 11459-11464.
Mayell, H. (February 25, 2004). Three high-altitude peoples, three adaptations to thin air. Retrieved October 12, 2010
from National Geographic News.
Moore, L. G., Young, D., McCullough, R. E., Droma, T., and Zamudio, S. (2001). Tibetan protection from intrauterine grown
restriction (IUGR) and reproductive loss at high altitude. American Journal of Human Biology 13: 635-644.
Storz, J. F. (2010) Genes for high altitudes. Science 329: 40-41.
Yi, X., Liang, Y., Huerta-Sanchez, E., Jin, X., Cuo, Z. X. P., Pool, J. E. . . .Wang, J. (2010). Sequencing of 50 human exomes reveals
adaptation to high altitude. Science 329: 75-78.
Adapted from Evolving Altitude Aptitude: http://evolution.berkeley.edu/evolibrary/news/101001_altitude
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