Convergent Evolution Linked To Adaptation To High Altitudes
December 8, 2012

Different Genes Result In Shared Adaptation To High Altitudes

April Flowers for - Your Universe Online

A shared biological adaptation enables highlanders in both Tibet and Ethiopia to thrive in the low oxygen of high altitudes. A new study from Case Western Reserve University reveals that the ability to pass on this trait, however, appears to be linked to different genes in the two groups.

The shared adaptation is the ability to maintain relatively low levels of hemoglobin for high altitudes. Hemoglobin is the oxygen-carrying protein in red blood cells. Increased hemoglobin production is the natural response of a member of an ethnic group that historically lives at low altitudes — such as most Americans — when subjected to high altitude conditions. While this response can help draw oxygen into the body, it also increases blood viscosity and the risks for thrombosis, stroke and difficulties with pregnancies.

"At 4000 meters, every lungful of air only has 60% of the oxygen molecules that people at sea level have," said Cynthia Beall, professor of anthropology at Case Western Reserve University.

"Altitude affects your thinking, your breathing, and your ability to sleep. But high-altitude natives don't have these problems," said Beall, who has studied high altitude adaptation in different populations in Ethiopia, Peru and Tibet for more than 20 years.

"They don't wheeze like we do. Their thinking is fine. They sleep fine. They don't complain of headaches. They're able to live a healthy life, and they do it completely comfortably," she added.

The research may provide insight into managing high-altitude sickness and new treatments for low blood-oxygen conditions such as asthma, sleep apnea and heart problems among both high and low altitude populations by understanding how the populations living in such severe environments have adapted.

Scientists do not know how long it takes such physiological and genetic changes to take place. The research team found the adaptation in an ethnic group that has lived in the mountains of Ethiopia for at least 5,000 years. However, another related group that has lived in the mountains for only 500 years did not have the adaptation.

The team found the genes responsible for hemoglobin levels in Tibetans do not influence an ethnic group called the Amhara, who have lived more than a mile high in the Semien Mountains of northern Ethiopia for 5,000 to 70,000 years. The variant on the Amhara genome that is associated with their low hemoglobin levels is far away from the location of the Tibetan variant.

"All indications are we're seeing convergent evolution," said Beall,.

When two separate populations change biologically in a similar way to adapt to a similar environment yet use different mechanisms, that is convergent evolution. It is still unclear to the team if the initial settlers in the Tibetan and Ethiopian populations carried different genetic variants when they arrived in their respective regions, or if the different mutations occurred after they arrived. What is clear is that each group followed a different evolutionary path.

"They have a similar physiologic solution, but that doesn't necessarily amount to a similar genetic solution," Anna Di Rienzo of the University of Chicago said.

"These were two different evolutionary experiments," Beall said of the mountain dwellers in Tibet and Ethiopia. "On one level–the biological response–they are the same. On another level–the changes in the gene pool–they are different."

Beall and her colleagues from the University of Chicago and Addis Ababa University investigated the adaptations and genetic links by analyzing the genomes of nearly 260 Ethiopian villagers extracted from blood and saliva samples. The team looked for changes in physiology and genetics among a related ethnic group in addition to studying the Amhara. The second group, the Oromo, have lived more than a mile above sea level in the Bale Mountains of southern Ethiopia for 500 years.

The team used a statistical technique called a genome-wide association study to scan the genomes of highland and lowland Ethiopians from both ethnic groups in search of variants associated with hemoglobin levels in the blood.

In the Oromo, the team found no long-term adaptation and no genetic changes related to a low-oxygen environment. The Omoro have high levels of hemoglobin, the team found, as would be expected for a lowland population.

The team is continuing their study using the samples collected from the Amhara and the Oromo populations, looking at biological traits such as ventilation. They are also investigating the influence of vasoconstrictors and vasodilators on blood flow, and searching for associations with genes.

The team plans to study blood flow, especially through the heart and lungs of the highlanders. They will test the metabolic rate of mitochondria that use oxygen to create the energy on which our cells and we operate.

"We also want to find whether people with the variants for low hemoglobin levels have more children and a higher survival rate," Beall said. "That's the evolutionary payoff."

The findings of this study have been reported online in the journal PLOS Genetics.