July 01, 2010
Tibetans Adapted To Altitude In 3000 Years
The Tibetans are packing lots of genetic mutations that adapt them to high altitude living.
A comparison of the genomes of 50 Tibetans and 40 Han Chinese shows that ethnic Tibetans split off from the Han less than 3,000 years ago and since then rapidly evolved a unique ability to thrive at high altitudes and low oxygen levels.
A common misperception is that evolutionary changes require tens or hundreds of thousands of years. Not so. Given strong enough selective pressures mutations can spread very rapidly. Humans have developed many adaptations to adjust them to local conditions. Altitude is just one of the environmental challenges which humans have evolved to handle in local regions. Among the many local adaptations: lactase enzyme upregulation among pastoral populations that derived a substantial fraction of their calories from animal milk. Differences in alcohol tolerance in northern and southern European peoples also probably comes as a result of selective pressures in areas where more alcohol was produced.
Many genes were involved in the evolution of Tibetan adaptation to high altitude living.
The genome-wide comparison, performed by evolutionary biologists at the University of California, Berkeley, uncovered more than 30 genes with DNA mutations that have become more prevalent in Tibetans than Han Chinese, nearly half of which are related to how the body uses oxygen. One mutation in particular spread from fewer than 10 percent of the Han Chinese to nearly 90 percent of all Tibetans.
Evolution happens due to different death rates between carriers of different genetic variations. Lots of people died early while others died later (after reproducing) to adapt Tibetans to their ecological niche.
"This is the fastest genetic change ever observed in humans," said Rasmus Nielsen, UC Berkeley professor of integrative biology, who led the statistical analysis. "For such a very strong change, a lot of people would have had to die simply due to the fact that they had the wrong version of a gene."
The fastest change observed in human evolution took place in 3000 miles? See this book for an example of a human evolutionary adaptation that took place in several hundred years.
The continued decline in DNA sequencing costs is set to unleash a huge flood of discoveries about human genetic adaptations which developed in ecological niches around the globe. I expect that every organ in the body has genetic variations that have adapted humans to different diets, weather, diseases, elevations, temperature ranges, local prey, and other local conditions.
Also see my previous post Tibetans Genetically Adapted To High Altitude.
My favorite Adaptation Conundrum:
Al Gore (et al) : Plants and Animals cannot adapt to swings in average temperature of .6C per century. They are all going to die.
Charles Darwin : Plants and Animals CAN ADAPT to climates where the temperature can change 10-20C per day and swings of temperature of 60-70C per year.
There are some problems with the statement "Evolution happens due to different death rates between carriers of different genetic variations"
1st, death is not required; rather only differential reproductive success is required and mortality is only one mechanism that generates differential reproduction rates of certain variations. There are a large number of other mechanisms also at work, like female selection criteria and other behavioral inputs, non-lethal disease incidence, differential reproductive failure or rates, etc.
Numerous studies with insects for more than 30 years have demonstrated that it takes roughly 25 generations to propagate a particular genetic variation throughout a closed population (see the works of D. Pimentel and his students at Cornell U). For insects, 25 generations can be only a few years but for humans it is more like [age of 1st reproduction =] ~18 x 25 gen = ~450 years. So it would be possible for humans to adapt to high elevation in some manner in as little as 450 years, assuming age of 1st reproduction of the population is 18. Of course there are numerous other factors like how closed the population is, how likely the adaptation is to be found already existing or newly generated within the population, etc. I would guess that evolution of some trait very much faster than 450 years is pretty unlikely with a natural population of any moderate size.
If you want to get idiotic about it, when you have a population of 2 and one person has an adaptation and lives and the other person does not have it and dies, the evolution of the population adaptation is virtually instantaneous, rofl.
I don't argue that such selective pressure isn't fast. It is just 100% spread of a mutation isn't likely to be faster than 25 generations or approximately 450 years in extant humans populations. Your citation actually records such evolution over thousands of years and NOT under 450 years with initial selection during smallpox continued by black death, and following by other diseases and that continuous selection by smallpox is more effective than the waves of selection by plague. In actuality, none of these selective pressures resulted in the kind of 100% spread of a particular allele demonstrated in the lab experiments with insect populations because the human populations in question were quite large and NOT closed populations. Rather, such alleles are only found in much less than 100% of the population.
In any case, my main point is that death is only ONE possible way of generating differential reproduction required to spread a new adaptation thru a population and not the ONLY way.
If this variation was already present in the original Han population, is this really evolution (change)? This is simply selection for a pre-existing variant- not the addition of a new adaptation. Or is it proposed that the 10% of Han Chinese were due to crosses from Tibet after a new trait evolved there?