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NYTime article: good layman's review of microevolution in humans

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  • Popplestone, Ann
    June 26, 2007 Humans Have Spread Globally, and Evolved Locally By NICHOLAS WADE
    Message 1 of 2 , Jun 26, 2007
      June 26, 2007

      Humans Have Spread Globally, and Evolved Locally


      Historians often assume that they need pay no attention to human
      evolution because the process ground to a halt in the distant past. That
      assumption is looking less and less secure in light of new findings
      based on decoding human DNA
      pics/geneticsandheredity/index.html?inline=nyt-classifier> .

      People have continued to evolve since leaving the ancestral homeland in
      northeastern Africa some 50,000 years ago, both through the random
      process known as genetic drift and through natural selection. The genome
      bears many fingerprints in places where natural selection has recently
      remolded the human clay, researchers have found, as people in the
      various continents adapted to new diseases, climates, diets and,
      perhaps, behavioral demands.

      A striking feature of many of these changes is that they are local. The
      genes under selective pressure found in one continent-based population
      or race are mostly different from those that occur in the others. These
      genes so far make up a small fraction of all human genes.

      A notable instance of recent natural selection is the emergence of
      lactose tolerance - the ability to digest lactose in adulthood - among
      the cattle-herding people of northern Europe some 5,000 years ago.
      Lactase, the enzyme that digests the principal sugar of milk, is usually
      switched off after weaning. But because of the great nutritional benefit
      for cattle herders of being able to digest lactose in adulthood, a
      genetic change that keeps the lactase gene switched on spread through
      the population.

      Lactose tolerance is not confined to Europeans. Last year, Sarah
      Tishkoff of the University of Maryland
      versity_of_maryland/index.html?inline=nyt-org> and colleagues tested 43
      ethnic groups in East Africa and found three separate mutations, all
      different from the European one, that keep the lactase gene switched on
      in adulthood. One of the mutations, found in peoples of Kenya and
      Tanzania, may have arisen as recently as 3,000 years ago.

      That lactose tolerance has evolved independently four times is an
      instance of convergent evolution. Natural selection has used the
      different mutations available in European and East African populations
      to make each develop lactose tolerance. In Africa, those who carried the
      mutation were able to leave 10 times more progeny, creating a strong
      selective advantage.

      Researchers studying other single genes have found evidence for recent
      evolutionary change in the genes that mediate conditions like skin
      color, resistance to malaria
      pics/malaria/index.html?inline=nyt-classifier> and salt retention.

      The most striking instances of recent human evolution have emerged from
      a new kind of study, one in which the genome is scanned for evidence of
      selective pressures by looking at a few hundred thousand specific sites
      where variation is common.

      Last year Benjamin Voight, Jonathan Pritchard and colleagues at the
      University of Chicago
      versity_of_chicago/index.html?inline=nyt-org> searched for genes under
      natural selection in Africans, Europeans and East Asians. In each race,
      some 200 genes showed signals of selection, but without much overlap,
      suggesting that the populations on each continent were adapting to local

      Another study, by Scott Williamson of Cornell University
      nell_university/index.html?inline=nyt-org> and colleagues, published in
      PLoS Genetics this month, found 100 genes under selection in Chinese,
      African-Americans and European-Americans.

      In most cases, the source of selective pressure is unknown. But many
      genes associated with resistance to disease emerge from the scans,
      confirming that disease is a powerful selective force. Another category
      of genes under selective pressure covers those involved in metabolism,
      suggesting that people were responding to changes in diet
      pics/diet/index.html?inline=nyt-classifier> , perhaps associated with
      the switch from hunting and gathering to agriculture.

      Several genes involved in determining skin color have been under
      selective pressure in Europeans and East Asians. But Dr. Pritchard's
      study detected skin color genes only in Europeans, and Dr. Williamson
      found mostly genes selected in Chinese.

      The reason for the difference is that Dr. Pritchard's statistical screen
      detects genetic variants that have become very common in a population
      but are not yet universal. Dr. Williamson's picks up variants that have
      already swept through a population and are possessed by almost everyone.

      The findings suggest that Europeans and East Asians acquired their pale
      skin through different genetic routes and, in the case of Europeans,
      perhaps as recently as around 7,000 years ago.

      Another puzzle is presented by selected genes involved in brain
      function, which occur in different populations and could presumably be
      responses to behavioral challenges encountered since people left the
      ancestral homeland in Africa.

      But some genes have more than one role, and some of these brain-related
      genes could have been selected for other properties.

      Two years ago, Bruce Lahn, a geneticist at the University of Chicago,
      reported finding signatures of selection in two brain-related genes of a
      type known as microcephalins, because when mutated, people are born with
      very small brains. Two of the microcephalins had come under selection in
      Europeans and one in Chinese, Dr. Lahn reported.

      He suggested that the selected forms of the gene had helped improved
      cognitive capacity and that many other genes, yet to be identified,
      would turn out to have done the same in these and other populations.

      Neither microcephalin gene turned up in Dr. Pritchard's or Dr.
      Williamson's list of selected genes, and other researchers have disputed
      Dr. Lahn's claims. Dr. Pritchard found that two other microcephalin
      genes were under selection, one in Africans and the other in Europeans
      and East Asians.

      Even more strikingly, Dr. Williamson's group reported that a version of
      a gene called DAB1 had become universal in Chinese but not in other
      populations. DAB1 is involved in organizing the layers of cells in the
      cerebral cortex, the site of higher cognitive functions.

      Variants of two genes involved in hearing have become universal, one in
      Chinese, the other in Europeans.

      The emerging lists of selected human genes may open new insights into
      the interactions between history and genetics. "If we ask what are the
      most important evolutionary events of the last 5,000 years, they are
      cultural, like the spread of agriculture, or extinctions of populations
      through war or disease," said Marcus Feldman, a population geneticist at
      Stanford. These cultural events are likely to have left deep marks in
      the human genome.

      A genomic survey of world populations by Dr. Feldman, Noah Rosenberg and
      colleagues in 2002 showed that people clustered genetically on the basis
      of small differences in DNA into five groups that correspond to the five
      continent-based populations: Africans, Australian aborigines, East
      Asians, American Indians and Caucasians, a group that includes
      Europeans, Middle Easterners and people of the Indian subcontinent. The
      clusterings reflect "serial founder effects," Dr. Feldman said, meaning
      that as people migrated around the world, each new population carried
      away just part of the genetic variation in the one it was derived from.

      The new scans for selection show so far that the populations on each
      continent have evolved independently in some ways as they responded to
      local climates, diseases and, perhaps, behavioral situations.

      The concept of race as having a biological basis is controversial, and
      most geneticists are reluctant to describe it that way. But some say the
      genetic clustering into continent-based groups does correspond roughly
      to the popular conception of racial groups.

      "There are difficulties in where you put boundaries on the globe, but we
      know now there are enough genetic differences between people from
      different parts of the world that you can classify people in groups that
      correspond to popular notions of race," Dr. Pritchard said.

      David Reich, a population geneticist at the Harvard Medical School, said
      that the term "race" was scientifically inexact and that he preferred
      "ancestry." Genetic tests of ancestry are now so precise, he said, that
      they can identify not just Europeans but can distinguish between
      northern and southern Europeans. Ancestry tests are used in trying to
      identify genes for disease risk by comparing patients with healthy
      people. People of different races are excluded in such studies. Their
      genetic differences would obscure the genetic difference between
      patients and unaffected people.

      No one yet knows to what extent natural selection for local conditions
      may have forced the populations on each continent down different
      evolutionary tracks. But those tracks could turn out to be somewhat
      parallel. At least some of the evolutionary changes now emerging have
      clearly been convergent, meaning that natural selection has made use of
      the different mutations available in each population to accomplish the
      same adaptation.

      This is the case with lactose tolerance in European and African peoples
      and with pale skin in East Asians and Europeans.

      Ann Popplestone AAB, BA, MA

      CCC Metro TLC



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    • Kaupp, Ann
      WASHINGTON, D.C. IS THE SITE OF THE 2008 SACC ANNUAL MEETING Save the dates: March 14-18, 2008 (Friday - Tuesday) The meeting will be held at the Holiday Inn
      Message 2 of 2 , Jun 26, 2007

        Save the dates: March 14-18, 2008 (Friday - Tuesday)

        The meeting will be held at the Holiday Inn Central at 1501 Rhode Island
        Ave., N.W., Washington, DC 20005; (202) 483-2000. Rooms have been held
        at the special rate of $169 plus tax for SACC members.

        The Smithsonian will be the focus of our field trip on Monday, March 17.

        More details to follow. Ann Kaupp

        P. Ann Kaupp, Head

        Anthropology Outreach Office

        Smithsonian Institution

        PO Box 37012

        NHB MRC 112

        Washington, DC 20012-7012

        (202) 633-1917


        <http://www.nmnh.si.edu/anthro/outreach/outrch1.html> June 26, 2007

        [Non-text portions of this message have been removed]
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