Free A Troublesome Inheritance: Genes, Race and Human History by Nicholas Wade Page B

raw material for natural selection, the second evolutionary force. Most mutations affect only the copious regions of DNA that lie between the genes and are of little consequence. It’s the sequence of bases in the genes that codes the information that specifies proteins and other working parts of the cell. This coding DNA, as it is called, occupies less than 2% of the human genome. Mutations that do not meaningfully alter the coding DNA or the nearby promoter regions of DNA, used to activate the coding DNA, generally have no effect on the organism. Natural selection has no reason to bother about them, and for this reason geneticists call them neutral mutations.
    Of the mutations that do change the genetic sequence, most degrade or even destroy the function of the protein specified by the gene. These mutations are detrimental and need to be eliminated. “Purifying selection” is the phrase geneticists use for the action of natural selection in ridding the genome of harmful mutations. The bearer of the mutation fails to live or has few or no offspring.
    It’s just a handful of mutations that have a beneficial effect, and these become more common in the population with each succeeding generation as the lucky owners are better able to survive and breed.
    The individuals with a beneficial mutation possess a new gene, or rather a new allele—a version of the old gene with the new mutation embedded in it. It’s because of mutations and alleles that there exists a third force of evolutionary change, called genetic drift. Each generation is a genetic lottery. Your father and mother each have two copies of every gene. Each parent bequeaths one of their two copies to you. The other is left on the cutting room floor. Suppose that witha particular gene there are just two versions, called alleles A and B, in a population. Suppose too that 60% of a present population carries allele A and 40% allele B. In the next generation, these proportions will change because, by the luck of the draw, allele A will be passed on to children more often than allele B, or the other way round.
    If you follow the fate of allele A down the generations, it does a random walk in terms of its frequency in the population, from 60% in one generation, say, to 67% in the next to 58% to 33% and so on. But the walk cannot continue forever, because sooner or later it will hit one of two numbers, either 0% or 100%. If the frequency falls to 0%, allele A is permanently lost from the population. If it hits 100%, it’s allele B that is lost and allele A that becomes the permanent form of the gene, at least until a new and better mutation crops up. This fluctuation in frequency is a random process known as genetic drift, and when the walk ends in allele A hitting 100%, geneticists say that it has become fixed or has gone to fixation, meaning it’s the only game in town.
    An important part of the genome that has gone to fixation is the DNA of the energy-producing mitochondria, former bacteria that were captured and enslaved long ago by the ancestor of all animal and plant cells. The mitochondria, little organelles within every cell, are inherited through the egg and passed down from a mother to her children. At some early stage in modern human evolution, one woman’s mitochondrial DNA went to fixation by edging out all other versions of mitochondrial DNA.
    The same winner-take-all victory was attained by a particular version of the Y chromosome, which men alone carry because it includes the male-determining gene. At a time when the human population was quite small, a single individual’s Y chromosome increased in frequency until it became the only one left. As described below, the genetic legacies of the mitochondrial Eve and the Y-chromosomalAdam have proved immensely useful for tracing the migration of their descendants around the globe.
    This rise and fall of the alleles depends on the blind chance of which are cast aside and which pass into the next generation