Gene Variation in Nature Evolution within a species may result from any process that causes a change in the genetic composition of a population In considering this theory of population genetics, it is best to start by looking at the genetic variation present among individuals within a species. This is the raw material avail able for the selective process Measuring Levels of Genetic Variation As we saw in chapter 13, a natural population can contain a great deal of genetic variation. This is true not only of hu mans, but of all organisms. How much variation usually oc- curs: Biologists have looked at many different genes in an effort to answer this question: 1. Blood groups. Chemical analysis has revealed the ex- istence of more than 30 blood group genes in humans, in addition to the abo locus. At least a third of these genes are routinely found in several alternative allelic forms in human populations. In addition to these, there FIGURE 20.3 are more than 45 variable genes encoding other pro- Polymorphic variation. These Australian snails, all of the species teins in human blood cells and plasma which are not Bankivia fasciata, exhibit considerable variation in pattern and considered blood groups. Thus, there are more than 75 color. Individual variations are heritable and passed on to genetically variable genes in this one system alone. offspring 2. Enzymes. Alternative alleles of genes specifying particular enzymes are easy to distinguish by measur ing how fast the alternative proteins migrate in an than 5%)at more than half of their enzyme-encoding loci electric field(a process called electrophoresis).A although vertebrates are somewhat less polymorphic. Het- eat deal of variation exists at enzyme specifying erozygosity(that is, the probability that a randomly se- loci. About 5% of the enzyme loci of a typical human lected gene will be heterozygous for a randomly selected are heterozygous: if you picked an individual at individual) is about 15% in Drosophila and other inverte- random, and in turn selected one of the enzyme brates. between 5% and 8% in vertebrates and around 8% encoding i in 20(5%)that the gene you selected ity prowide ang plants. These high levels of genetic variabil- nes of that individual at random the chances are mple supplies of raw material for evolution rould be heterozygous in that individual Considering the entire human genome, it is fair to say DNA Sequence polymorphism that almost all people are different from one another. This With the advent of gene technology, it has become possible is also true of other organisms, except for those that repro- to assess genetic var variation even more directly by seq ing the DNA itself. In a pioneering study in 1989, Martin Kreitman sequenced ADH genes isolated from 11 individu Enzyme Polymorphism ls of the fruit fly Drosophila melanogaster. He found 43 var Many loci in a given population have more than one allele able sites, only one of which had been detected by protein at frequencies significantly greater than would occur from electrophoresis! In the following decade, numerous other studies of variation at the dna level have confirmed these mutation alone. Researchers refer to a locus with more findings: abundant variation exists in both the codig variation than can be explained by mutation as polymor phic(pob,“many," morphic,“ forms”)( figure20.3).The gions of genes and in their nontranslated introns--consic tent of such variation within natural populations was not erably more variation than we can detect examining en even suspected a few decades ago, until modern techniques zymes with electrophoresis such as gel electrophoresis made it possible to exa amine en- rmes and other proteins directly. We now know that most Natural populations contain considerable amounts of populations of insects and plants are polymorphic(that is, genetic variation---more than can be accounted for by ave more than one allele occurring at a frequenc Chapter 20 Go ene Populations 423Gene Variation in Nature Evolution within a species may result from any process that causes a change in the genetic composition of a population. In considering this theory of population genetics, it is best to start by looking at the genetic variation present among individuals within a species. This is the raw material avail￾able for the selective process. Measuring Levels of Genetic Variation As we saw in chapter 13, a natural population can contain a great deal of genetic variation. This is true not only of hu￾mans, but of all organisms. How much variation usually oc￾curs? Biologists have looked at many different genes in an effort to answer this question: 1. Blood groups. Chemical analysis has revealed the ex￾istence of more than 30 blood group genes in humans, in addition to the ABO locus. At least a third of these genes are routinely found in several alternative allelic forms in human populations. In addition to these, there are more than 45 variable genes encoding other pro￾teins in human blood cells and plasma which are not considered blood groups. Thus, there are more than 75 genetically variable genes in this one system alone. 2. Enzymes. Alternative alleles of genes specifying particular enzymes are easy to distinguish by measur￾ing how fast the alternative proteins migrate in an electric field (a process called electrophoresis). A great deal of variation exists at enzyme-specifying loci. About 5% of the enzyme loci of a typical human are heterozygous: if you picked an individual at random, and in turn selected one of the enzyme￾encoding genes of that individual at random, the chances are 1 in 20 (5%) that the gene you selected would be heterozygous in that individual. Considering the entire human genome, it is fair to say that almost all people are different from one another. This is also true of other organisms, except for those that repro￾duce asexually. In nature, genetic variation is the rule. Enzyme Polymorphism Many loci in a given population have more than one allele at frequencies significantly greater than would occur from mutation alone. Researchers refer to a locus with more variation than can be explained by mutation as polymor￾phic (poly, “many,” morphic, “forms”) (figure 20.3). The ex￾tent of such variation within natural populations was not even suspected a few decades ago, until modern techniques such as gel electrophoresis made it possible to examine en￾zymes and other proteins directly. We now know that most populations of insects and plants are polymorphic (that is, have more than one allele occurring at a frequency greater than 5%) at more than half of their enzyme-encoding loci, although vertebrates are somewhat less polymorphic. Het￾erozygosity (that is, the probability that a randomly se￾lected gene will be heterozygous for a randomly selected individual) is about 15% in Drosophila and other inverte￾brates, between 5% and 8% in vertebrates, and around 8% in outcrossing plants. These high levels of genetic variabil￾ity provide ample supplies of raw material for evolution. DNA Sequence Polymorphism With the advent of gene technology, it has become possible to assess genetic variation even more directly by sequenc￾ing the DNA itself. In a pioneering study in 1989, Martin Kreitman sequenced ADH genes isolated from 11 individu￾als of the fruit fly Drosophila melanogaster. He found 43 vari￾able sites, only one of which had been detected by protein electrophoresis! In the following decade, numerous other studies of variation at the DNA level have confirmed these findings: abundant variation exists in both the coding re￾gions of genes and in their nontranslated introns—consid￾erably more variation than we can detect examining en￾zymes with electrophoresis. Natural populations contain considerable amounts of genetic variation—more than can be accounted for by mutation alone. Chapter 20 Genes within Populations 423 FIGURE 20.3 Polymorphic variation. These Australian snails, all of the species Bankivia fasciata, exhibit considerable variation in pattern and color. Individual variations are heritable and passed on to offspring