ABSTRACT
Geographic maps of the gene frequencies often show distinct clinal patterns. These clinal patterns graphically indicate some of the forces at work in producing biological variation. Human variation can be viewed at these various levels: race and sex, age and race subgroups, and individual. Gene frequencies are often drawn on maps by use of computer graphics. Phenotypes represent the action of one or more sets of genes. Phenotype is distinguished from genotype; genotype is the more specific term. Phenotypes come about in a variety of ways; but genotypes are specific to a single set of alleles. Population gene frequencies remain stable over generations, provided that no mutation, gene flow, genetic drift, or selection occurs. This principle is called the Hardy-Weinberg Equilibrium Law, the basic law of population genetics. Two major types of selection produce effects on the frequency of genes in a population: directional and stabilizing selection.
