BIOLOGY Volume 3 - A Guide to General Biology - 2004

27. MECHANISMS OF SPECIATION

27.1. Population Genetics

27.1.5. Implications of the Hardy-Weinberg Equation

The Hardy-Weinberg equation shows that a significant proportion of the recessive alleles present in a population are carried by heterozygotes. In fact, heterozygous genotypes serve as an important potential source of genetic variation. As a result, only a very small fraction of recessive alleles can be eliminated from a population in each generation. Only those recessive alleles present in the homozygous state will be expressed in the phenotype, thereby becoming subject to environmental Selection pressures and potential elimination.

Many recessive alleles are eliminated because they are disadvantageous to the phenotype. Such elimination may occur either through the death of the Organism before it can reproduce, or through so-called “genetic death,” i.e., sterility. However, not all recessive alleles are detrimental to the population. For instance, in humans, Blood group O—which corresponds to homozygosity for a recessive allele—is the most common blood group. Another example is Sickle-Cell Anemia. This inherited blood disorder is widespread in parts of Africa and India, certain Mediterranean countries, and among the African American population in North America. Individuals homozygous for the relevant recessive allele usually die before reaching sexual maturity, thus eliminating two recessive alleles from the population. Heterozygotes, on the other hand, do not die. It has been established that the frequency of the sickle-cell allele remains relatively stable in many PARTS OF THE globe. In some African tribes, the frequency of the heterozygous genotype reaches 40%. It was previously thought that this level was maintained by the appearance of new mutants. However, subsequent research revealed that the reality is quite different: it turned out that in many parts of Africa where malaria is a major health threat, individuals carrying the sickle-cell allele exhibit increased resistance to the disease. In the malaria-endemic regions of Central Africa, this selective advantage of the heterozygous genotype maintains the frequency of the sickle-cell allele at 10–20%.

The maintenance of a potentially harmful recessive allele at a fairly constant level is known as heterozygote advantage. Among North American Blacks, who have not experienced the selective pressure of malaria for 200–300 years, the frequency of the sickle-cell allele has dropped to 5%. This decline can be partially attributed to Gene flow resulting from intermarriage between Black and white populations, but an equally important factor is the absence of malaria in North America, which removes the selective pressure favoring heterozygotes. As a result, the recessive allele is slowly being eliminated from the population. This is an example of evolution in action. It clearly demonstrates the selective Influence of the environment on allele frequencies—a mechanism that disrupts the genetic equilibrium predicted by the Hardy-Weinberg law. It is precisely mechanisms of this kind that cause shifts in populations leading to evolutionary change.