A clear view of the cell cycle David O.MorganThe Cell Cycle: Principles of Control2007New Science PressISBN-13:978-0-19-920610-0

cause of mortality. However, another hemoglobin allele C has recently been estimated to have even higher fitness, and now appears to be replacing allele S, suggesting that the sickle-cell polymorphism may be evolutionarily short-lived. When alleles are maintained by balancing selection for a long evolutionary time and are consequently shared by related species, it is referred to as 'trans-species polymorphism'. There are only a few widely accepted examples of trans-species polymorphism, which include the major histocompatibility complex (MHC) loci that are part of the vertebrate immune system, the self-incompatibility loci that prevent self-fertilization in a number of plant species, and the complementary sex determination (csd) locus that determines sex in a number of hymenoptera species. How is balancing selection different from directional selection? 'Directional selection' causes adaptively important genetic variants to increase in frequency. The initial stages of increase for a mutant, or a low frequency allele, under directional and balancing selection are hard to distinguish from each other because, in both instances, the allele increases to some intermediate frequency at a rate that depends upon the selective advantage. Directional selection then leads to a continued increase in frequency and to fixation, or near fixation, causing the appearance of a 'selective sweep' and reduced variability in the genomic region nearby the selected locus. Balancing selection, on the other hand, brings the favored allele to an intermediate equilibrium, where it is maintained as a genetic polymorphism that potentially increases the variability in the genomic region nearby the selected locus. If directional selection does not result in a complete selective sweep and stops when the allele is at intermediate frequency, then it may have somewhat similar effects as balancing selection. The terms 'positive selection' or 'positive Darwinian selection' are often used to include both directional and balancing selection. Has genomics uncovered examples of balancing selection? There have been several recent genomic screens in humans which have attempted to find loci exhibiting balancing selection, but they have revealed few new examples of trans-species polymorphism or of high single nucleotide polymorphism (SNP) density. These studies suggest either that a low proportion of loci in the human genome are under long-term balancing selection, or that these screens are not efficiently detecting such loci. Future genomic studies in humans and similar studies with other species should help resolve these basic questions about the role of balancing selection in the maintenance of genetic polymorphism. et al. (2006). …