Molecular markers and natural selection

Natural selection is defined as the differential reproduction of genotypes. Advances in molecular methodology now enable genotypes to be identified wi th allozyme polymorphisms, cleaved polymorphic amplified fragments, anonymo us nuclear DNA polymorphisms, and mitochondrial DNA and chloroplast DNA hap lotypes. Most molecular markers are expected to be neutral, but intense sel ection has been detected in some allozyme loci. It is always difficult to d etermine whether documented examples involve selection for allozyme genotyp es, or selection detected by allozyme genotypes. Similarly, some studies ha ve reported components of fitness to increase with allozyme heterozygosity, but the interpretation of these correlations is controversial. Comprehensive studies of allozyme polymorphisms have revealed that the alte rnate genotypes at a locus often differ kinetically, and may have substanti al impacts on physiological variation, growth rate, viability, fecundity, a nd male mating success. Two recent research programs are featured here. The alcohol dehydrogenase (ADH) polymorphism in the tiger salamander interacts with levels of oxygen in ponds to produce variability in growth and surviv al. In pinyon pine, the glycerate dehydrogenase polymorphism (CLY) is assoc iated with microgeographic variation in soil moisture, and both growth rate s and viabilities on dry sites. Demographic and genetic studies in red deer and harbour seals have reported that birth weight and neonatal survival increase with variation at microsa tellite loci. The high mutation rates of microsatellites may detect differe nces in levels of inbreeding among individuals. Loci sharing the same mode of inheritance but revealing strikingly differen t patterns of geographic variation must be subject to different evolutionar y pressures. Studies that compare the geographic variation of allozymes and DNA markers often show discordant patterns of variation. When estimates of gene flow are estimated from values of Fn, allozyme markers usually return higher estimates of gene flow than do DNA markers. Molecular markers have provided new insights into sexual selection, and pro vided evidence in support of female choice of "good genes". Studies of sexu al selection in butterflies, brine shrimp, and marine snails have revealed male mating success to increase with allozyme heterozygosity.