Genetic variation as a test of natural selection

Statistical Studies on Protein Polymorphism in Natural Populations. III. Distribution of Allele Frequencies and the Number of Alleles per Locus

With the aim of understanding the mechanism of maintenance of protein polymorphism, we have studied the properties of allele frequency distribution and the number of alleles per locus, using gene-frequency data from a wide range of organisms (mammals, birds, reptiles, amphibians, Drosophila and non-Drosophila invertebrates) in which 20 or more loci with at least 100 genes were sampled. The observed distribution of allele frequencies was U-shaped in all of the 138 populations (mostly species or subspecies) examined and generally agreed with the theoretical distribution expected under the mutation-drift hypothesis, though there was a significant excess of rare alleles (gene frequency, 0 approximately 0.05) in about a quarter of the populations. The agreement between the mutation-drift theory and observed data was quite satisfactory for the numbers of polymorphic (gene frequency, 0.05 approximately 0.95) and monomorphic (0.95 approximately 1.0) alleles.-The observed pattern of allele-frequency distribution was incompatible with the prediction from the overdominance hypothesis. The observed correlations of the numbers of rare alleles, polymorphic alleles and monomorphic alleles with heterozygosity were of the order of magnitude that was expected under the mutation-drift hypothesis. Our results did not support the view that intracistronic recombination is an important source of genetic variation. The total number of alleles per locus was positively correlated with molecular weight in most of the species examined, and the magnitude of the correlation was consistent with the theoretical prediction from mutation-drift hypothesis. The correlation between molecular weight and the number of alleles was generally higher than the correlation between molecular weight and heterozygosity, as expected.

Relationship between allozyme heterozygosity and rates of divergence

The relationship between locus variability and genetic identity is analysed in pooled data from a collection ofDrosophilaand vertebrate allozyme surveys. For vertebrates the analysis provides evidence that loci with high expected heterozygosity diverge at a faster rate than loci with low heterozygosity in comparisons involving pairs of populations and pairs of species. This relationship is not observed in comparisons ofDrosophilaspecies pairs.

Evolutionary dynamics of transferrin in Notropis

This study reports on the development of comparative data for the transferrin (TF) gene in cyprinid fishes, focusing on the genus Notropis. While previous studies have suggested varied patterns of adaptation influencing the diversity at this gene locus both within and among species, sequence data for (TF) in Notropis exhibit limited evidence for selection. However, there are significant effects of sequence variation associated with Notropis lutipinnis, suggesting some form of diversifying selection acting among populations of this species. Overall, the gene performs well as a second locus for phylogenetic and biogeographic inference and may help improve description of the pattern and the process of diversification in Notropis.

Genetic diversity and ecological relationships of marsh frog populations in Israel

Allozymic variation in proteins encoded by 28 loci was analyzed electrophoretically in 340 mostly adult specimens representing 11 populations, 8 central and 3 isolated, of aquatic marsh frogs, Rana ridibunda in Israel, along a north-south transect of generally increasing aridity. In addition, geographic variation in 3 morphological variables of 144 frogs and in vertebral stripe color polymorphism of 262 frogs were also studied. The results indicate that. (a) Of the 28 loci examined, 12 (= 43%) are largely monomorphic in all populations; out of the remaining loci, 6 were locally and weakly polymorphic and 10 regionally and strongly polymorphic. (b) No fixation of alternative alleles was found in any of the 28 loci and 11 populations studied. The commonest allele predominated across all populations, central as well as isolates, (c) Clinal patterns associated with increasing aridity southwards and eastwards occurred in polymorphism, P; heterozygosity, H; and in allele frequencies of Esterase-1, Xanthine dehydrogenase, Aldehyde oxidase and Albumin. (d) In the 3 estimates of genie variation, mean number of alleles per locus, A, mean proportion of polymorphic loci per population, P, and heterozygous loci per individual, H, marsh frogs displayed average estimates of genetic variation. The 3 estimates were: A =1.14 (range, 1.18-1.57); P = 0.33 (range, 0.14-0.54): H = 0.069 (range, 0.032-0.094). (e) Central populations harbored distinctly more genic variation than isolated populations. (f) Genic similarity between populations was high. (g) Significant deviations from Hardy-Weinberg equilibrium were found in 8 out of 11 populations involving 8 loci, (h) P, H, and allozymic variation in several gene loci were significantly correlated and predictable by environmental variables, primarily those related to water and temperature. (i) A significant amount of morphological variation was found between localities for body length, foot length, and weight in both sexes. Body weight in females was negatively correlated with temperature; and all three morphological variables in females were predicted significantly by a combination of temperature and humidity. (j) The three vertebral stripe color phenotypes, gray, green and red occurred in the following frequencies: 0.59, 0.24, 0.17, respectively. The red morph increased clinally southwards and was significantly correlated with most temperature and water variables. The geographic variation in both the green and red morphs was predicted significantly by climatic variables, both colors blending with local substrates.The spatial patterns and environmental correlates of genetic and morphological variation in Rana ridibunda in Israel suggest that (i) protein polymorphisms are at least partly adaptive and that part is moulded by natural selection rather than by stochastic processes or neutrality; (ii) the environmental variation model seems to be a good predictor of genetic variation in marsh frogs; (iii) body size varies adaptively, presumably determined primarily through thermoregulation; (iv) the spatial pattern of the color polymorphism seems to be adaptively selected by at least two factors: visual predation and climatic determinants.

Statistical studies on protein polymorphism in natural populations. I. Distribution of single locus heterozygosity

Surveying the literature, the frequency distribution of single-locus heterozygosity among protein loci was examined in 95 vertebrate and 34 invertebrate species with the aim of testing the validity of the mutation-drift hypothesis. This distribution did not differ significantly from that expected under the mutation-drift hypothesis for any of the species examined when tested by the Kolmogorov-Smirnov goodness-of-fit statistic. The agreement between the observed interlocus variance of heterozygosity and its theoretical expectation was also satisfactory. There was an indication that variation in the mutation rate among loci inflates the interlocus variance of heterozygosity. The variance of heterozygosity for a homologous locus among different species was also studied. This variance generally agreed with the theoretical value very well, though in some groups of Drosophila species there was a significant discrepancy. The observed relationship between average heterozygosity and the proportion of polymorphic loci was in good agreement with the theoretical relationship. It was concluded that, with respect to the pattern of distribution of heterozygosity, the majority of data on protein polymorphisms are consistent with the mutation-drift hypothesis. After examining alternative possible explanations involving selection, it was concluded that the present data cannot be explained adequately without considering a large effect of random genetic drift, whether there is selection or not.

Relationship between enzyme heterozygosity and quaternary structure

The need for proteins to maintain particular quarternary structures constrains variability in amino acid sequence. Monomeric enzymes are then expected to be more variable than dimeric forms, which in turn are expected to be more variable than tetrameric forms. These predictions are confirmed by analysis of available data on enzyme variation. Theories relating enzyme heterozygosity to metabolic function are discussed in the light of these findings.

High genetic variability in an ecologically variable vertebrate, Bufo viridis

Allozymic variation in proteins encoded by 26 loci was analyzed electrophoretically in 517 specimens of green toads from 11 populations from Israel and one population from Vis Island in the Adriatic Sea. Genetic variation in this toad is the highest yet reported in any vertebrate. All three genetic parameters, mean number of alleles per locus (A), mean proportion of loci polymorphic per population (P), and mean number of heterozygous loci per individual (H), are very high (A = 1.65, range 1.38-2.04; P = 0.423, range 0.346-0.615; H = 0.133, range 0.108-0.159). Central and marginal mainland populations are only slightly more variable than desert isolates, but much more variable than the Vis Island population. Genetic similarity is very high between mainland populations (S = 0.951, range 0.93-0.97). Frequencies of two alleles (Icd-lc and Tfa) are correlated with an ecological gradient of increasing aridity. Regulatory enzymes appeared to contribute more to overall polymorphism than non-regulatory enzymes. The genetic variation observed suggests that selection for heterozygosity as an adaptive strategy is operating in the ecologically variable environment in which green toads live.