Showing papers on "Genetic drift published in 1972"

The effect of selection on esterase allozymes in a barley population

Abstract: Changes in gene and genotypic frequencies at four esterase loci were monitored over 25 generations in Composite Cross V, an experimental population of barley, to obtain experimental evidence concerning the balance of forces responsible for: (1) the marked differences in allelic frequencies among barleys from different ecogeographical regions of the world; and (2) the extensive allelic variation found within local populations of barley. Analyses of the highly significant changes in allelic frequencies which occurred in CCV showed they were due to directional selection favoring particular alleles and not to mutation, migration or genetic drift. The results show that intense balancing selection, featuring consistent excesses of heterozygotes, also occurred in CCV. It is concluded that among the factors of neo-Darwinian evolution, natural selection plays the predominant role in determining the observed patterns of allelic variation in the barley species as a whole.

Selection in Finite Populations with Multiple Alleles. III. Genetic Divergence with Centripetal Selection and Mutation

Abstract: Natural selection for an intermediate level of gene or enzyme activity has been shown to lead to a high frequency of heterotic polymorphisms in populations subject to mutation and random genetic drift. The model assumes a symmetrical spectrum of mutational variation, with the majority of variants having only minor effects on the probability of survival. Each mutational event produces a variant which is novel to the population. Allelic effects are assumed to be additive on the scale of enzyme activity, heterosis arising whenever a heterozygote has a mean level of activity closer to optimal than that of other genotypes in the population.—A new measure of genetic divergence between populations is proposed, which is readily interpreted genetically, and increases approximately linearly with time under centripetal selection, drift and mutation. The parameter is closely related to the rate of accumulation of mutational changes in a cistron over an evolutionary time span.—A survey of published data concerning polymorphic loci in man and Drosophila suggests than an alternative model, based on the superiority of hybrid molecules, is not of general importance. Thirteen loci giving rise to hybrid zones on electrophoresis have a mean heterozygote frequency of 0.22 ±.06, compared with a value of 0.23 ±.04 for 16 loci classified as producing no hybrid enzyme.

The biological relationships of some prehistoric and historic Pueblo populations.

Abstract: Genetic evidence from the skeletal remains of three Pueblo populations, those of Hawikuh, Pueblo Bonito, and Puye, does not indicate that important racial differences arose between these groups, either through genetic influx or selection. They formed a unified group when compared with several non-Southwestern skeletal samples. Significant genetic variability, however, exists between each pair of populations, contradicting the idea of their belonging to a unified Pueblo Indian gene pool or to a fixed physical type. No differences can be detected between prehistoric populations and those contacted by Europeans. Genetic drift, supplemented by the action of non-random cultural associations and disease, provides a better explanation of the biological variability of Pueblo Indian populations than gene flow or directional selection.

Evidence for the Neutral Hypothesis of Protein Polymorphism

Abstract: Data for some 400 polymorphic proteins were examined with special reference to molecular evolution, by using a statistic that depends on neither mutation rate, population structure, nor other ecological factors. The result indicates that most of these polymorphisms are maintained in a population by mutation and random genetic drift.

Computer Simulation of a Sporophytic Self-Incompatibility Breeding System

Abstract: A sporophytic self· incompatibility system, such as occurs in Carthamus jlavescens Spreng., was studied by computer simulation. Equilibrium gene fre-quencies in an infinite population were estimated for three-allele and six-allele systems and found to be independent of initial frequencies. In a species existing as a series of more or less isolated small populations genetic drift caused rapid loss of alleles. Maintenance of the S allele system was enhanced by increased population size, and particularly by migration, by which alleles lost from one local population could be re-introduced from adjacent populations. Hard seed carryover had a lesser effect.

Natural selection and genetic drift in protein polymorphism.

Abstract: THE Kimura theory1 that protein polymorphism is mainly due to random genetic drift acting on a number of neutral isoalleles has been recently discussed by many authors2–4 The electrophoretic study of populations of the same species having different geographical origin and showing great fluctuations in number offers possibilities for testing the relative importance of natural selection and genetic drift in the context We report a study of two species of mosquitoes, Aedes aegypti and Aedes mariae, for the phosphoglucomutase (PGM) locus This is an autosomal locus belonging in A aegypti to the linkage group 2 and showing in both species a very high number of electrophoretically detectable alleles (Bullini et al, refs 5 and 6, and unpublished data)

Natural selection for enzyme variants among parthenogenetic Daphnia magna.

Abstract: The frequencies of genetically determined electrophoretic variants of two enzyme systems in the parthenogenetic crustacean D. magna have been followed in two isolated populations. In both populations a marked excess of hetero-zygotes was found in the later samples. It is concluded that the observed changes in gene and genotypic frequencies are due to natural selection as both migration and genetic drift can be excluded.

Genetic drift and polygenic inheritance.

Abstract: The interaction of random gene drift and selection was studied by computer simulation for two quantitative traits, which were considered to approximate stature and skin color differences in human populations. The expected effects of gene drift, fixation of alleles and reduction of genotypic and phenotypic variances, were found in the simulation. Stabilizing selection, which seems to be the type of selection operating on these traits, was found to increase the effects of gene drift. Since there seems to be no evidence of reduction in phenotypic and presumably genotypic variability in small human populations, the applicability of these simple genetic models to human traits raises problems for which several possible solutions exist.

Variation in protein structure and function: Primate hemoglobins.

Abstract: Variation in structure among primate hemoglobins is associated with variation in function This supports the hypothesis that most substitutions observed among homologous proteins in different species have been fixed by natural selection because they contribute to the fitness of the genotype It does not support the concept that most substitutions result from the fixation of neutral alleles by genetic drift