Rapid appearance of epistasis during adaptive divergence following colonization (original) (raw)
Related papers
A Test of the Role of Epistasis in Divergence Under Uniform Selection
Evolution, 1989
Five populations of Drosophila melanogaster have previously been shown to be replicably different in their responses to artificial selection for knockdown resistance to ethanol fumes (Cohan and Hoffmann, 1986). The present study tests whether this divergence could be attributed to the epistatic mechanism assumed by Wright's shifting-balance model of evolution, in which alleles favored in the genetic background of one population are not favored in that of another. If this were the mechanism of divergence, crosses between selected lines from different populations would be expected to yield an epistatic loss of the selected phenotype. However, all such crosses showed a good fit to an additive model with dominance. Divergence by an epistatic mechanism may also be associated with epistatic variance within populations, but no evidence for such epistasis was found. The populations therefore appear to have responded in different ways to selection not because of epistasis but because knockdown-resistance alleles that were common in some populations were absent (or at least less common) in others.
Natural selection for modifiers of heterozygote fitness
Journal of Theoretical Biology, 1963
The paper elaborates the theoretical analyses which formed the basis of Parsons' & Bodmer's (1961) suggested mechanism for the evolution of overdominance. In a random mating population a new advantageous gene occurs initially much more often in heterozygotes than in homozygotes. Calculations are given for this relative preponderance of heterozygotes over homozygotes during the initial progress of a new advantageous gene. Following an argument similar to that used by Fisher (1928) for the evolution of dominance, it seems likely that during this initial stage modifiers will be selected principally for their effect on the heterozygote and their selection may result in the evolution of overdominance. When, on the other hand, overdominance has been achieved and has given rise to a balanced polymorphism, there is no longer the same differential selection in favor of the heterozygote. The results are discussed in relation to the significance of linkage and interaction between loci for the evolution of linked overdominant gene complexes.
Genetic architecture of fitness and nonfitness traits: empirical patterns and development of ideas
Heredity, 1999
Comparative studies of the genetic architecture of dierent types of traits were initially prompted by the expectation that traits under strong directional selection (®tness traits) should have lower levels of genetic variability than those mainly under weak stabilizing selection (non®tness traits). Hence, early comparative studies revealing lower heritabilities of ®tness than non®tness traits were ®rst framed in terms of giving empirical support for this prediction, but subsequent treatments have eectively reversed this view. Fitness traits seem to have higher levels of additive genetic variance than non®tness traits Ð an observation that has been explained in terms of the larger number loci in¯uencing ®tness as compared to non®tness traits. This hypothesis about the larger functional architecture of ®tness than non®tness traits is supported by their higher mutational variability, which is hard to reconcile without evoking capture of mutational variability over many loci. The lower heritabilities of ®tness than non®tness traits, despite the higher additive genetic variance of the former, occur because of their higher residual variances. Recent comparative studies of dominance contributions for dierent types of traits, together with theoretical predictions and a large body of indirect evidence, suggest an important role of dominance variance in determining levels of residual variance for ®tness-traits. The role of epistasis should not be discounted either, since a large number of loci increases the potential for epistatic interactions, and epistasis is strongly implicated in hybrid breakdown.
Evolution, 2003
Plant traits that show little variation across higher taxa are often used as diagnostic traits, but the reason for the stasis of such traits remains unclear. Wild radish, Raphanus raphanistrum, exhibits tetradynamous stamens (four long and two short, producing a dimorphism in anther height within each flower), as do the vast majority of the more than 3000 species in the Brassicaceae. Here we examine the hypothesis that selection maintains the stasis of dimorphic anther height by investigating the effects of this trait on pollen removal, seed siring success, and seed set in R. raphanistrum using both experimental and observational methods. Observational selection gradient analysis based on lifetime seed siring success provided evidence for an optimum dimorphism that was greater than zero in one of three years. In both experimentally manipulated and unmanipulated flowers, more pollen was removed in single visits from flowers with less dimorphism. There was no significant effect of anther dimorphism on female fitness (seed set). Therefore, there is some evidence to suggest that selection is maintaining anther dimorphism in wild radish, and that higher male fitness might result from restriction of single-visit pollen removal. We discuss these results in light of pollen presentation theory.
The Effects of Intraspecific Competition and Stabilizing Selection on a Polygenic Trait
Genetics, 2004
The equilibrium properties of an additive multilocus model of a quantitative trait under frequency-and density-dependent selection are investigated. Two opposing evolutionary forces are assumed to act: (i) stabilizing selection on the trait, which favors genotypes with an intermediate phenotype, and (ii) intraspecific competition mediated by that trait, which favors genotypes whose effect on the trait deviates most from that of the prevailing genotypes. Accordingly, fitnesses of genotypes have a frequency-independent component describing stabilizing selection and a frequency-and density-dependent component modeling competition. We study how the equilibrium structure, in particular, number, degree of polymorphism, and genetic variance of stable equilibria, is affected by the strength of frequency dependence, and what role the number of loci, the amount of recombination, and the demographic parameters play. To this end, we employ a statistical and numerical approach, complemented by analytical results, and explore how the equilibrium properties averaged over a large number of genetic systems with a given number of loci and average amount of recombination depend on the ecological and demographic parameters. We identify two parameter regions with a transitory region in between, in which the equilibrium properties of genetic systems are distinctively different. These regions depend on the strength of frequency dependence relative to pure stabilizing selection and on the demographic parameters, but not on the number of loci or the amount of recombination. We further study the shape of the fitness function observed at equilibrium and the extent to which the dynamics in this model are adaptive, and we present examples of equilibrium distributions of genotypic values under strong frequency dependence. Consequences for the maintenance of genetic variation, the detection of disruptive selection, and models of sympatric speciation are discussed.
The Action of Purifying Selection, Mutation and Drift on Fitness Epistatic Systems
Genetics, 2009
For different fitness mutational models, with epistasis introduced, we simulated the consequences of drift (D scenario) or mutation, selection, and drift (MSD scenario) in populations at the MSD balance subsequently subjected to bottlenecks of size N ¼ 2, 10, 50 during 100 generations. No ''conversion'' of nonadditive into additive variance was observed, all components of the fitness genetic variance initially increasing with the inbreeding coefficient F and subsequently decreasing to zero (D) or to an equilibrium value (MSD). In the D scenario, epistasis had no appreciable effect on inbreeding depression and that on the temporal change of variance components was relevant only for high rates of strong epistatic mutation. In parallel, between-line differentiation in mean fitness accelerated with F and that in additive variance reached a maximum at F $ 0.6-0.7, both processes being intensified by strong epistasis. In the MSD scenario, however, the increase in additive variance was smaller, as it was used by selection to purge inbreeding depression (N $ 10), and selection prevented between-line differentiation. Epistasis, either synergistic or antagonistic (this leading to multiple adaptive peaks), had no appreciable effect on MSD results nor, therefore, on the evolutionary rate of fitness change.