It is often assumed that the mutation rate is an evolutionarily optimized property of a taxon. The relevant mutation rate is for mutations that affect fitness, U, but the strength of selection on the mutation rate depends on the average effect of a mutation. Determination of U is complicated by the possibility that mutational effects depend on the particular environmental context in which the organism exists. It has been suggested that the effects of deleterious mutations are typically magnified in stressful environments, but most studies confound genotype with environment, so it is unclear to what extent environmental specificity of mutations is specific to a particular starting genotype. We report a study designed to separate effects of species, genotype, and environment on the degradation of fitness resulting from new mutations. Mutations accumulated for .200 generations at 20°in two strains of two species of nematodes that differ in thermal sensitivity. Caenorhabditis briggsae and C. elegans have similar demography at 20°, but C. elegans suffers markedly reduced fitness at 25°. We find little evidence that mutational properties differ depending on environmental conditions and mutational correlations between environments are close to those expected if effects were identical in both environments.T HE importance of deleterious mutations to the evolutionary process is well appreciated (Morgan 1903;Haldane 1927;Fisher 1930;Sturtevant 1937;Kondrashov 1988), and much effort has been expended in understanding the processes by which new mutations arise and their effects on the phenotype and on fitness (reviewed by Simmons and Crow 1977;Drake et al. 1998;Keightley and Eyre-Walker 1999;Lynch et al. 1999;Houle and Kondrashov 2006). Drake, especially, has emphasized the remarkable consistency of the per-genome mutation rate across very broad taxonomic categories, but has also noted that there is considerable variation within those broad taxa (e.g., Drake et al. 1998). The idea that certain mutational properties vary between related species and even within species is venerable (Sturtevant 1937 and references therein), but there is as yet nothing approaching a comprehensive understanding of the variation in mutational properties-rate, distribution of effects, environmental sensitivity, molecular spectrum-of any species or group of closely related species, with the possible exception of the bacterium Escherichia coli (Matic et al. 1997;Sniegowski et al. 1997;Bjedov et al. 2003).An intriguing but almost completely unsubstantiated possibility (but see Nöthel 1987;Bjedov et al. 2003) is that mutation rates are themselves an evolutionarily optimized property (Fisher 1930;Sturtevant 1937;Kimura 1960Kimura , 1967Leigh 1970Leigh , 1973 Kondrashov 1995a,b;Dawson 1998). Because the vast majority of mutations with observable effects are deleterious, it is generally accepted that direct selection (almost) always favors a reduction in the mutation rate, with an optimal mutation rate of zero, at least in sexual taxa (e.g., Leig...

The brown algae are one of the largest and most important groups of primary producers in benthic coastal marine environments. Despite their biological importance, consensus regarding their taxonomic or evolutionary relationships remains elusive. Our goal was to produce a taxon-rich two-gene (rbcL and LSU rDNA) phylogeny. Key species were sequenced to represent each order and family in the analyses across all 19 orders and ∼40 families, including selected outgroups Schizocladiophyceae and Xanthophyceae. Our results are in sharp contrast to traditional phylogenetic concepts; the Ectocarpales are not an early diverging clade, nor do the Fucales diverge early from other brown algae. Rather, Choristocarpus is sister to the remaining brown algae. Other groups traditionally considered to have primitive features are actually recently diverged lineages, turning traditional phylogenetic concepts upside down. Additionally, our results allow for the assessment, in the broadest context, of many of the historical and more recent taxonomic changes, resulting in several emended groups along with proposals for two new orders (Onslowiales, Nemodermatales) and one new family (Phaeosiphoniellaceae).

Mutational bias is a potentially important agent of evolution, but it is difficult to disentangle the effects of mutation from those of natural selection. Mutation-accumulation experiments, in which mutations are allowed to accumulate at very small population size, thus minimizing the efficiency of natural selection, are the best way to separate the effects of mutation from those of selection. Body size varies greatly among species of nematode in the family rhabditidae; mutational biases are both a potential cause and a consequence of that variation. We report data on the cumulative effects of mutations that affect body size in three species of rhabditid nematode that vary fivefold in adult size. Results are very consistent with previous studies of mutations underlying fitness in the same strains: two strains of Caenorhabditis briggsae decline in body size about twice as fast as two strains of C. elegans, with a concomitant higher point estimate of the genomic mutation rate; the confamilial Oscheius myriophila is intermediate. There is an overall mutational bias, such that mutations reduce size on average, but the bias appears consistent between species. The genetic correlation between mutations that affect size and those underlying fitness is large and positive, on average.T HE importance of mutation to the evolutionary process is universally appreciated by biologists, both in terms of the deleterious effects on fitness (Morgan 1903;Fisher 1930;Haldane 1937;Sturtevant 1937) and as the ultimate source of potentially adaptive genetic variation. It has been recognized for a long time that there is substantial variation in the mutational process at a variety of taxonomic levels, even among genotypes within species (Sturtevant 1937 and references therein;Woodruff et al. 1984;Fry 2004b;Baer et al. 2005;Á vila et al. 2006;Haag-Liautard et al. 2007). The factors responsible for that variation are poorly understood, but there are two classes of potential explanations. First, the mutation rate may be primarily a by-product of intrinsic or extrinsic environmental factors, e.g., temperature, metabolic rate, UV exposure, etc. (Martin and Palumbi 1993;Hebert et al. 2002;Gillooly et al. 2005). Alternatively, the mutation rate may be an evolutionarily optimized property, with either the optimum or the deviation from the optimum varying among taxa (Kimura 1967;Leigh 1973;Kondrashov 1995;Dawson 1998). Elucidating the taxonomic distribution of variation in mutational properties may provide important insights into several disparate areas of evolutionary biology, among them the causes of adaptive radiation (Bjedov et al. 2003;Sikorski and Nevo 2005) and cladogenesis (Shpak 2005), the rate of molecular evolution (Martin and Palumbi 1993;Gillooly et al. 2005), the nature of selection on modifier loci (Kondrashov 1995), the evolution of genetic architecture underlying the phenotype ( Jones et al. 2003), and the evolution of mating system and sexual reproduction (Kondrashov 1988(Kondrashov , 1995Keightley and Otto 2006).Of part...