Genome size correlates with reproductive fitness in seed beetles

The ultimate cause of genome size (GS) evolution in eukaryotes remains a major and unresolved puzzle in evolutionary biology. Large-scale comparative studies have failed to find consistent correlations between GS and organismal properties, resulting in the ‘C-value paradox’. Current hypotheses for the evolution of GS are based either on the balance between mutational events and drift or on natural selection acting upon standing genetic variation in GS. It is, however, currently very difficult to evaluate the role of selection because within-species studies that relate variation in life-history traits to variation in GS are very rare. Here, we report phylogenetic comparative analyses of GS evolution in seed beetles at two distinct taxonomic scales, which combines replicated estimation of GS with experimental assays of life-history traits and reproductive fitness. GS showed rapid and bidirectional evolution across species, but did not show correlated evolution with any of several indices of the relative importance of genetic drift. Within a single species, GS varied by 4–5% across populations and showed positive correlated evolution with independent estimates of male and female reproductive fitness. Collectively, the phylogenetic pattern of GS diversification across and within species in conjunction with the pattern of correlated evolution between GS and fitness provide novel support for the tenet that natural selection plays a key role in shaping GS evolution.     

Exploring among-site rate variation models in a maximum likelihood framework using empirical data: effects of model assumptions on estimates of topology,branch lengths,and bootstrap support

We have investigated the effects of different among-site rate variation models on the estimation of substitution model parameters, branch lengths, topology, and bootstrap proportions under minimum evolution (ME) and maximum likelihood (ML). Specifically, we examined equal rates, invariable sites, gamma-distributed rates, and site-specific rates (SSR) models, using mitochondrial DNA sequence data from three protein-coding genes and one tRNA gene from species of the New Zealand cicada genus Maoricicada. Estimates of topology were relatively insensitive to the substitution model used; however, estimates of bootstrap support, branch lengths, and R-matrices (underlying relative substitution rate matrix) were strongly influenced by the assumptions of the substitution model. We identified one situation where ME and ML tree building became inaccurate when implemented with an inappropriate among-site rate variation model. Despite the fact the SSR models often have a better fit to the data than do invariable sites and gamma rates models, SSR models have some serious weaknesses. First, SSR rate parameters are not comparable across data sets, unlike the proportion of invariable sites or the alpha shape parameter of the gamma distribution. Second, the extreme among-site rate variation within codon positions is problematic for SSR models, which explicitly assume rate homogeneity within each rate class. Third, the SSR models appear to give severe underestimates of R-matrices and branch lengths relative to invariable sites and gamma rates models in this example. We recommend performing phylogenetic analyses under a range of substitution models to test the effects of model assumptions not only on estimates of topology but also on estimates of branch length and nodal support.     

Epistasis Can Facilitate the Evolution of Reproductive Isolation by Peak Shifts: A Two-Locus Two-Allele Model

The influence of epistasis on the evolution of reproductive isolation by peak shifts is studied in a two-locus two-allele model of a quantitative genetic character under stabilizing selection. Epistasis is introduced by a simple multiplicative term in the function that maps gene effects onto genotypic values. In the model with only additive effects on the trait, the probability of a peak shift and the amount of reproductive isolation are always inversely related, i.e., the higher the peak shift rate, the lower the amount of reproductive isolation caused by the peak shift. With epistatic characters there is no consistent relationship between these two values. Interestingly, there are causes where both transition rates as well as the amount of reproductive isolation are increased relative to the additive model. This effect has two main causes: a shift in the location of the transition point, and the hybrids between the two alternative optimal genotypes have lower average fitness in the epistatic case. A review of the empirical literature shows that the fitness relations resulting in higher peak shift rates and more reproductive isolation are qualitatively the same as those observed for genes causing hybrid inferiority.     

Experimental evolution: Assortative mating and sexual selection,independent of local adaptation,lead to reproductive isolation in the nematode Caenorhabditis remanei

Using experimental evolution, we investigated the contributions of ecological divergence, sexual selection, and genetic drift to the evolution of reproductive isolation in Caenorhabditis remanei. The nematodes were reared on two different environments for 100 generations. They were assayed for fitness on both environments after 30, 64, and 100 generations, and hybrid fitness were analyzed after 64 and 100 generations. Mating propensity within and between populations was also analyzed. The design allowed us to determine whether local adaptation was synchronous with pre‐ and postzygotic reproductive isolation. Prezygotic isolation evolved quickly but was unconnected with adaptation to the divergent environments. Instead, prezygotic isolation was driven by mate preferences favoring individuals from the same replicate population. A bottleneck treatment, meant to enhance the opportunity for genetic drift, had no effect on prezygotic isolation. Postzygotic isolation occurred in crosses where at least one population had a large fitness advantage in its “home” environment. Taken together, our results suggest that prezygotic isolation did not depend on drift or adaptation to divergent environments, but instead resulted from differences in sexual interactions within individual replicates. Furthermore, our results suggest that postzygotic isolation can occur between populations even when only one population has greater fitness in its home environment.     

Pleiotropic effects of environment-sensitive genes affecting fitness in relation to postmating reproductive isolation

With regard to speciation in sexually reproducing organisms, some population geneticists continue to argue about the relative merits of sympatry versus allopatry. However, all workers seem quite comfortable with the conventional scenario depicting how reproductive isolation arises between subpopulations in the state of incipient speciation. This view according to which the evolution of reproductive isolation mainly results from some genetic divergence consecutive to a substantial restriction in gene flow is questioned here. A verbal model is described in which gene flow is no longer seen as being first interrupted by a mere physical barrier. The model is based on limited genetic changes at loci influencing fitness but it places two important constraints on the properties of the genetic elements involved in it. One of them is concerned with the environment-sensitivity of the mutations implicated in the process, and the other with their presumed pleiotropic action on a behavioural trait.     

Phylogenetic analysis and intraspecific variation: performance of parsimony,likelihood, and distance methods

Intraspecific variation is abundant in all types of systematic characters but is rarely addressed in simulation studies of phylogenetic method performance. We compared the accuracy of 15 phylogenetic methods using simulations to (1) determine the most accurate method(s) for analyzing polymorphic data (under simplified conditions) and (2) test if generalizations about the performance of phylogenetic methods based on previous simulations of fixed (nonpolymorphic) characters are robust to a very different evolutionary model that explicitly includes intraspecific variation. Simulated data sets consisted of allele frequencies that evolved by genetic drift. The phylogenetic methods included eight parsimony coding methods, continuous maximum likelihood, and three distance methods (UPGMA, neighbor joining, and Fitch-Margoliash) applied to two genetic distance measures (Nei's and the modified Cavalli-Sforza and Edwards chord distance). Two sets of simulations were performed. The first examined the effects of different branch lengths, sample sizes (individuals sampled per species), numbers of characters, and numbers of alleles per locus in the eight-taxon case. The second examined more extensively the effects of branch length in the four-taxon, two-allele case. Overall, the most accurate methods were likelihood, the additive distance methods (neighbor joining and Fitch-Margoliash), and the frequency parsimony method. Despite the use of a very different evolutionary model in the present article, many of the results are similar to those from simulations of fixed characters. Similarities include the presence of the "Felsenstein zone," where methods often fail, which suggests that long-branch attraction may occur among closely related species through genetic drift. Differences between the results of fixed and polymorphic data simulations include the following: (1) UPGMA is as accurate or more accurate than nonfrequency parsimony methods across nearly all combinations of branch lengths, and (2) likelihood and the additive distance methods are not positively misled under any combination of branch lengths tested (even when the assumptions of the methods are violated and few characters are sampled). We found that sample size is an important determinant of accuracy and affects the relative success of methods (i.e., distance and likelihood methods outperform parsimony at small sample sizes). Attempts to generalize about the behavior of phylogenetic methods should consider the extreme examples offered by fixed-mutation models of DNA sequence data and genetic-drift models of allele frequencies.     

GENETIC ARCHITECTURE AND POSTZYGOTIC REPRODUCTIVE ISOLATION: EVOLUTION OF BATESON–DOBZHANSKY–MULLER INCOMPATIBILITIES IN A POLYGENIC MODEL

The Bateson–Dobzhansky–Muller model predicts that postzygotic isolation evolves due to the accumulation of incompatible epistatic interactions, but few studies have quantified the relationship between genetic architecture and patterns of reproductive divergence. We examined how the direction and magnitude of epistatic interactions in a polygenic trait under stabilizing selection influenced the evolution of hybrid incompatibilities. We found that populations evolving independently under stabilizing selection experienced suites of compensatory allelic changes that resulted in genetic divergence between populations despite the maintenance of a stable, high‐fitness phenotype. A small number of loci were then incompatible with multiple alleles in the genetic background of the hybrid and the identity of these incompatibility loci changed over the evolution of the populations. For F₁ hybrids, reduced fitness evolved in a window of intermediate strengths of epistatic interactions, but F₂ and backcross hybrids evolved reduced fitness across weak and moderate strengths of epistasis due to segregation variance. Strong epistatic interactions constrained the allelic divergence of parental populations and prevented the development of reproductive isolation. Because many traits with varying genetic architectures must be under stabilizing selection, our results indicate that polygenetic drift is a plausible hypothesis for the evolution of postzygotic reproductive isolation.     

The importance of the timescale of the fitness metric for estimates of selection on phenotypic traits during a period of demographic change

Although fitness is central to the evolutionary process, metrics vary by timescale. Different timescales may give rise to different estimates of selection, especially during demographic transitions caused by rapid environmental and socioeconomic change. In this study, we used a dataset of a human population in Finland from 1775 to 1950 to compare two fitness metrics and their estimates of selection pressures, before and during a demographic transition. Both metrics, lifetime reproductive success and an annual metric of individual performance, declined while selection on the ages at first and last reproduction remained nearly constant, favouring individuals with wider reproductive windows. The ability to partition the annual metric into contributions from reproduction and survival revealed the short‐term effects of a famine and the reversal of selection pressure via the survival component of annual fitness. Although the metrics generally agreed, the annual metric detected the effects of environmental variation and demographic change occurring within a generation.     

QTL mapping reveals the genetic architecture of loci affecting pre- and post-zygotic isolating barriers in Louisiana Iris

ABSTRACT: BACKGROUND: Hybridization among Louisiana Irises has been well established and the genetic architecture of reproductive isolation is known to affect the potential for and the directionality of introgression between taxa. Here we use co-dominant markers to identify regions where QTL are located both within and between backcross maps to compare the genetic architecture of reproductive isolation and fitness traits across treatments and years. RESULTS: QTL mapping was used to elucidate the genetic architecture of reproductive isolation between Iris fulva and Iris brevicaulis. Homologous co-dominant EST-SSR markers scored in two backcross populations between I. fulva and I. brevicaulis were used to generate genetic linkage maps. These were used as the framework for mapping QTL associated with variation in 11 phenotypic traits likely responsible for reproductive isolation and fitness. QTL were dispersed throughout the genome, with the exception of one region of a single linkage group (LG) where QTL for flowering time, sterility, and fruit production clustered. In most cases, homologous QTL were not identified in both backcross populations, however, homologous QTL for flowering time, number of growth points per rhizome, number of nodes per inflorescence, and number of flowers per node were identified on several linkage groups. CONCLUSION: Two different traits affecting reproductive isolation, flowering time and sterility, exhibit different genetic architectures, with numerous QTL across the Iris genome controlling flowering time and fewer, less distributed QTL affecting sterility. QTL for traits affecting fitness are largely distributed across the genome with occasional overlap, especially on LG 4, where several QTL increasing fitness and decreasing sterility cluster. Given the distribution and effect direction of QTL affecting reproductive isolation and fitness, we have predicted genomic regions where introgression may be more likely to occur (those regions associated with an increase in fitness and unlinked to loci controlling reproductive isolation) and those that are less likely to exhibit introgression (those regions linked to traits decreasing fitness and reproductive isolation).     

Quantifying patterns in the evolution of reproductive isolation

We present a likelihood-based statistical method for examining the pattern or rate of evolution of reproductive isolation. The method uses large empirical datasets to estimate, for a given clade, the average duration of two phases in the divergence of populations. The first phase is a lag phase and refers to the period during which lineages diverge but no detectable reproductive isolation evolves. The second is an accumulation phase, referring to the period during which the magnitude of reproductive isolation between diverging lineages increases. The pattern of evolution is inferred from the relative durations of these two phases. Results of analyses of postzygotic isolation data indicate significant differences among taxa in the pattern of evolution of postzygotic isolation that are consistent with predictions based on genetic differences among these groups. We also examine whether the evolution of postzygotic isolation is best explained by either of two models for the rate of accumulation: a linear model or a quadratic function as may be suggested by recent studies. Our analysis indicates that the appropriateness of either model varies among taxa.     

Patterns of reproductive isolation in Mediterranean deceptive orchids

The evolution of reproductive isolation is of central interest in evolutionary biology. In plants, this is typically achieved by a combination of pre- and postpollination mechanisms that prevent, or limit, the amount of interspecific gene flow. Here, we investigated and compared two ecologically defined groups of Mediterranean orchids that differ in pollination biology and pollinator specificity: sexually deceptive orchids versus food-deceptive orchids. We used experimental crosses to assess the strength of postmating prezygotic, and postzygotic reproductive isolation, and a phylogenetic framework to determine their relative rates of evolution. We found quantitative and qualitative differences between the two groups. Food-deceptive orchids have weak premating isolation but strong postmating isolation, whereas the converse situation characterizes sexually deceptive orchids. Only postzygotic reproductive isolation among food-deceptive orchids was found to evolve in a clock-like manner. Comparison of evolutionary rates, within a common interval of genetic distance, showed that the contribution of postmating barriers was more relevant in the food-deceptive species than in the sexually deceptive species. Asymmetry in prezygotic isolation was found among food-deceptive species. Our results indicate that postmating barriers are most important for reproductive isolation in food-deceptive orchids, whereas premating barriers are most important in sexually deceptive orchids. The different rate of evolution of reproductive isolation and the relative strength of pre- and postmating barriers may have implication for speciation processes in the two orchid groups.     

On the evolution of premating isolation after a founder event

We present a new simple model for the evolution of premating reproductive isolation. Using this model we first analyze the level of genetic variability maintained by mutation in a large stable population. Then we consider the plausibility of the evolution of strong premating reproductive isolation after a founder event. We demonstrate that after a founder event a new adaptive combination of genes may rise to high frequencies in the presence of an old combination of genes. We compare the probabilities of speciation after a founder event with those in a stable population and with those when reproductive isolation is due to viability selection against hybrids. We argue that premating reproductive isolation is more efficient than postmating reproductive isolation in maintaining the integrity of sympatric species. This might have contributed to the pattern of stronger premating isolation than postmating isolation between closely related pairs of sympatric species.     

Mosaicism, modules, and the evolution of birds: results from a Bayesian approach to the study of morphological evolution using discrete character data

The study of morphological evolution after the inferred origin of active flight homologous with that in Aves has historically been characterized by an emphasis on anatomically disjunct, mosaic patterns of change. Relatively few prior studies have used discrete morphological character data in a phylogenetic context to quantitatively investigate morphological evolution or mosaic evolution in particular. One such previously employed method, which used summed unambiguously optimized synapomorphies, has been the basis for proposing disassociated and sequential "modernizing" or "fine-tuning" of pectoral and then pelvic locomotor systems after the origin of flight ("pectoral early-pelvic late" hypothesis). We use one of the most inclusive phylogenetic data sets of basal birds to investigate properties of this method and to consider the application of a Bayesian phylogenetic approach. Bayes factor and statistical comparisons of branch length estimates were used to evaluate support for a mosaic pattern of character change and the specific pectoral early-pelvic late hypothesis. Partitions were defined a priori based on anatomical subregion (e.g., pelvic, pectoral) and were based on those hypothesized using the summed synapomorphy approach. We compare 80 models all implementing the M(k) model for morphological data but varying in the number of anatomical subregion partitions, the models for among-partition rate variation and among-character rate variation, as well as the branch length prior. Statistical analysis reveals that partitioning data by anatomical subregion, independently estimating branch lengths for partitioned data, and use of shared or per partition gamma-shaped among-character rate distribution significantly increases estimated model likelihoods. Simulation studies reveal that partitioned models where characters are randomly assigned perform significantly worse than both the observed model and the single-partition equal-rate model, suggesting that only partitioning by anatomical subregion increases model performance. The preference for models with partitions defined a priori by anatomical subregion is consistent with a disjunctive pattern of character change for the data set investigated and may have implications for parameterization of Bayesian analyses of morphological data more generally. Statistical tests of differences in estimated branch lengths from the pectoral and pelvic partitions do not support the specific pectoral early-pelvic late hypothesis proposed from the summed synapomorphy approach; however, results suggest limited support for some pectoral branch lengths being significantly longer only early at/after the origin of flight.     

Size and architectural traits as ontogenetic determinants of fitness in a phenotypically plastic annual weed (Amaranthus albus)

Abstract Plasticity of size and architectural traits, and their importance to reproductive fitness, were estimated in relation to nutrient availability in the annual Amaranthus albus . Seeds from seven field-collected genotypes were used to rear a first generation under uniform conditions. Seed families (inbred lines) from four first-generation plants per genotype were used to rear a second generation in a glasshouse for 10 weeks. Nine plants per family were regularly fertilized, while nine others were unfertilized. Size was estimated at 5, 8, and 10 weeks; architectural traits were recorded at 8 weeks (no. branches) and 10 weeks (no. branches and branch length). Fitness was assessed by the number of seeds per plant. Because traits were intercorrelated, size-scaled architectural traits were generated and allometric analyses performed. Genetic variation was analyzed for the second generation among inbred lines of the original genotypes, and among families within each genotype. For fertilized and unfertilized groups, early size (volume occupied at 5 weeks) and branch length per mass were significant determinants of fitness. The number of branches per size explained a smaller proportion of the variance in fitness. Genotype by treatment interaction was apparent for some traits, indicating genetic variation for plasticity, but plasticity of size did not change over ontogeny. Significant effects of genotype on size and architectural traits, and fitness, were detected mostly in the unfertilized group. Thus, selection is most likely to differentiate among genotypes in nutrient-poor environments. Lengthening of multiple branches increases seed output throughout the season, and genotypes with longer branches per unit mass have a selective advantage. Because early size is correlated with seed output, ontogenetic plasticity in response to improved soil resources allows opportunistic increases in fitness.     

QUANTITATIVE GENETICS OF SIZE AND PHENOLOGY OF LIFE-HISTORY TRAITS IN CHAMAECRISTA FASCICULATA

Despite numerous adaptive scenarios concerning the evolution of plant life-history phenologies few studies have examined the heritable basis for and genetic correlations among these phenologies. Documentation of genetic variation for and covariation among reproductive phenologies is important because it is this variation/covariation that will determine the potential for response to evolutionary forces. To address this problem, I conducted a breeding experiment to determine narrow-sense heritabilities for and genetic correlations among the phenologies of life-history events and plant size in Chamaecristafasciculata, a temperate summer annual plant species. Paternal families showed no evidence of heritable variation for two estimates of plant size, six measures of reproductive phenology or two fitness components. Similarly, paternal estimates of genetic correlations among these traits were low or zero. In contrast, maternal estimates of heritability suggested the influence of maternal parent on one estimate of plant size and four phenological traits. Likewise, maternal effects influenced maternal estimates of genetic correlations. These maternal effects can arise from three sources: endosperm nuclear, cytoplasmic genetic and/or maternal phenotypic. The degree to which the phenology of one life-history trait acts as a constraint on the evolution of other phenological traits depends on the source of the maternal influence in this species.     

Behavioural reproductive isolation and speciation in Drosophila

The origin of premating reproductive isolation continues to help elucidate the process of speciation and is the central event in the evolution of biological species. Therefore, during the process of species formation the diverging populations must acquire some means of reproductive isolation so that the genes from one gene pool are prevented from dispersing freely into a foreign gene pool. In the genus Drosophila, the phenomenon of behavioural reproductive isolation, which is an important type of premating (prezygotic) reproductive isolating mechanisms, has been extensively studied and interesting data have been documented. In many cases incomplete sexual isolation has been observed and the pattern and degree of isolation within and between the species have often been used to elucidate the phylogenetic relationships. The present review documents an overview of speciation mediated through behavioural incompatibility in different species groups of Drosophila with particular reference to the models proposed on the basis of one-sided ethological isolation to predict the direction of evolution. This study is crucial for understanding the mechanism of speciation through behavioural incompatibility and also for an understanding of speciation genetics in future prospects.     

Frequency-dependent selection and the evolution of assortative mating

A long-standing goal in evolutionary biology is to identify the conditions that promote the evolution of reproductive isolation and speciation. The factors promoting sympatric speciation have been of particular interest, both because it is notoriously difficult to prove empirically and because theoretical models have generated conflicting results, depending on the assumptions made. Here, we analyze the conditions under which selection favors the evolution of assortative mating, thereby reducing gene flow between sympatric groups, using a general model of selection, which allows fitness to be frequency dependent. Our analytical results are based on a two-locus diploid model, with one locus altering the trait under selection and the other locus controlling the strength of assortment (a "one-allele" model). Examining both equilibrium and nonequilibrium scenarios, we demonstrate that whenever heterozygotes are less fit, on average, than homozygotes at the trait locus, indirect selection for assortative mating is generated. While costs of assortative mating hinder the evolution of reproductive isolation, they do not prevent it unless they are sufficiently great. Assortative mating that arises because individuals mate within groups (formed in time or space) is most conducive to the evolution of complete assortative mating from random mating. Assortative mating based on female preferences is more restrictive, because the resulting sexual selection can lead to loss of the trait polymorphism and cause the relative fitness of heterozygotes to rise above homozygotes, eliminating the force favoring assortment. When assortative mating is already prevalent, however, sexual selection can itself cause low heterozygous fitness, promoting the evolution of complete reproductive isolation (akin to "reinforcement") regardless of the form of natural selection.     

Phylogeny of Eastern North American Coreopsis (Asteraceae-Coreopsideae): insights from nuclear and plastid sequences, and comments on character evolution

A molecular phylogenetic study of eastern North American Coreopsis and representatives of other genera of tribe Coreopsideae was conducted using combined sequences from nuclear ITS and two plastid regions (matK, rpl16). A total of 25-30 species has been recognized in five sections of Coreopsis in eastern North America. Based on morphological characters, these taxa have generally been considered a monophyletic group. Our well-resolved phylogeny supports the monophyly of sections that have been recognized in Coreopsis, but the sections collectively do not comprise a monophyletic group because species of north temperate Bidens occur within one of the two major Coreopsis clades. The most notable departure of present results from prior views of relationships among sections is the lack of a sister group relationship between sections Calliopsis and Eublepharis; the shared presence of four-lobed disk floret corollas had been used to support a close relationship between these two sections. Relationships within sections show both similarities and differences with the results of previous studies based primarily on morphological characters. Mapping of morphological characters used taxonomically in Coreopsis and related genera onto the phylogeny indicates that the evolution of these characters has been complex, and this compromises their value for defining monophyletic groups. Examples include the annual habit, alternate leaves, winged fruits, red or brown basal spots on the yellow ligules, and four-lobed disk floret corollas.     

Estimating the Rate of Adaptive Molecular Evolution When the Evolutionary Divergence Between Species is Small

We investigate the extent by which the estimates of the rate of adaptive molecular evolution obtained by extending the McDonald-Kreitman test are biased if the species, subjected to analysis, diverged recently. We show that estimates can be biased if the nucleotide divergence between the species is low relative to within species variation, and that the magnitude of the bias depends on the rate of adaptive evolution and the distribution of fitness effects of new mutations. Bias appears to be because of three factors: (1) misattribution of polymorphism to divergence; (2) the contribution of ancestral polymorphism to divergence; and (3) different rates of fixation of neutral and advantageous mutations. If there is little adaptive molecular evolution, then slightly deleterious mutations inflate estimates of the rate of adaptive evolution, because these contribute proportionately more to polymorphism than to nucleotide divergence than neutral mutations. However, if there is substantial adaptive evolution, polymorphism contributing to apparent divergence may downwardly bias estimates. We propose a simple method for correcting the different contributions of slightly deleterious and neutral mutations to polymorphism and divergence, and apply it to datasets from several species. We find that estimates of the rate of adaptive molecular evolution from closely related species may be underestimates by ~10% or more. However, after the contribution of polymorphism to divergence is removed, the rate of adaptive evolution may still be overestimated as a consequence of ancestral polymorphism and time for fixation effects. This bias may be substantial if branch lengths are less than 10N (e) generations.     

SEXUAL SPECIES ARE SEPARATED BY LARGER GENETIC GAPS THAN ASEXUAL SPECIES IN ROTIFERS

Why organisms diversify into discrete species instead of showing a continuum of genotypic and phenotypic forms is an important yet rarely studied question in speciation biology. Does species discreteness come from adaptation to fill discrete niches or from interspecific gaps generated by reproductive isolation? We investigate the importance of reproductive isolation by comparing genetic discreteness, in terms of intra‐ and interspecific variation, between facultatively sexual monogonont rotifers and obligately asexual bdelloid rotifers. We calculated the age (phylogenetic distance) and average pairwise genetic distance (raw distance) within and among evolutionarily significant units of diversity in six bdelloid clades and seven monogonont clades sampled for 4211 individuals in total. We find that monogonont species are more discrete than bdelloid species with respect to divergence between species but exhibit similar levels of intraspecific variation (species cohesiveness). This pattern arises because bdelloids have diversified into discrete genetic clusters at a faster net rate than monogononts. Although sampling biases or differences in ecology that are independent of sexuality might also affect these patterns, the results are consistent with the hypothesis that bdelloids diversified at a faster rate into less discrete species because their diversification does not depend on the evolution of reproductive isolation.