Phylogenomics, Lineage Diversification Rates, and the Evolution of Diadromy in Clupeiformes (Anchovies, Herrings, Sardines, and Relatives).

Migration independently evolved numerous times in animals, with a myriad of ecological and evolutionary implications. In fishes, perhaps the most extreme form of migration is diadromy, the migration between marine and freshwater environments. Key and longstanding questions are: how many times has diadromy evolved in fishes, how frequently do diadromous clades give rise to non-diadromous species, and does diadromy influence lineage diversification rates? Many diadromous fishes have large geographic ranges with constituent populations that use isolated freshwater habitats. This may limit gene flow among some populations, increasing the likelihood of speciation in diadromous lineages relative to non-diadromous lineages. Alternatively, diadromy may reduce lineage diversification rates if migration is associated with enhanced dispersal capacity that facilitates gene flow within and between populations. Clupeiformes (herrings, sardines, shads and anchovies) is a model clade for testing hypotheses about the evolution of diadromy because it includes an exceptionally high proportion of diadromous species and several independent evolutionary origins of diadromy. However, relationships among major clupeiform lineages remain unresolved and existing phylogenies sparsely sampled diadromous species, limiting the resolution of phylogenetically-informed statistical analyses. We assembled a phylogenomic dataset and used multi-species coalescent and concatenation-based approaches to generate the most comprehensive, highly-resolved clupeiform phylogeny to date, clarifying associations among several major clades and identifying recalcitrant relationships needing further examination. We determined that variation in rates of sequence evolution (heterotachy) and base-composition (non-stationarity) had little impact on our results. Using this phylogeny, we characterized evolutionary patterns of diadromy and tested for differences in lineage diversification rates between diadromous, marine, and freshwater lineages. We identified thirteen transitions to diadromy, all during the Cenozoic Era (ten origins of anadromy, two origins of catadromy, and one origin of amphidromy), and seven losses of diadromy. Two diadromous lineages rapidly generated non-diadromous species, demonstrating that diadromy is not an evolutionary dead-end. We discovered considerably faster transition rates out of diadromy than to diadromy. The largest lineage diversification rate increase in Clupeiformes was associated with a transition to diadromy, but we uncovered little statistical support for categorically faster lineage diversification rates in diadromous versus non-diadromous fishes. We propose that diadromy may increase the potential for accelerated lineage diversification, particularly in species that migrate long distances. However, this potential may only be realized in certain biogeographic contexts, such as when diadromy allows access to ecosystems in which there is limited competition from incumbent species.

Intermittent Search, Not Strict Lévy Flight, Evolves under Relaxed Foraging Distribution Constraints.

AbstractThe survival of an animal depends on its success as a forager, and understanding the adaptations that result in successful foraging strategies is an enduring endeavour of behavioral ecology. Random walks are one of the primary mathematical descriptions of foraging behavior. Power law distributions are often used to model random walks, as they can characterize a wide range of behaviors, including Lévy walks. Empirical evidence indicates the prevalence and efficiency of Lévy walks as a foraging strategy, and theoretical work suggests an evolutionary origin. However, previous evolutionary models have assumed a priori that move lengths are drawn from a power law or other families of distributions. Here, we remove this restriction with a model that allows for the evolution of any distribution. Instead of Lévy walks, our model unfailingly results in the evolution of intermittent search, a random walk composed of two disjoint modes-frequent localized walks and infrequent extensive moves-that consistently outcompeted Lévy walks. We also demonstrate that foraging using intermittent search may resemble a Lévy walk because of interactions with the resources within an environment. These extrinsically generated Lévy-like walks belie an underlying behavior and may explain the prevalence of Lévy walks reported in the literature.

The evolutionary maintenance of Lévy flight foraging.

Lévy flight is a type of random walk that characterizes the behaviour of many natural phenomena studied across a multiplicity of academic disciplines; within biology specifically, the behaviour of fish, birds, insects, mollusks, bacteria, plants, slime molds, t-cells, and human populations. The Lévy flight foraging hypothesis states that because Lévy flights can maximize an organism's search efficiency, natural selection should result in Lévy-like behaviour. Empirical and theoretical research has provided ample evidence of Lévy walks in both extinct and extant species, and its efficiency across models with a diversity of resource distributions. However, no model has addressed the maintenance of Lévy flight foraging through evolutionary processes, and existing models lack ecological breadth. We use numerical simulations, including lineage-based models of evolution with a distribution of move lengths as a variable and heritable trait, to test the Lévy flight foraging hypothesis. We include biological and ecological contexts such as population size, searching costs, lifespan, resource distribution, speed, and consider both energy accumulated at the end of a lifespan and averaged over a lifespan. We demonstrate that selection often results in Lévy-like behaviour, although conditional; smaller populations, longer searches, and low searching costs increase the fitness of Lévy-like behaviour relative to Brownian behaviour. Interestingly, our results also evidence a bet-hedging strategy; Lévy-like behaviour reduces fitness variance, thus maximizing geometric mean fitness over multiple generations.

Delusions of grandeur: Seed count is not a good fitness proxy under individual variation in phenology.

The concept of fitness is central to evolutionary biology, yet it is difficult to define and to measure. In plant biology, fitness is often measured as seed count. However, under an array of circumstances, seed count may be a biased proxy of fitness, for example when individuals vary in allocation to sexual versus asexual reproduction. A more subtle example, but also likely to be important in natural populations, is when interindividual variation in conditions during development results in variation in offspring quality among seed parents. In monocarpic (semelparous) plants, this is expected to result from variation in effective season length experienced among individuals that reach reproductive maturity at different times. Here, we manipulate growing season length to ask whether seed count is an accurate representation of parental fitness in the monocarpic herb Lobelia inflata. Simple seed count suggests a paradoxical fitness advantage under constrained-season length. However, we find that the apparent fitness advantage of a constrained-season length is overridden by low relative per-seed fitness. Furthermore, the fitness deficit in the constrained environment is associated primarily with an accelerating decrease in viability and seedling survival in seeds derived from fruits produced progressively later in the season. In this study, the overall fitness value of a seed under a constrained season is 0.774 of that observed under a long season.

Latitudinal variation in norms of reaction of phenology in the greater duckweed Spirodela polyrhiza.

Variable environments may result in the evolution of adaptive phenotypic plasticity when cues reliably indicate an appropriate phenotype-environment match. Although adaptive plasticity is well established for phenological traits expressed across environments, local differentiation in norms of reaction is less well studied. The switch from the production of regular fronds to overwintering 'turions' in the greater duckweed Spirodela polyrhiza is vital to fitness and is expressed as a norm of reaction induced by falling temperatures associated with the onset of winter. However, the optimal norm of reaction to temperature is expected to differ across latitudes. Here, we test the hypothesis that a gradient in the length and predictability of growing seasons across latitudes results in the evolution of reaction norms characterized by earlier turion production at higher latitudes. We test this by collecting S. polyrhiza from replicate populations across seven latitudes from Ontario to Florida and then assessing differentiation in thermal reaction norms of turion production along a common temperature gradient. As predicted, northern populations produce turions at a lower birth order and earlier; a significant latitude-by-temperature interaction suggests that reaction norm differentiation has occurred. Our results provide evidence of differentiation in reaction norms across latitudes in a phenological trait, and we discuss how the adaptive significance of this plasticity might be further tested.

Understanding Maladaptation by Uniting Ecological and Evolutionary Perspectives.

Evolutionary biologists have long trained their sights on adaptation, focusing on the power of natural selection to produce relative fitness advantages while often ignoring changes in absolute fitness. Ecologists generally have taken a different tack, focusing on changes in abundance and ranges that reflect absolute fitness while often ignoring relative fitness. Uniting these perspectives, we articulate various causes of relative and absolute maladaptation and review numerous examples of their occurrence. This review indicates that maladaptation is reasonably common from both perspectives, yet often in contrasting ways. That is, maladaptation can appear strong from a relative fitness perspective, yet populations can be growing in abundance. Conversely, resident individuals can appear locally adapted (relative to nonresident individuals) yet be declining in abundance. Understanding and interpreting these disconnects between relative and absolute maladaptation, as well as the cases of agreement, is increasingly critical in the face of accelerating human-mediated environmental change. We therefore present a framework for studying maladaptation, focusing in particular on the relationship between absolute and relative fitness, thereby drawing together evolutionary and ecological perspectives. The unification of these ecological and evolutionary perspectives has the potential to bring together previously disjunct research areas while addressing key conceptual issues and specific practical problems.

Molecules and morphology reveal 'new' widespread North American freshwater mussel species (Bivalvia: Unionidae).

In the Family Unionidae, the greatest radiation of freshwater mussels, malacologists have been misled by extreme intraspecific shell variation and conversely interspecific conchological stasis or convergence. We characterized the genetic and morphological diversity of two phenotypes of Lampsilis teres from specimens (n = 108) collected across its distribution using geometric and traditional morphometrics and multilocus molecular phylogenetics to test the hypothesis that phenotypes represent separate species. Results from our morphometric and molecular phylogenetic analyses unanimously indicate that L. teres sensu lato is made up of two divergent, widespread species with overlapping distributions. We describe a new species and provide a revised description of L. teres sensu stricto. We use morphometrics and machine-learning classification algorithms to test if shell morphology alone can be used to discriminate between these species. Classification percentages of 97.02% and 93.86% demonstrate that shell morphology is highly informative for species identification. This study highlights our lack of understanding of species diversity of freshwater mussels and the importance of multiple characters and quantitative approaches to species delimitation.

Phylogenetic analysis of trophic niche evolution reveals a latitudinal herbivory gradient in Clupeoidei (herrings, anchovies, and allies).

Biotic and abiotic forces govern the evolution of trophic niches, which profoundly impact ecological and evolutionary processes and aspects of species biology. Herbivory is a particularly interesting trophic niche because there are theorized trade-offs associated with diets comprised of low quality food that might prevent the evolution of herbivory in certain environments. Herbivory has also been identified as a potential evolutionary "dead-end" that hinders subsequent trophic diversification. For this study we investigated trophic niche evolution in Clupeoidei (anchovies, sardines, herrings, and their relatives) and tested the hypotheses that herbivory is negatively correlated with salinity and latitude using a novel, time-calibrated molecular phylogeny, trophic guilds delimited using diet data and cluster analysis, and standard and phylogenetically-informed statistical methods. We identified eight clupeoid trophic guilds: molluscivore, terrestrial invertivore, phytoplanktivore, macroalgivore, detritivore, piscivore, crustacivore, and zooplanktivore. Standard statistical methods found a significant negative correlation between latitude and the proportion of herbivorous clupeoids (herbivorous clupeoid species/total clupeoid species), but no significant difference in the proportion of herbivorous clupeoids between freshwater and marine environments. Phylogenetic least squares regression did not identify significant negative correlations between latitude and herbivory or salinity and herbivory. In clupeoids there were five evolutionary transitions from non-herbivore to herbivore guilds and no transitions from herbivore to non-herbivore guilds. There were no transitions to zooplanktivore, the most common guild, but it gave rise to all trophic guilds, except algivore, at least once. Transitions to herbivory comprised a significantly greater proportion of diet transitions in tropical and subtropical (<35°) relative to temperate areas (>35°). Our findings suggest cold temperatures may constrain the evolution of herbivory and that herbivory might act as an evolutionary "dead-end" that hinders subsequent trophic diversification, while zooplanktivory acts as an evolutionary "cradle" that facilitates trophic diversification.

Phylogeny and polyploidy: resolving the classification of cyprinine fishes (Teleostei: Cypriniformes).

Cyprininae is the largest subfamily (>1300 species) of the family Cyprinidae and contains more polyploid species (∼400) than any other group of fishes. We examined the phylogenetic relationships of the Cyprininae based on extensive taxon, geographical, and genomic sampling of the taxa, using both mitochondrial and nuclear genes to address the phylogenetic challenges posed by polyploidy. Four datasets were analyzed in this study: two mitochondrial gene datasets (465 and 791 taxa, 5604bp), a mitogenome dataset (85 taxa, 14,771bp), and a cloned nuclear RAG1 dataset (97 taxa, 1497bp). Based on resulting trees, the subfamily Cyprininae was subdivided into 11 tribes: Probarbini (new; Probarbus+Catlocarpio), Labeonini Bleeker, 1859 (Labeo & allies), Torini Karaman, 1971 (Tor, Labeobarbus & allies), Smiliogastrini Bleeker, 1863 (Puntius, Enteromius & allies), Poropuntiini (Poropuntius & allies), Cyprinini Rafinesque, 1815 (Cyprinus & allies), Acrossocheilini (new; Acrossocheilus & allies), Spinibarbini (new; Spinibarbus), Schizothoracini McClelland, 1842 (Schizothorax & allies), Schizopygopsini Mirza, 1991 (Schizopygopsis & allies), and Barbini Bleeker, 1859 (Barbus & allies). Phylogenetic relationships within each tribe were discussed. Two or three distinct RAG1 lineages were identified for each of the following tribes Torini, Cyprinini, Spinibarbini, and Barbini, indicating their hybrid origin. The hexaploid African Labeobarbus & allies and Western Asian Capoeta are likely derived from two independent hybridization events between their respective maternal tetraploid ancestors and Cyprinion.

Does peer use influence adoption of efficient cookstoves? Evidence from a randomized controlled trial in Uganda.

The authors examined the effect of peer usage on consumer demand for efficient cookstoves with a randomized controlled trial in rural Uganda. The authors tested whether the neighbors of buyers who ordered and received a stove are more likely to purchase an efficient cookstove than the neighbors of buyers who ordered but have not yet received a stove. The authors found that neighbors of buyers who have experience with the stove are not detectably more likely to purchase a stove than neighbors of buyers who have not yet received their stove. The authors found evidence of peer effects in opinions about efficient cookstoves. Knowing that a prominent member of the community has the efficient stove predicts 17-22 percentage points higher odds of strongly favoring the stove. However, this more favorable opinion seemingly has no effect on purchase decisions.

The continuum between semelparity and iteroparity: plastic expression of parity in response to season length manipulation in Lobelia inflata.

BACKGROUND: Semelparity and iteroparity are considered to be distinct and alternative life-history strategies, where semelparity is characterized by a single, fatal reproductive episode, and iteroparity by repeated reproduction throughout life. However, semelparous organisms do not reproduce instantaneously; typically reproduction occurs over an extended time period. If variation in reproductive allocation exists within such a prolonged reproductive episode, semelparity may be considered iteroparity over a shorter time scale.This continuity hypothesis predicts that "semelparous" organisms with relatively low probability of survival after age at first reproduction will exhibit more extreme semelparity than those with high probability of adult survival. This contrasts with the conception of semelparity as a distinct reproductive strategy expressing a discrete, single, bout of reproduction, where reproductive phenotype is expected to be relatively invariant. Here, we manipulate expected season length--and thus expected adult survival--to ask whether Lobelia inflata, a classic "semelparous" plant, exhibits plasticity along a semelparous-iteroparous continuum. RESULTS: Groups of replicated genotypes were manipulated to initiate reproduction at different points in the growing season in each of three years. In lab and field populations alike, the norm of reaction in parity across a season was as predicted by the continuity hypothesis: as individuals bolted later, they showed shorter time to, and smaller size at first reproduction, and multiplied their reproductive organs through branching, thus producing offspring more simultaneously. CONCLUSIONS: This work demonstrates that reproductive effort occurs along a semelparous-iteroparous continuum within a "semelparous" organism, and that variation in parity occurs within populations as a result of phenotypic plasticity.

Experimental evolution of bet hedging under manipulated environmental uncertainty in Neurospora crassa.

All organisms are faced with environmental uncertainty. Bet-hedging theory expects unpredictable selection to result in the evolution of traits that maximize the geometric-mean fitness even though such traits appear to be detrimental over the shorter term. Despite the centrality of fitness measures to evolutionary analysis, no direct test of the geometric-mean fitness principle exists. Here, we directly distinguish between predictions of competing fitness maximization principles by testing Cohen's 1966 classic bet-hedging model using the fungus Neurospora crassa. The simple prediction is that propagule dormancy will evolve in proportion to the frequency of 'bad' years, whereas the prediction of the alternative arithmetic-mean principle is the evolution of zero dormancy as long as the expectation of a bad year is less than 0.5. Ascospore dormancy fraction in N. crassa was allowed to evolve under five experimental selection regimes that differed in the frequency of unpredictable 'bad years'. Results were consistent with bet-hedging theory: final dormancy fraction in 12 genetic lineages across 88 independently evolving samples was proportional to the frequency of bad years, and evolved both upwards and downwards as predicted from a range of starting dormancy fractions. These findings suggest that selection results in adaptation to variable rather than to expected environments.

Cryptic speciation reversal in the Etheostoma zonale (Teleostei: Percidae) species group, with an examination of the effect of recombination and introgression on species tree inference.

Mitochondrial and nuclear introgression among closely related taxa can greatly complicate the process of determining their phylogenetic relationships. In the Central Highlands of North America, many fish taxa have undergone introgression; in this study, we demonstrate the existence of an unusual introgression event in the Etheostoma zonale species group. We used one mitochondrial and seven nuclear loci to determine the relationships of the taxa within the E. zonale group, and their degree of differentiation. We found evidence of multiple divergent populations within each species; much of the divergence within species has taken place during the Pleistocene. We also found evidence of a previously unknown cryptic species in the Upper Tennessee River which diverged from the remainder of the group during the Pliocene, and has undergone mitochondrial and nuclear introgression with E. zonale, in an apparent process of speciation reversal. We examined the effects that using varying types of recombination tests to eliminate the signal of recombination from nuclear loci would have on the phylogenetic placement of this introgressed lineage in our species tree analyses.

A multilocus molecular phylogeny of combtooth blennies (Percomorpha: Blennioidei: Blenniidae): multiple invasions of intertidal habitats.

The combtooth blennies (f. Blenniidae) is a diverse family of primarily marine fishes with approximately 387 species that inhabit subtidal, intertidal, supralittoral habitats in tropical and warm temperate regions throughout the world. The Blenniidae has typically been divided into six groups based on morphological characters: Blenniini, Nemophini, Omobranchini, Phenablenniini, Parablenniini, and Salariini. There is, however, considerable debate over the validity of these groups and their relationships. Since little is known about the relationships in this group, other aspects of their evolutionary history, such as habitat evolution and remain unexplored. Herein, we use Bayesian and maximum likelihood analyses of four nuclear loci (ENC1, myh6, ptr, and tbr1) from 102 species, representing 41 genera, to resolve the phylogeny of the Blenniidae, determine the validity of the previously recognized groupings, and explore the evolution of habitat association using ancestral state reconstruction. Bayesian and maximum likelihood analyses of the resulting 3100bp of DNA sequence produced nearly identical topologies, and identified many well-supported clades. Of these clades, Nemophini was the only traditionally recognized group strongly supported as monophyletic. This highly resolved and thoroughly sampled blenniid phylogeny provides strong evidence that the traditional rank-based classification does not adequately delimit monophyletic groups with the Blenniidae. This phylogeny redefines the taxonomy of the group and supports the use of 13 unranked clades for the classification of blenniids. Ancestral state reconstructions identified four independent invasions of intertidal habitats within the Blenniidae, and subsequent invasions into supralittoral and freshwater habitats from these groups. The independent invasions of intertidal habitats are likely to have played an important role in the evolutionary history of blennies.

Molecular systematics and historical biogeography of the Nocomis biguttatus species group (Teleostei: Cyprinidae): nuclear and mitochondrial introgression and a cryptic Ozark species.

The Nocomis biguttatus species group ranges widely across North America from the Red River in Oklahoma and Arkansas north to Minnesota and east-west from Wyoming to Ontario. The group includes three traditionally recognized allopatric species: the wide-ranging N. biguttatus and two geographically more restricted species, N. asper from the western Ozarks (Arkansas River system) and two disjunct locations in the Red River system, and N. effusus from the Green, Cumberland, and lower Tennessee rivers. Separate analyses of the mitochondrial cytb gene and two nuclear genes (S7 intron 1 and a portion of the gene for growth hormone, GH), each resolved a cryptic species previously treated as N. biguttatus from the southern Ozarks (White River). Relationships among the four species were unresolved because of conflicts between cytb and S7 and a lack of resolution for GH. A previously indicated N. biguttatus-N. effusus sister-relationship appears to reflect past hybridization and mtDNA capture by N. effusus. Nocomis biguttatus includes four primary cytb clades with unresolved inter-relationships. A Northern Ozarks-Great Plains-Upper Midwest Clade and an Ohio River-Eastern Great Lakes Clade presumably represent late Quaternary dispersal from glacial refugia in, respectively, the northern Ozarks and an unglaciated portion of the Ohio River system. Other clades include one from the Meramec River and a Black River-St. Francis River Clade. There was evidence in N. effusus for a phylogeographic break between the lower Tennessee River and the Green-Cumberland basins. Geographic structure is weak in N. asper, indicating relatively recent contact between now disjunct populations in the Arkansas and Red river basins. The Blue River population of N. asper appears to reflect late Pleistocene or Holocene hybridization and genetic swamping of a resident native population of N. biguttatus by an invading population of N. asper. This postulates past occurrence of N. biguttatus far south of its present range.

Changing reproductive effort within a semelparous reproductive episode.

UNLABELLED: • PREMISE OF THE STUDY: Life-history theory predicts a trade-off between current and future reproduction for iteroparous organisms-as individuals age, the expected value of future reproduction declines, and thus reproductive effort is expected to be higher in later clutches than in earlier. In contrast, models explaining the evolution of semelparity treat semelparous reproduction as instantaneous, with no scope for intraindividual variation. However, semelparous reproduction is also extended, but over shorter time scales; whether there are similar age- or stage-specific changes in reproductive effort within a semelparous episode is unclear. In this study, we assessed whether semelparous individuals increase reproductive effort as residual reproductive value declines by comparing the reproductive phenotype of flowers at five different floral positions along a main inflorescence.• METHODS: Using the herbaceous monocarp Lobelia inflata, we conducted a longitudinal study of 409 individuals including both laboratory and field populations over three seasons. We recorded six reproductive traits-including the length of three phenological intervals as well as fruit size, seed size, and seed number-for all plants across floral positions produced throughout the reproductive episode.• KEY RESULTS: We found that while the rate of flower initiation did not change, flowers at distal (late) floral positions developed more quickly and contained larger seed than flowers at basal (early) floral positions did.• CONCLUSIONS: Our results were consistent with the hypothesis that, like iteroparous organisms, L. inflata increases reproductive effort in response to low residual reproductive value.

Secondary reproduction in the herbaceous monocarp Lobelia inflata: time-constrained primary reproduction does not result in increased deferral of reproductive effort.

BACKGROUND: Although semelparity is a life history characterized by a single reproductive episode within a single reproductive season, some semelparous organisms facultatively express a second bout of reproduction, either in a subsequent season ("facultative iteroparity") or later within the same season as the primary bout ("secondary reproduction"). Secondary reproduction has been explained as the adaptive deferral of reproductive potential under circumstances in which some fraction of reproductive success would otherwise have been lost (due, for example, to inopportune timing). This deferral hypothesis predicts a positive relationship between constraints on primary reproduction and expression of secondary reproduction. The herbaceous monocarp Lobelia inflata has been observed occasionally to express a secondary reproductive episode in the field. However, it is unknown whether secondary reproduction is an example of adaptive reproductive deferral, or is more parsimoniously explained as the vestigial expression of iteroparity after a recent transition to semelparity. Here, we experimentally manipulate effective season length in each of three years to test whether secondary reproduction is a form of adaptive plasticity consistent with the deferral hypothesis. RESULTS: Our results were found to be inconsistent with the adaptive deferral explanation: first, plants whose primary reproduction was time-constrained exhibited decreased (not increased) allocation to subsequent secondary reproduction, a result that was consistent across all three years; second, secondary offspring-although viable in the laboratory-would not have the opportunity for expression under field conditions, and would thus not contribute to reproductive success. CONCLUSIONS: Although alternative adaptive explanations for secondary reproduction cannot be precluded, we conclude that the characteristics of secondary reproduction found in L. inflata are consistent with predictions of incomplete or transitional evolution to annual semelparity.

Taxonomic revision of the Pirate Perches, Aphredoderus, (Percopsiformes: Aphredoderidae) with descriptions of two new species.

Pirate Perches, Aphredoderus, are a widespread lowland freshwater fish native to the Eastern half of the United States. Aphredoderus was thought to contain a single species divided into an Eastern and Western subspecies on either side of the Appalachian Mountains with a widespread intergrade zone through much of the Eastern Gulf of Mexico and Southern Atlantic drainages. We use morphology and genetic data from specimens spanning the entire range of the genus to determine species limits within Aphredoderus. We find evidence of five species, four of which exhibit widespread sympatry in the Southeastern United States. We elevate A. sayanus sayanus and A. sayanus gibbosus to species, redescribe A. mesotrema, supplement previous descriptions, and describe two new species, A. retrodorsalis, and A. ornatus.

Riverine flow rate drives widespread convergence in the shell morphology of imperiled freshwater mussels.

Frequent and strong morphological convergence suggests that determinism tends to supersede historical contingencies in evolutionary radiations. For many lineages living within the water column of rivers and streams, hydrodynamic forces drive widespread morphological convergence. Living below the sediment-water interface may release organisms from these hydrodynamic pressures, permitting a broad array of morphologies, and thus less convergence. However, we show here that the semi-infaunal freshwater mussels have environmentally determined convergence in shell morphology. Using 3D morphometric data from 715 individuals among 164 Nearctic species, we find that species occurring in rivers with high flow rates have evolved traits that resist dislodgement from their burrowed position in the streambed: thicker shells for their body size, with the thickest sector of the shell being the most deeply buried. Species occurring in low flow environments have evolved thinner and more uniformly thickened shells, corresponding to an alternative adaptation to dislodgement: increased burrowing efficiency. Within species, individuals also show increased shell thickness for their body size at higher flow rates, suggesting that ecophenotypy may, in part, be an important mechanism for establishing populations in new environments and thus evolutionary divergence in this highly imperiledinvertebrate group.

Explosive diversification following a benthic to pelagic shift in freshwater fishes.

BACKGROUND: Interspecific divergence along a benthic to pelagic habitat axis is ubiquitous in freshwater fishes inhabiting lentic environments. In this study, we examined the influence of this habitat axis on the macroevolution of a diverse, lotic radiation using mtDNA and nDNA phylogenies for eastern North America's most species-rich freshwater fish clade, the open posterior myodome (OPM) cyprinids. We used ancestral state reconstruction to identify the earliest benthic to pelagic transition in this group and generated fossil-calibrated estimates of when this shift occurred. This transition could have represented evolution into a novel adaptive zone, and therefore, we tested for a period of accelerated lineage accumulation after this historical habitat shift. RESULTS: Ancestral state reconstructions inferred a similar and concordant region of our mtDNA and nDNA based gene trees as representing the shift from benthic to pelagic habitats in the OPM clade. Two independent tests conducted on each gene tree suggested an increased diversification rate after this inferred habitat transition. Furthermore, lineage through time analyses indicated rapid early cladogenesis in the clade arising after the benthic to pelagic shift. CONCLUSIONS: A burst of diversification followed the earliest benthic to pelagic transition during the radiation of OPM cyprinids in eastern North America. As such, the benthic/pelagic habitat axis has likely influenced the generation of biodiversity across disparate freshwater ecosystems.