Evolutionary Trends in the Ornamentation of Cambrian Solenopleuropsine Trilobites

A phylogeny of the Cambrian solenopleuropsine trilobites is constructed to evaluate the importance of differential speciation, extinction and developmental constraints in the evolutionary history of the clade. The transformation of the Pardailhania–Solenopleuropsis ( Manublesia )– Solenopleuropsis ( Solenopleuropsis ) lineages occurred during the mid Caesaraugustian–early Languedocian interval, with all measured morphological variables exhibiting continuous and gradual changes. Several evolutionary trends show tendencies towards an increase in tuberculation and its complexity on the anterior part of the cephalon. A mid Caesaraugustian anagenetic tendency is related to the influence of constant environmental conditions, whereas a late Caesaraugustian–early Languedocian cladogenetic tendency seems to have been an adaptative response to unstable environments. The Solenopleuropsinae clade has a well-preserved continuous fossil record over its full biogeographical range in the Mediterranean region. The sympatric evolution in centrally located populations (Mediterranean area) follows the model of phyletic gradualism, whilst a peripheral population in Avalonia gave rise (by allopatric speciation) to a distinct species, characterized by a variation in the relative size of the palpebral lobes, which may be evidence of a parallel evolution in the trends of ornamentation.     

The importance of size-frequency relationships for predicting ecological impact of zebra mussel populations

Although most physiological processes of bivalves are highly size-dependent in a non-linear manner, often only total densities of populations of freshwater bivalves such as the zebra mussel are reported rather than size-frequency information. This can cause serious errors when trying to predict or assess the environmental impacts of these filter feeders on planktonic communities or the role of their pseudofeces in transferring materials from the plankton to the benthos. We used a bioenergetics model to examine the effect that differing size-frequency distribution has on influencing total phytoplankton consumption and pseudofeces production. We constructed different size-frequency distributions of 1000 zebra mussels with the same mean length or same mean body mass for comparison. In addition, we used several size-frequency distributions from the published literature. The size-frequency distribution of a population had a tremendous impact on both total consumption and pseudofeces production with rates varying by more than an order of magnitude (43.5 g consumption by 1000 smaller mussels to 654 g for a population dominated by large mussels). These data emphasize the importance of knowing not only population density, but population size structure in order to accurately understand and predict the impacts of zebra mussels, or any filter feeder on pelagic and benthic communities. This work also demonstrates the usefulness of a tool such as our bioenergetics model for partitioning the relative impacts of densities and size on a variety of factors such as consumption and pseudofeces production.     

The life history legacy of evolutionary body size change in carnivores

We estimate the body sizes of direct ancestors of extant carnivores, and examine selected aspects of life history as a function not only of species' current size, but also of recent changes in size. Carnivore species that have undergone marked recent evolutionary size change show life history characteristics typically associated with species closer to the ancestral body size. Thus, phyletic giants tend to mature earlier and have larger litters of smaller offspring at shorter intervals than do species of the same body size that are not phyletic giants. Phyletic dwarfs, by contrast, have slower life histories than nondwarf species of the same body size. We discuss two possible mechanisms for the legacy of recent size change: lag (in which life history variables cannot evolve as quickly as body size, leading to species having the 'wrong' life history for their body size) and body size optimization (in which life history and hence body size evolve in response to changes in energy availability); at present, we cannot distinguish between these alternatives. Our finding that recent body size changes help explain residual variation around life history allometries shows that a more dynamic view of character change enables comparative studies to make more precise predictions about species traits in the context of their evolutionary background.     

Within‐generation dynamics of leatherjackets (Tipula paludosa Meig.)

Larvae of Tipula paludosa (leatherjackets) are pests of grassland and cereals, and important prey items for a range of farmland birds. Their population dynamics from year to year have been the subject of study, but within‐generation dynamics have received much less attention. The duration and occurrence of instars and size of the population is important to both pest management and bird conservation. Leatherjackets were collected monthly from four grass fields over five generations (October to September). Count data were analysed for sources of variability and used to estimate the timing and duration of instars and population changes over a single generation. Instar durations were correlated with temperature. From the first to third instars, sample counts were strongly influenced by the location of the site and the generation. Fourth instar larvae showed a similar decline in numbers across all sites and generations. The duration and timing of first to third instars was more variable than that of the fourth instar, but there was limited evidence that this was influenced by temperature. Only the durations of the third and fourth instars were correlated. The population decline from April to September coincided with the onset of the fourth instar. Our results show that the factors influencing leatherjacket numbers change over the life cycle, but by the fourth instar, these are biotically driven. The predictability of its numerical decline indicates defined windows for crop damage and availability as prey. The stability of its timing suggests that day length may synchronize the annual cycle. Fixed timing of the fourth instar will have important consequences for the timing of crop damage and the conservation of farmland birds as climate changes.     

Population structure, growth and distribution of Lingula anatina (Brachiopoda) in Queensland, Australia

Size-frequency data collected in large samples from five populations of Lingula anatina from North Queensland intertidal sandflats conformed to previously known brachiopod size-frequency distributions. Probability paper analysis of the samples and constituent sub-samples confirmed that all the distributions had a hidden poly-modality resulting from a long but defined breeding season. Two patterns of annual recruitment were revealed; the first is the periodic failure of the populations to gain recruits and the second is one of patchy distribution of settlement cohorts. These patterns introduce sampling bias into the derivation of lingulid size frequency curves unless compensated by more extensive sampling methods than have previously been considered adequate. The interpretation of year classes has enabled a growth curve to be derived. The populations inhabited predominantly sandy sub-strates and displayed aggregated distributions. Littoral distribution of the species is regarded as optimal rather than marginal.     

Latitudinal variation in the shape of the species body size distribution: an analysis using freshwater fishes

Many taxonomic and ecological assemblages of species exhibit a right-skewed body size-frequency distribution when characterized at a regional scale. Although this distribution has been frequently described, factors influencing geographic variation in the distribution are not well understood, nor are mechanisms responsible for distribution shape. In this study, variation in the species body size-frequency distributions of 344 regional communities of North American freshwater fishes is examined in relation to latitude, species richness, and taxonomic composition. Although the distribution of all species of North American fishes is right-skewed, a negative correlation exists between latitude and regional community size distribution skewness, with size distributions becoming left-skewed at high latitudes. This relationship is not an artifact of the confounding relationship between latitude and species richness in North American fishes. The negative correlation between latitude and regional community size distribution skewness is partially due to the geographic distribution of families of fishes and apparently enhanced by a nonrandom geographic distribution of species within families. These results are discussed in the context of previous explanations of factors responsible for the generation of species size-frequency distributions related to the fractal nature of the environment, energetics, and evolutionary patterns of body size in North American fishes.     

Evidence for positive density-dependent effects in recovering Diadema antillarum populations

Recovering populations may experience positive density-dependent feedbacks that contribute to population increases. Diadema antillarum, a keystone herbivore on Caribbean coral reefs, suffered a well-documented mass mortality in 1983-84. High densities of adults of this long-spined urchin could provide effective refuge from predation for juveniles under a spine canopy, as has been suggested for other urchin species. We evaluated the effect of adult density on juvenile persistence of D. antillarum experimentally, and examined size-frequency distributions of recovering local populations for evidence of positively density-dependent juvenile persistence at St. Croix, U.S. Virgin Islands. Juvenile persistence was significantly higher in high adult density treatments, and bimodal population size distributions also suggest potential positive effects of adult density on juveniles. This positive feedback could accelerate the recovery of this important coral reef grazer.     

SPECIES RECOGNITION AND PATTERNS OF POPULATION VARIATION IN THE REPRODUCTIVE STRUCTURES OF A DAMSELFLY GENUS

The selection pressures imposed by mate choice for species identity should impose strong stabilizing selection on traits that confer species identity to mates. Thus, we expect that such traits should show nonoverlapping distributions among closely related species, but show little to no variance among populations within a species. We tested these predictions by comparing levels of population differentiation in the sizes and shapes of male cerci (i.e., the clasper structures used for species identity during mating) of six Enallagma damselfly species. Cerci shapes were nonoverlapping among Enallagma species, and five of six Enallagma species showed no population variation across their entire species ranges. In contrast, cerci sizes overlapped among species and varied substantially among populations within species. These results, taken with previous studies, suggest that cerci shape is a primary feature used in species recognition used to discriminate conspecific from heterospecifics during mating.     

The Cambrian radiation of bilaterians: Evolutionary origins and palaeontological emergence; earth history change and biotic factors

Evidence from a variety of research areas, including phylogenetic palaeobiogeographic studies of trilobites, indicates that there may be a fuse to the Cambrian radiation, with a duration on the order of 20–70 myr. Evolution in trilobites appears to have been powerfully influenced by the tectonic changes occurring at the end of the Neoproterozoic: especially the breakup of Pannotia. This continental fragmentation may have also elevated opportunities for vicariance and speciation in trilobites, and other metazoans, given that speciation rates at this time period were high, though not phenomenally so. This provides clear evidence that abiotic factors played an important role in motivating evolution during this key episode in the history of life; biotic factors probably also played a role. The evidence for the role of biotic factors is considered in light of information from some problematic Cambrian taxa. These may show affinities with modern problematic pseudocoelomate phyla, although Cambrian and modern exponents differ dramatically in body size.     

Comparative ontogeny of different species of agnostina

This study is focused on the growth pattern of agnostids, a group of organisms that were very abundant in the Cambrian. The developmental patterns of six agnostid species are compared. It is shown that, like other trilobites, agnostids have two developmental stages, i.e., meraspid degrees 0 and 1 and the holaspid stage. Meraspid degrees are named according to the number of released thoracic segments. Several molts took place during every meraspid stage resulting in successive instars with a unique set of features. Meraspid degree 0 includes two or three instars, the first and last of which have a distinctive morphology and have been encountered in most of the species studied. During meraspid degree 1, the animal molted from two to four times. The number of molt stages appears to be species-specific. The first and last instars are easily distinguished from other developmental forms. Early holaspids of the majority of agnostid species studied also have certain features in common.     

Phenetic and phylogenetic analysis of Reinhardtia (Palmae)

Principal component analysis and cluster analysis of morphometric data divide specimens of Reinhardtia into six groups, corresponding to the six species recognized in the most recent revisions. Discriminant analysis classifies specimens into these six species with 100% success. Five species occur in lowland to montane moist forests in Central America, from Mexico to Panama, and just reach Colombia; one species occurs in montane moist forests in Hispaniola. Three species have large stems and are rare, patchily distributed, and seldom collected. The other three species have small stems, are common and frequently collected, but also patchily distributed. One species of small plants, R. gracilis, exhibits considerable variability. Within this species, seven distinct groups can be recognized, although sample size is limited. Among species, there is a phyletic decrease in size of plants, from the basal species with large stems to derived species with small stems. For leaves and inflorescences there is also an associated decrease in size, but one species does not follow this trend. In this species, R. latisecta, there is evidence of a large ontogenetic change in leaf development. Phyletic decrease in size corresponds to a latitudinal and elevational gradient suggesting speciation has taken place from north to south and from high to low elevation. However, this pattern is obscured disjunct distributions in some species.     

Lateral variations of size-frequency distribution in a fossil echinoid community and their palaeoecological significance

The analysis of size-frequency spatial distribution of three species of Cenomanian echinoids in a single fossil assemblage shows both intraspecific and interspecific horizontal segregation of the individuals parallel to the variations in silt and organic matter levels of the substrate. The exceptional preservation of echinoids. fossilized in the living position with part of their spines. and their multimodal size-frequency distribution, with no evidence of physical transportation. suggest that a catastrophic event resulted in the fossilization of the demographic structure of the populations. The assemblage. being the result of mass mortality of a community, corresponds to a census assemblage. Thus the patchy segregation of echinoids could not primarily be influenced by size-selective taphonomy (current winnowing, biological or mechanical destruction) but more likely determined by biological phcnomena like mortality and recruitment patterns, high juvenile growth, substrate selection and herd mode of life. Recent examples of spatial separation of adults and juveniles. given for different species of echinoids. show that these patchy distributions are widespread, and therefore may be a privileged source of fossil preservation of cohort's size-frequency distribution in catastrophic mortality. □ Palaeoecology, size-frequency distribution, demography, echinoids. spatangoids, cassiduloids, M ecaster , C atopygus , N ucleopygus .     

Formation of the hydrophobic core of ribonuclease A through sequential coordinated conformational transitions

With steady-state and time-resolved fluorescence energy-transfer measurements, we determined the distributions of intramolecular distances in nine mutants to study the conformations of wild-type ribonuclease A in the reduced state under folding conditions. Although far-UV-CD measurements show no evidence for a secondary-structure transition, temperature- and GdnHCl-induced changes in intramolecular distance distributions in the reduced state revealed evidence for long-range subdomain structures in the denatured protein. These poorly defined structures, reflected here by wide distributions corresponding to a wide range of energies, form during refolding in a complex sequence of multiple subdomain transitions. A more well-defined structure emerges only when this structural framework, which directs the successive steps in the folding process, matures and is reinforced by stronger interactions such as disulfide bonds.     

Gene tree distributions under the coalescent process

Under the coalescent model for population divergence, lineage sorting can cause considerable variability in gene trees generated from any given species tree. In this paper, we derive a method for computing the distribution of gene tree topologies given a bifurcating species tree for trees with an arbitrary number of taxa in the case that there is one gene sampled per species. Applications for gene tree distributions include determining exact probabilities of topological equivalence between gene trees and species trees and inferring species trees from multiple datasets. In addition, we examine the shapes of gene tree distributions and their sensitivity to changes in branch lengths, species tree shape, and tree size. The method for computing gene tree distributions is implemented in the computer program COAL.     

The ecology of south-east tasmanian phytal animal communities. IV. Factors affecting the distribution of ampithoid amphipods among algae

Adults of four abundant species of phytal ampithoid amphipod which occur sympatrically at Fancy Point, Tasmania, were found to partition the environment by size and plant species. A series of motility, predation, and algal selection experiments was carried out to help explain the observed distribution and life-history patterns of these animals. The restricted distributions among algae of two of these amphipods were exceptional for animals within the phytal community but followed a general pattern of increasing specialization of amphipods as the number of co-occurring congeneric species increased.

Sampling over a 24-h period indicated that a large proportion of the total amphipod population moved away from Zonaria turneriana J. Ag. at night but that no noticeable changes were evident in the densities of animals collected from Sargassum verruculosum (Mert.) C. Ag. Recolonization experiments, however, showed that some movement of animals occurred between S. verruculosum plants and also, presumably, between the other erect algae.

Selection experiments showed that large Cymadusa sp. 1 preferred Zonaria to the other algae but were forced onto Sargassum bracteolosum J. Ag. and then the more filamentous S. verruculosum and Cystophora retroflexa (Labill.) J. Ag. as crowding increased. Juveniles preferred Cystophora and Sargassum verruculosum to Zonaria and Sargassum bracteolosum.

The omnivorous fish Acanthaluteres spilomelanurus Quoy & Gaimard was found to capture significantly more amphipods from among the open branches of Cystophora than from the more compact Zonaria in the laboratory. The fish were also found to consume more large than small amphipods residing on Cystophora over a 2-h feeding period.

An analysis of the size-frequency distributions of the four amphipod species, in conjunction with the results of the laboratory experiments, provided circumstantial evidence that the ampithoid guild was structured by competitive constraints and differing predation pressures at two size levels.     

Animal body size distributions: patterns, mechanisms and implications

Documenting the shape of the frequency distribution of species body sizes for an animal taxon appears at first sight a straightforward task. However, a variety of patterns has been reported, and a consensus is only now being reached through an understanding of how potential biases may affect observed shapes of distributions. A new body of evidence suggests that, at large scales, size distributions are right-skewed, even on logarithmic axes. If body size distributions can be described with certainty, this will allow assessment of the mechanisms proposed to generate them, and will be an important step towards understanding the structure and dynamics of animal assemblages.     

The Random Nature of Genome Architecture: Predicting Open Reading Frame Distributions

Background

A better understanding of the size and abundance of open reading frames (ORFS) in whole genomes may shed light on the factors that control genome complexity. Here we examine the statistical distributions of open reading frames (i.e. distribution of start and stop codons) in the fully sequenced genomes of 297 prokaryotes, and 14 eukaryotes.

Methodology/Principal Findings

By fitting mixture models to data from whole genome sequences we show that the size-frequency distributions for ORFS are strikingly similar across prokaryotic and eukaryotic genomes. Moreover, we show that i) a large fraction (60–80%) of ORF size-frequency distributions can be predicted a priori with a stochastic assembly model based on GC content, and that (ii) size-frequency distributions of the remaining “non-random” ORFs are well-fitted by log-normal or gamma distributions, and similar to the size distributions of annotated proteins.

Conclusions/Significance

Our findings suggest stochastic processes have played a primary role in the evolution of genome complexity, and that common processes govern the conservation and loss of functional genomics units in both prokaryotes and eukaryotes.     

How species longevity, intraspecific morphological variation, and geographic range size are related: a comparison using late Cambrian trilobites

Phenotypic variation is fundamental to evolutionary change. Variation not only evinces the connectivity of populations but it is also associated with the adaptability and evolvability of taxa. Despite the potential importance of morphological variation in structuring evolutionary patterns, little is known about how relative differences in intraspecific morphological variation and its geographic structure are linked to differences in species longevity. This study offers a novel combination of analyses that reveal the quantitative relationships among intraspecific variation, geographic range size and duration in the fossil record using late Cambrian trilobites. Results show that geographic range size and duration are positively correlated. Surprisingly, longer lived species tend to have less intraspecific variation. Phylogenetic effects were also explored and found not to determine the association between these variables. However, the distribution of geographic range sizes shows strong phylogenetic signal. In light of previous work, one possible explanation for these results is that species with shorter durations have comparatively higher rates of morphological evolution, reflected in higher phenotypic variation overall.     

Understanding gregariousness in a larval Lepidopteran: the roles of host plant, predation, and microclimate

Abstract.  1. Many moth and butterfly larvae are gregarious early in development, but become solitary in late instars. This ontogenetic variation in behaviour is probably the result of temporal changes in the costs and benefits associated with gregariousness. This study provides observational and experimental evidence that, in one particular moth species, a series of different ecological factors influence larval behaviour at different times during development.
2. Field observations show that young caterpillars of the limocodid Doratifera casta form large aggregations while foraging, but that mature larvae are largely solitary.
3. A field experiment revealed that individual first to third instar larvae in larger groups develop more rapidly, but that group size had no detectable influence on survival. The developmental advantage associated with gregariousness is affected by host plant species, but not by predator exclusion, suggesting that group living in these cryptic early instar larvae promotes feeding facilitation, but does not provide individuals with protection from natural enemies.
4. Laboratory experiments revealed that aposematic fourth instar caterpillars in large groups were less likely to be attacked by a generalist insect predator than those in small groups.
5. Field observations provided no evidence that group living affects body temperature, suggesting that microclimatic factors do not favour gregariousness in this species.
6. It is concluded that gregariousness in D. casta confers at least two different advantages on larvae at different stages early in development, but that these advantages disappear, or are outweighed by costs associated with intraspecific competition, in final instars.