Population genomic structure of a widespread, urban-dwelling mammal: The eastern grey squirrel (Sciurus carolinensis).

Urbanization is a persistent and widespread driver of global environmental change, potentially shaping evolutionary processes due to genetic drift and reduced gene flow in cities induced by habitat fragmentation and small population sizes. We tested this prediction for the eastern grey squirrel (Sciurus carolinensis), a common and conspicuous forest-dwelling rodent, by obtaining 44K SNPs using reduced representation sequencing (ddRAD) for 403 individuals sampled across the species' native range in eastern North America. We observed moderate levels of genetic diversity, low levels of inbreeding, and only a modest signal of isolation-by-distance. Clustering and migration analyses show that estimated levels of migration and genetic connectivity were higher than expected across cities and forested areas, specifically within the eastern portion of the species' range dominated by urbanization, and genetic connectivity was less than expected within the western range where the landscape is fragmented by agriculture. Landscape genetic methods revealed greater gene flow among individual squirrels in forested regions, which likely provide abundant food and shelter for squirrels. Although gene flow appears to be higher in areas with more tree cover, only slight discontinuities in gene flow suggest eastern grey squirrels have maintained connected populations across urban areas in all but the most heavily fragmented agricultural landscapes. Our results suggest urbanization shapes biological evolution in wildlife species depending strongly on the composition and habitability of the landscape matrix surrounding urban areas.

Naming the menagerie: creativity, culture and consequences in the formation of scientific names.

The coining of scientific names for newly described species is one of the most creative acts in science. We briefly review the history of species naming, with an emphasis on constraints and freedoms in the choice of new names and how they came to be. We then consider patterns in etymologies and linguistic origins of scientific names across clades and through time. Use of 'non-classical' languages (those other than Latin and Greek) in naming species has increased, as has the use of eponymous names (despite recent controversy around the practice). Finally, we consider ways in which creativity in naming has consequences for the conduct and outcome of scientific work. For example, sale of naming rights has funded research and conservation, while naming species after celebrities has increased media attention to the science of species discovery. Other consequences of naming are more surprising, including a strong effect of species-name etymology on the kinds of scientific studies conducted for plant-feeding arthropods. Scientific naming is a clear example of how science and scientists are socially situated, and how culturally influenced decisions such as what to name a new species can affect both public perception of science and the conduct of science itself.

Can species naming drive scientific attention? A perspective from plant-feeding arthropods.

How do researchers choose their study species? Some choices are based on ecological or economic importance, some on ease of study, some on tradition-but could the name of a species influence researcher decisions? We asked whether phytophagous arthropod species named after their host plants were more likely to be assayed for host-associated genetic differentiation (or 'HAD'; the evolution of cryptic, genetically isolated host specialists within an apparently more generalist lineage). We chose 30 arthropod species (from a Google Scholar search) for which a HAD hypothesis has been tested. We traced the etymologies of species names in the 30 corresponding genera, and asked whether HAD tests were more frequent among species whose etymologies were based on host-plant names (e.g. Eurosta solidaginis, which attacks Solidago) versus those with other etymologies (e.g. Eurosta fenestrata, from Latin fenestra, 'window'). Species with host-derived etymologies were more likely to feature in studies of HAD than those with other etymologies. We speculate that the etymology of a scientific name can draw a researcher's attention to aspects of life-history and thus influence the direction of our scientific gaze.

Site-selection bias and apparent population declines in long-term studies.

Detecting population declines is a critical task for conservation biology. Logistical difficulties and the spatiotemporal variability of populations make estimation of population declines difficult. For statistical reasons, estimates of population decline may be biased when study sites are chosen based on abundance of the focal species. In this situation, apparent population declines are likely to be detected even if there is no decline. This site-selection bias is mentioned in the literature but is not well known. We used simulations and real population data to examine the effects of site-selection biases on inferences about population trends. We used a left-censoring method to detect population-size patterns consistent with site-selection bias. The site-selection bias is an important consideration for conservation biologists, and we offer suggestions for minimizing or mitigating it in study design and analysis. Article impact statement: Estimates of population declines are biased if studies begin in large populations, and time-series data show a signature of such an effect.

Sesgos en la Selección de Sitios y las Declinaciones Poblacionales Aparentes en los Estudios a Largo Plazo Resumen La detección de las declinaciones poblacionales es una tarea muy importante para la biología de la conservación. Las dificultades logísticas y la variabilidad espacio-temporal de las poblaciones complican la estimación de las declinaciones poblacionales. Por razones estadísticas, las estimaciones de las declinaciones poblacionales pueden estar sesgadas cuando se eligen los sitios de estudio con base en la abundancia de la especie focal. En esta situación, las declinaciones poblacionales aparentes probablemente sean detectadas sin que exista dicha declinación. Este sesgo en la selección del sitio está mencionado en la literatura, pero sabe poco sobre él. Usamos simulaciones y datos de poblaciones reales para examinar los efectos que tienen los sesgos en la selección de sitio sobre las inferencias que se tienen sobre las tendencias poblacionales. Usamos un método censurado por la izquierda para detectar los patrones en el tamaño poblacional consistentes con el sesgo en la selección de sitios. Este sesgo es una consideración importante para los biólogos de la conservación y ofrecemos sugerencias para minimizarlo o mitigarlo en el análisis y diseño de los estudios.

Impact of an Invasive Longhorned Beetle, Tetropium fuscum (Coleoptera: Cerambycidae), on Community Structure of Subcortical and Wood-Associated Insects in Eastern Canada.

Tetropium fuscum (Fabricius) (Coleoptera: Cerambycidae), a phloem-feeding and wood-boring beetle introduced from Eurasia, attacks spruce in eastern Canada alongside its native congener Tetropium cinnamopterum Kirby. We reared phloem- and wood-feeding insects (and their predators) from bolts of red and Norway spruce (Picea rubens and Picea abies) in Nova Scotia, comparing insect communities between bolts with added eggs of T. fuscum or T. cinnamopterum and bolts without added Tetropium (controls). We tested for impacts of each Tetropium on insect community structure (Simpson's diversity, richness, and evenness). We also asked whether, consistent with Darwin's Naturalization Hypothesis, Tetropium spp. would have greater impacts on emergence of its closer relatives (which might be most likely to compete and/or share natural enemies). Addition of Tetropium eggs (either species) to bolts lowered insect diversity in both host trees. Both richness and evenness components of diversity were always lower in +Tetropium treatments, although different components reached statistical significance in different Tetropium species × host combinations. Addition of Tetropium spp. significantly reduced emergence of some species: Evodinus monticola (Randall) (Coleoptera: Cerambycidae) was reduced by T. fuscum on both hosts and by T. cinnamopterum on Norway spruce; Hylobius congener Dalla Torre, Schenkling, and Marshall was reduced by T. fuscum on red spruce; and Xylophagus sp. (Diptera: Xylophagidae) was reduced by T. cinnamopterum on Norway spruce. However, there was no relationship between Tetropium's impact on a community member and their phylogenetic relatedness, and the overall impacts of Tetropium presence were not very different between T. fuscum and T. cinnamopterum.

The shapes of phylogenetic trees of clades, faunas, and local assemblages: exploring spatial pattern in differential diversification.

Life on Earth is characterized by strong diversity skewness: related lineages typically show pronounced variation in diversification success, and clades contain hyperdiverse and depauperate subclades. Previous studies have documented diversity skewness only for entire (global) clades. We demonstrate methods for measurement and significance testing of diversity skewness of local assemblages and regional biotas; we illustrate this with an analysis of geographic structure in diversity skewness of primate assemblages. For primates, continental faunas differ in diversity skewness from expectations based on the global phylogeny: South American faunas have significantly low skewness and African faunas have significantly high skewness. However, no local assemblage has diversity skewness different from that expected based on sampling the continental fauna. We also document a latitudinal gradient in diversity skewness for the African assemblages and test for (but do not find) associations of skewness with longitude, local species richness, and net primary productivity. Our data suggest that continental-scale biogeographic events rather than local-scale processes have shaped diversity skewness in modern primate faunas.

Bringing ecology blogging into the scientific fold: measuring reach and impact of science community blogs.

The popularity of science blogging has increased in recent years, but the number of academic scientists who maintain regular blogs is limited. The role and impact of science communication blogs aimed at general audiences is often discussed, but the value of science community blogs aimed at the academic community has largely been overlooked. Here, we focus on our own experiences as bloggers to argue that science community blogs are valuable to the academic community. We use data from our own blogs (n = 7) to illustrate some of the factors influencing reach and impact of science community blogs. We then discuss the value of blogs as a standalone medium, where rapid communication of scholarly ideas, opinions and short observational notes can enhance scientific discourse, and discussion of personal experiences can provide indirect mentorship for junior researchers and scientists from underrepresented groups. Finally, we argue that science community blogs can be treated as a primary source and provide some key points to consider when citing blogs in peer-reviewed literature.

Cascading host-associated genetic differentiation in parasitoids of phytophagous insects.

The extraordinary diversity of phytophagous insects may be attributable to their narrow specialization as parasites of plants, with selective tradeoffs associated with alternate host plants driving genetic divergence of host-associated forms via ecological speciation. Most phytophagous insects in turn are attacked by parasitoid insects, which are similarly specialized and may also undergo host-associated differentiation (HAD). A particularly interesting possibility is that HAD by phytophagous insects might lead to HAD in parasitoids, as parasitoids evolve divergent lineages on the new host plant-specific lineages of their phytophagous hosts. We call this process 'cascading host-associated differentiation' (cascading HAD). We tested for cascading HAD in parasitoids of two phytophagous insects, each of which consists of genetically distinct host-associated lineages on the same pair of goldenrods (Solidago). Each parasitoid exhibited significant host-associated genetic divergence, and the distribution and patterns of divergence are consistent with divergence in sympatry. Although evidence for cascading HAD is currently limited, our results suggest that it could play an important role in the diversification of parasitoids attacking phytophagous insects. The existence of cryptic host-associated lineages also suggests that the diversity of parasitoids may be vastly underestimated.

Paradigms in Eastern Spruce Budworm (Lepidoptera: Tortricidae) Population Ecology: A Century of Debate.

Three main hypotheses have been postulated over the past century to explain the outbreaking population dynamics of eastern spruce budworm, Choristoneura fumiferana (Clemens). The Silviculture Hypothesis first arose in the 1920s, with the idea that outbreaks were driven by forestry practices favoring susceptible softwood species. In the 1960s, it was proposed that populations were governed by Multiple Equilibria, with warm weather conditions releasing low-density populations from the regulatory control of natural enemies. Dispersal from outbreak foci, or "epicenters," was seen as causing widespread outbreaks that eventually collapsed following resource depletion. However, in the 1980s, following the re-analysis of data from the 1940s outbreak in New Brunswick, this interpretation was challenged. The alternative Oscillatory Hypothesis proposed that budworm population dynamics were governed by a second-order density-dependent process, with oscillations being driven by natural enemy-victim interactions. Under this hypothesis, weather and resource availability contribute to secondary fluctuations around the main oscillation, and weather and moth dispersal serve to synchronize population cycles regionally. Intensive, independent population studies during the peak and declining phases of the 1980s outbreak supported the principal tenet of the Oscillatory Hypothesis, but concluded that host plant quality played a more important role than this hypothesis proposed. More recent research on the early phase of spruce budworm cycles suggests that mate-finding and natural-enemy-driven Allee effects in low-density populations might be overcome by immigration of moths, which can facilitate the onset of outbreaks. Even more recent research has supported components of all three hypotheses attempting to explain spruce budworm dynamics. In the midst of a new rising outbreak (2006-present), we discuss the evolution of debates surrounding these hypotheses from a historic perspective, examine gaps in current knowledge, and suggest avenues for future research (e.g., intensive studies on low-density populations) to better understand and manage spruce budworm populations.

Displacement of Tetropium cinnamopterum (Coleoptera: Cerambycidae) by Its Invasive Congener Tetropium fuscum.

We examined the native community of insects interacting with an invasive species, Tetropium fuscum (F.) (Coleoptera: Cerambycidae), in its new range to explore reasons for the invader's relatively slow spread. Tetropium fuscum is a European spruce borer established in Nova Scotia since at least 1990, but it has spread only about 125 km from its site of introduction. We compared the densities of Tetropium spp., their known parasitoids, and the community of wood-boring insects at sites located within the invasion zone in Nova Scotia versus well outside this zone, in New Brunswick, Canada. Using red spruce trees stressed by girdling or felling, we tested whether: 1) T. fuscum had altered the native wood-boring community; 2) T. fuscum displaced a native congener, Tetropium cinnamopterum (Kirby); and 3) parasitism rates of Tetropium spp. differed between the invaded and noninvaded zones. Both Tetropium spp. and their parasitoid wasps emerged exclusively from felled trees as opposed to girdled trees. We found no difference in community diversity inside versus outside the invasion zone. The combined densities of both Tetropium spp. and their overall parasitism rates also did not differ between zones, but T. cinnamopterum density was significantly greater outside the invasion zone, suggesting T. fuscum may displace the native congener where they are sympatric. Our results suggest that the native and invasive Tetropium spp. act as a single functional species in the invasion zone. We speculate that natural control agents (predators, parasitoids, and competitors) might be limiting the rate of spread of T. fuscum.

Host-associated genetic differentiation in phytophagous insects: general phenomenon or isolated exceptions? Evidence from a goldenrod-insect community.

There is growing awareness of the importance of natural selection in driving genetic divergence and speciation, and several of the most apparent cases of this ecological speciation are provided by the existence of genetically distinct host forms in phytophagous insects. Such examples of host-associated differentiation (HAD) have become increasingly documented, and the implications of this phenomenon for the diversification of insects are becoming widely appreciated. However, instances of HAD remain rare relative to insect diversity and are sparsely distributed both ecologically and taxonomically. We sought to assess the frequency of HAD in a model herbivore community by examining genetic divergence in a variety of herbivores that feed on two closely related and broadly sympatric species of goldenrod (Solidago altissima and S. gigantea). Using mitochondrial DNA and allozyme data, in conjunction with previously published studies, we found that four of nine herbivores exhibited evidence of HAD, including possible host races or cryptic species. Using a range of reasonable substitution rate estimates for cytochrome oxidase I mitochondrial DNA, we found that HAD appears to have proceeded asynchronously across taxa. This pattern, along with the broadly sympatric distribution of host plants and the specialized life histories of the phytophagous insects, is consistent with sympatric divergence in some or all of these taxa. Although further behavioral and ecological study is needed, our survey of HAD in a community of herbivores indicates that ecological (perhaps sympatric) speciation may have been responsible for generating a significant fraction of the extant diversity of phytophagous insects.

Host-associated genetic differentiation in the goldenrod elliptical-gall moth, Gnorimoschema gallaesolidaginis (Lepidoptera: Gelechiidae).

Careful study of apparently generalist phytophagous insects often reveals that they instead represent complexes of genetically differentiated host races or cryptic species. The goldenrod elliptical-gall moth, Gnorimoschema gallaesolidaginis, attacks two goldenrods in the Solidago canadensis complex: S. altissima and S. gigantea (Asteraceae). We tested for host-associated genetic differentiation in G. gallaesolidaginis via analysis of variation at 12 allozyme loci among larvae collected at six sites in Iowa, Minnesota, and Nebraska. Gnorimoschema gallaesolidaginis from each host are highly polymorphic (3.6-4.7 alleles/locus and expected heterozygosity 0.28-0.38 within site-host combinations). Although there were no fixed differences between larvae from S. altissima and S. gigantea at any site, these represent well differentiated host forms, with 11 of 12 loci showing significantly different allele frequencies between host-associated collections at one or more sites. Host plant has a larger effect on genetic structure among populations than does location (Wright's FST = 0.16 between host forms vs. F(ST) = 0.061 and 0.026 among altissima and gigantea populations, respectively). The estimated F(ST) between host forms suggests that the historical effective rate of gene flow has been low (N(e)m approximately 1.3). Consistent with this historical estimate is the absence of detectable recombinant (hybrid and introgressant between host form) individuals in contemporary populations (none of 431 genotyped individuals). Upper 95% confidence limits for the frequency of recombinant individuals range from 5% to 9%. Host association is tight, but imperfect, with only one likely example of a host mismatch (a larva galling the wrong host species). Our inferences about hybridization and host association are based on new maximum-likelihood methods for estimating frequencies of genealogical classes (in this case, two parental classes, F1 and F2 hybrids, and backcrosses) in a population and for assigning individuals to genealogical classes. We describe these new methods in the context of their application to genetic structure in G. gallaesolidaginis. Population phenograms are consistent with the origin of the host forms (at least in the midwestern United States) via a single host shift: altissima and gigantea moth populations form distinct lineages with 100% bootstrap support. Genetic structure in Gnorimoschema is of particular interest because another gallmaking insect attacking the same pair of hosts, the tephritid fly Eurosta solidaginis, includes a pair of host races with partial reproductive isolation. Gnorimoschema gallaesolidaginis and E. solidaginis therefore represent the first reported case of parallel host-associated differentiation, that is, differentiation by evolutionarily independent insect lineages across the same pair of host plants.

Ecological correlates of single-seededness in a woody tropical flora.

A reduction in seed number per fruit is recognized as a common evolutionary trend among flowering plants. In order to evaluate the ecological role of single-seededness and its possible adaptive significance, we used van Roosmalen's (1985) descriptions of fruits for woody species in the Guianan flora to identify fruit and seed characters and dispersal syndromes associated with the single-seeded trait. We coded the following characters: seed number (one seed or more than one seed), fleshiness (dry or fleshy), dehiscence (dehiscent or indehiscent), dispersal syndrome, seed size (length), and fruit size (length). We ordered the data in a multi-dimensional contingency table and used maximum likelihood logistic regression to identify traits significantly correlated with single-seededness. Seed size and fruit size were treated as quantitative variables. Indehiscence and endozoochory are positively associated with single-seededness, with indehiscence contributing most to the best-fit model. Fruit size and seed size are also important with the probability of single-seededness generally increasing with seed size and decreasing with fruit size, although a (fruit size) x (seed size) interaction term is significant. Dry fruits are positively associated with single-seededness and dispersal by synzoochory or myrmecochory negatively associated when the full data set is examined, but neither parameter is significant in two models constructed to remove effects of phylogeny. A nested ANOVA revealed that most variation occurs below the family level for almost all of the traits considered, with the exception of the dry vs. fleshy trait for which there is no variation within genera. We argue that the strong association between indehiscence and single-seededness suggests selective advantages for single-seeded dispersal units but acknowledge that energetic trade-offs between seed number and seed size probably also occur. We suggest that the post-dispersal fates of seeds - especially those deposited in clumps by endozoochory - should be examined with the idea of identifying selective pressures on seed number per fruit.

A simulation approach to assessing sampling strategies for insect pests: an example with the balsam gall midge.

Estimation of pest density is a basic requirement for integrated pest management in agriculture and forestry, and efficiency in density estimation is a common goal. Sequential sampling techniques promise efficient sampling, but their application can involve cumbersome mathematics and/or intensive warm-up sampling when pests have complex within- or between-site distributions. We provide tools for assessing the efficiency of sequential sampling and of alternative, simpler sampling plans, using computer simulation with "pre-sampling" data. We illustrate our approach using data for balsam gall midge (Paradiplosis tumifex) attack in Christmas tree farms. Paradiplosis tumifex proved recalcitrant to sequential sampling techniques. Midge distributions could not be fit by a common negative binomial distribution across sites. Local parameterization, using warm-up samples to estimate the clumping parameter k for each site, performed poorly: k estimates were unreliable even for samples of n ∼ 100 trees. These methods were further confounded by significant within-site spatial autocorrelation. Much simpler sampling schemes, involving random or belt-transect sampling to preset sample sizes, were effective and efficient for P. tumifex. Sampling via belt transects (through the longest dimension of a stand) was the most efficient, with sample means converging on true mean density for sample sizes of n ∼ 25-40 trees. Pre-sampling and simulation techniques provide a simple method for assessing sampling strategies for estimating insect infestation. We suspect that many pests will resemble P. tumifex in challenging the assumptions of sequential sampling methods. Our software will allow practitioners to optimize sampling strategies before they are brought to real-world applications, while potentially avoiding the need for the cumbersome calculations required for sequential sampling methods.

Phenology and spatial distribution of native and exotic Tetropium longhorned beetles (Coleoptera: Cerambycidae).

The co-existence of two closely related Tetropium species in eastern Canada, invasive T. fuscum and native T. cinnamopterum (TF and TC, respectively), provides a model system to investigate seasonal and spatial demographic parameters of biological invasions at the interspecific level. In this study, we take advantage of the similar semiochemical communication of TF and TC to evaluate the abundance of adults of the two species concurrently using grids of traps baited with pheromone and host volatiles in stands of spruce. Adult TF emerged on average 2 wk before TC both in the field and under controlled laboratory conditions. This observation, combined with the early reproduction of emergent females, implies that the smaller (younger) larvae of native TC may be at increased risk of intra-guild predation by TF. The high spatial association between male and female TF in dense, aggregated populations suggests that the rate of mate encounter is depressed in sparse populations toward the edge of the invasive range. The higher level of spatial aggregation for TF than TC, particularly at high population density, suggests a higher propensity of adult TF to congregate at "landmarks." Considering the broader range of host conditions, earlier seasonal emergence, and presumably more effective mate encounter for TF than TC, the exotic TF may be a superior competitor with the potential to displace or reduce the abundance of TC.

Origins, distribution, and local co-occurrence of polyploid cytotypes in Solidago altissima (Asteraceae).

There is growing realization that intraspecific polyploidy in plants has important implications for the evolution of plant diversity and for plant and plant-herbivore community dynamics. Nonetheless, the rate at which polyploid cytotypes arise and their fate in local mixed-cytotype populations are not well understood. Although within- and especially among-population cytotype variation has been documented in many plants, particularly at the largest (continental) spatial scales, local and regional distributions of cytotypes have been well characterized only for a handful of species. Furthermore, few if any studies have addressed both local and regional patterns in the same plant species with sufficient power to detect cytotype variation on both scales. We assessed the co-occurrence and frequency distributions of diploid, tetraploid, and hexaploid cytotypes of Solidago altissima (Asteraceae) at local and regional spatial scales, sampling across a zone of presumed ploidy-form overlap in the midwestern United States. Determining ploidy by flow cytometry, we found many local populations containing two or three cytotypes, with the relative frequencies of cytotypes varying dramatically even among neighboring populations. AFLP marker data suggest that polyploid cytotypes of S. altissima likely have multiple origins from different diploid lineages. Our results are consistent with recent perspectives on polyploidization as an evolutionarily dynamic phenomenon with frequent multiple origins of higher-ploidy cytotypes.

Differential attack on diploid, tetraploid, and hexaploid Solidago altissima L. by five insect gallmakers.

Genetic variation among plants can influence host choice and larval performance in insect herbivores. Ploidy (cytotype) variation is a particularly dramatic form of plant genetic variation, and where diploid and polyploid cytotypes of a species occur in sympatry, they may provide herbivores with choices that are distinguished by profound and genome-wide genetic differences. We tested for non-random attack by five gallmaking insect herbivores on diploid, tetraploid, and hexaploid cytotypes of the goldenrod Solidago altissima L., working in seven midwestern US populations where the ploidies co-occur on spatial scales relevant to insect host choice. For four of the five herbivores, attack was non-random with respect to ploidy at one or more sites. Ploidy effects on attack were complex: the ploidy subjected to highest attack varied both across herbivores within sites and (for most herbivores) across sites within herbivores. Ploidy effects on attack will alter rates of encounter between insect herbivores-either increasing or decreasing the likelihood of two herbivores sharing a host plant ramet, compared with the case with no effects of ploidy. Plant ploidy variation appears likely to have a major impact on insect community organization, and perhaps on plant-herbivore coevolution, but that impact is likely to be spatially heterogeneous.

Quantification of repertoire diversity of influenza-specific epitopes with predominant public or private TCR usage.

The H-2Db-restricted CD8 T cell immune response to influenza A is directed at two well-described epitopes, nucleoprotein 366 (NP366) and acid polymerase 224 (PA224). The responses to the two epitopes are very different. The epitope NP366-specific response is dominated by TCR clonotypes that are public (shared by most mice), whereas the epitope PA224-specific response is private (unique within each infected animal). In addition to being public, the NP366-specific response is dominated by a few clonotypes, when T cell clonotypes expressing the Vbeta8.3 element are analyzed. Herein, we show that this response is similarly public when the NP366+Vbeta4+ CD8 T cell response is analyzed. Furthermore, to determine whether these features resulted in differences in total TCR diversity in the NP366+ and PA224+ responses, we quantified the number of different CD8 T clonotypes responding to each epitope. We calculated that 50-550 clonotypes recognized each epitope in individual mice. Thus, although the character of the response to the two epitopes appeared to be different (private and diverse vs public and dominated by a few clonotypes), similar numbers of precursor cells responded to both epitopes and this number was of similar magnitude to that previously reported for other viral CD8 T cell epitopes. Therefore, even in CD8 T cell responses that appear to be oligoclonotypic, the total response is highly diverse.

On the elusiveness of enemy-free space: spatial, temporal, and host-plant-related variation in parasitoid attack rates on three gallmakers of goldenrods.

Host shifting by phytophagous insects may play an important role in generating insect diversity by initiating host-race formation and speciation. Models of the host shifting process often invoke reduced rates of natural enemy attack on a novel host in order to balance the maladaptation expected following the shift. Such "enemy-free space" has been documented for some insects, at some times and places, but few studies have assessed the occurrence of enemy-free space across years, among sites, or among insect species. We measured parasitoid attack rates on three insect herbivores of two goldenrods (Solidago altissima L. and Solidago gigantea Ait.), with data from multiple sites and multiple years for each herbivore. For each insect herbivore, there were times and sites at which parasitoid attack rates differed strongly and significantly between host plants (that is, enemy-free space existed on one host plant or the other). However, the extent and even the direction of the attack-rate difference varied strongly among sites and even among years at the same site. There was no evidence of consistent enemy-free space for any herbivore on either host plant. Our data suggest that enemy-free space, like many ecological and evolutionary forces, is likely to operate as a geographic and temporal mosaic, and that conceptual models of host shifting that include enemy-free space as a consequence of host novelty are likely too simple.

PATTERNS IN TREE BALANCE AMONG CLADISTIC, PHENETIC, AND RANDOMLY GENERATED PHYLOGENETIC TREES.

I examine patterns in tree balance for a sample of 208 cladograms and phenograms from the recent literature. I provide an expression for expected imbalance under a simple, uniform-rate random speciation model, and I estimate variances by simulation for the same model. Imbalance decreases with tree size (number of included taxa) in both theoretical and literature trees. In contrast to previous suggestions, I find cladistic trees to be no more imbalanced than phenetic trees when confounding variables are appropriately controlled. The degree of imbalance found in literature trees is inconsistent with the uniform-rate speciation model; this is most likely a result of variability in speciation and extinction rates among real lineages. The existence of such variation is a necessary (but not sufficient) condition for the operation of the macroevolutionary processes of species sorting and species selection.