Differences In Diversification Research Articles

Assessing portfolio diversification via two-sample graph kernel inference. A case study on the influence of ESG screening.

In this work we seek to enhance the frameworks practitioners in asset management and wealth management may adopt to asses how different screening rules may influence the diversification benefits of portfolios. The problem arises naturally in the area of Environmental, Social, and Governance (ESG) based investing practices as practitioners often need to select subsets of the total available assets based on some ESG screening rule. Once a screening rule is identified, one constructs a dynamic portfolio which is usually compared with another dynamic portfolio to check if it satisfies or outperforms the risk and return profile set by the company. Our study proposes a novel method that tackles the problem of comparing diversification benefits of portfolios constructed under different screening rules. Each screening rule produces a sequence of graphs, where the nodes are assets and edges are partial correlations. To compare the diversification benefits of screening rules, we propose to compare the obtained graph sequences. The method proposed is based on a machine learning hypothesis testing framework called the kernel two-sample test whose objective is to determine whether the graphs come from the same distribution. If they come from the same distribution, then the risk and return profiles should be the same. The fact that the sample data points are graphs means that one needs to use graph testing frameworks. The problem is natural for kernel two-sample testing as one can use so-called graph kernels to work with samples of graphs. The null hypothesis of the two-sample graph kernel test is that the graph sequences were generated from the same distribution, while the alternative is that the distributions are different. A failure to reject the null hypothesis would indicate that ESG screening does not affect diversification while rejection would indicate that ESG screening does have an effect. The article describes the graph kernel two-sample testing framework, and further provides a brief overview of different graph kernels. We then demonstrate the power of the graph two-sample testing framework under different realistic scenarios. Finally, the proposed methodology is applied to data within the SnP500 to demonstrate the workflow one can use in asset management to test for structural differences in diversification of portfolios under different ESG screening rules.

Immunity-driven evolution of virulence and diversity in respiratory diseases.

The time-honoured paradigm in the theory of virulence evolution assumes a positive relation between infectivity and harmfulness. However, the aetiology of respiratory diseases yields a negative relation, with diseases of the lower respiratory tract being less infective and more harmful. We explore the evolutionary consequences in a simple model incorporating cross-immunity between disease strains that diminishes with their distance in the respiratory tract, assuming that docking rate follows the match between the local mix of cell surface types and the pathogen's surface, and cross-immunity the similarity of the pathogens' surfaces. The assumed relation between fitness components causes virulent strains infecting the lower airways to evolve to milder, more transmissible variants. Limited cross-immunity generally causes a readiness to diversify that increases with host population density. In respiratory diseases, that diversity will be highest in the upper respiratory tract. More tentatively, emerging respiratory diseases are likely to start low and virulent, to evolve up and become milder. Our results extend to a panoply of realistic generalisations of the disease's ecology and to including additional epitope axes. These extensions allow us to apply our results quantitatively to elucidate the differences in diversification between rhino- and coronavirus caused common colds.

Traits related to efficient acquisition and use of phosphorus promote diversification in Proteaceae in phosphorus‐impoverished landscapes

Background and aimsPlant species richness increases with declining soil phosphorus (P) availability, especially for Proteaceae in old infertile landscapes. This difference in richness might be attributed to faster diversification in lineages adapted to P-impoverished soils, i.e. species that possess specialised P-acquisition strategies, and have lower leaf P concentration ([P]) and higher seed [P]. Alternatively, a longer time for species accumulation might contribute to high species richness in low-P environments due to the geological stability of the landscapes in which they evolved.MethodsWe assessed differences in diversification of Proteaceae in P-impoverished vs. nutrient-rich environments and whether these were linked to adaptations to P-impoverished soils. We explored mature leaf and seed [P] and investigated how these traits changed over the evolutionary history of the family, and within two species-rich genera (Banksia, Hakea).ResultsFaster diversification was correlated with lower leaf and higher seed [P] for species-rich genera across the Proteaceae. For Banksia and Hakea, diversification rates peaked at relatively low leaf [P], but not at the lowest leaf [P]. Ancestral state reconstructions indicated that low leaf [P] is a trait that was likely present in the early evolution of the Proteaceae, with recent transitions to higher leaf [P] across several species-poor rainforest genera.ConclusionsDiversification of Proteaceae correlated strongly with P-related traits. In an evolutionary context, functional cluster roots, low leaf [P] and high seed [P] were likely key innovations allowing diversification. Selection for low leaf [P] early in the evolutionary history of Proteaceae pre-adapted ancestors of this family to diversify into oligotrophic environments. We discuss how our findings are likely relevant for understanding diversification dynamics of other plant families that occur in P-impoverished environments.

Planktivores as trophic drivers of global coral reef fish diversity patterns

One of the most prominent features of life on Earth is the uneven number of species across large spatial scales. Despite being inherently linked to energetic constraints, these gradients in species richness distribution have rarely been examined from a trophic perspective. Here we dissect the global diversity of over 3,600 coral reef fishes to reveal patterns across major trophic groups. By analyzing multiple nested spatial scales, we show that planktivores contribute disproportionally to the formation of the Indo-Australian Archipelago (IAA) marine biodiversity hotspot. Besides being "hotter" at the hotspot, planktivorous fishes display the steepest decline in species numbers with distance from the IAA when compared to other trophic groups. Surprisingly, we did not detect differences in diversification, transition, and dispersal rates in extant species phylogenies that would explain this remarkable gradient in planktivorous fish richness. Thus, we identify two potential complementary drivers for this pattern. First, exceptional levels of partitioning among planktivorous coral reef fishes were driven by temporally stable oceanographic conditions and abundant planktonic resources in the IAA. Second, extinctions of planktivores outside the IAA have been particularly pronounced during Quaternary climate fluctuations. Overall, our results highlight trophic ecology as an important component of global species richness gradients.

Genomic Analyses of Human Sapoviruses Detected over a 40-Year Period Reveal Disparate Patterns of Evolution among Genotypes and Genome Regions.

Human sapovirus is a causative agent of acute gastroenteritis in all age groups. The use of full-length viral genomes has proven beneficial to investigate evolutionary dynamics and transmission chains. In this study, we developed a full-length genome sequencing platform for human sapovirus and sequenced the oldest available strains (collected in the 1970s) to analyse diversification of sapoviruses. Sequence analyses from five major genotypes (GI.1, GI.2, GII.1, GII.3, and GIV.1) showed limited intra-genotypic diversification for over 20–40 years. The accumulation of amino acid mutations in VP1 was detected for GI.2 and GIV.1 viruses, while having a similar rate of nucleotide evolution to the other genotypes. Differences in the phylogenetic clustering were detected between RdRp and VP1 sequences of our archival strains as well as other reported putative recombinants. However, the lack of the parental strains and differences in diversification among genomic regions suggest that discrepancies in the phylogenetic clustering of sapoviruses could be explained, not only by recombination, but also by disparate nucleotide substitution patterns between RdRp and VP1 sequences. Together, this study shows that, contrary to noroviruses, sapoviruses present limited diversification by means of intra-genotype variation and recombination.

Diatoms diversify and turn over faster in freshwater than marine environments.

Many clades that span the marine-freshwater boundary are disproportionately more diverse in the younger, shorter lived, and scarcer freshwater environments than they are in the marine realm. This disparity is thought to be related to differences in diversification rates between marine and freshwater lineages. However, marine and freshwaters are not ecologically homogeneous, so the study of diversification across the salinity divide should also account for other potentially interacting variables. In diatoms, freshwater and substrate-associated (benthic) lineages are several-fold more diverse than their marine and suspended (planktonic) counterparts. These imbalances provide an excellent system to understand whether these variables interact with diversification. Using multistate hidden-state speciation and extinction models, we found that freshwater lineages diversify faster than marine lineages regardless of whether they inhabit the plankton or the benthos. Freshwater lineages also had higher turnover rates (speciation + extinction), suggesting that habitat transitions impact speciation and extinction rates jointly. The plankton-benthos contrast was also consistent with state-dependent diversification, but with modest differences in diversification and turnover rates. Asymmetric and bidirectional transitions rejected hypotheses about the plankton and freshwaters as absorbing, inescapable habitats. Our results further suggest that the high turnover rate of freshwater diatoms is related to high turnover of freshwater systems themselves.

Diversification decisions among family firms: The role of family involvement and generational stage

While prior literature has focused on whether family firms are more or less inclined to diversification than non-family firms, the examination of differences in diversification among family firms has received much less attention. We analyze how family involvement (in ownership, control, and management) and the generational stage in the company (first versus later generations) influence diversification among family firms. The empirical evidence is provided by a sample of publicly listed family firms from the EU. Our results show that larger levels of family involvement in the firm are associated with lower diversification. Furthermore, first-generation family firms are found to be less diversified than their later-generation counterparts.

Stages of diversification in Africa

This paper investigates the relationship between manufacturing diversification and level of economic development for 29 African countries. Three diversification stages are identified. Considerable differences in diversification are found over the first stage at the low end of the per capita income scale. Diversification alone will not encourage development in poorer countries until obstacles such as weak institutions, poor governance and a lack of infrastructure are addressed. Diversification slows down over the remaining stages as countries develop. Results show countries further along the African economic development path successfully diversified manufacturing.

How Do Free-Living, Lerp-Forming, and Gall-Inducing Aphalaridae (Hemiptera: Psylloidea) Affect the Nutritional Quality ofEucalyptusLeaves?

About 380 described species of Psylloidea occur on Eucalyptus in Australia. These show differences in diversification, feeding behavior, and apparent patterns of development. We analyzed the quality of nutrients used by three species of Aphalaridae belonging to different feeding guilds on three species of Eucalyptus . We evaluated the quantity and quality of total nonstructural carbohydrates (TNC), δ13C and δ15N isotope ratios, amino acids, and fatty acids. In general, TNC levels were greater in infested leaves than in uninfested leaves of the three species of Eucalyptus . TNC levels in the leaves of E. macrorhyncha F. Mueller ex Bentham were the highest and in E. globulus Labillardiere the lowest. Total masses of amino acids and fatty acids were the greatest in the leaves of E. globulus infested by C. eucalypti (Maskell), followed by leaves of E. sideroxylon A. Cunningham ex. Woolls infested by the species of Glycaspis Taylor, and the lowest values were in the leaves of E. macrorhyncha infested by species of Synglycaspis Moore. In general, δ13C increased in C. eucalypti- infested leaves of E. globulus , Glycaspis sp. infested leaves of E. sideroxylon, and Synglycaspis sp. infested leaves of E. macrorhyncha . Nitrogen-isotope ratios (δ15N values) were not significantly different in infested and uninfested leaves. The free-living C. eucalypti stress E. globulus leaves more intensely by its group-feeding behavior, whereas the gall-inducing species of Synglycaspis stresses E. macrorhyncha leaves in such a way to elicit a response with a novel phenotypic expression, viz. , the gall. The lerp-forming species of Glycaspis utilize nutrients, especially sugars, the excess of which is secreted to build their characteristic lerp.

A likelihood inference of historical biogeography in the world’s most diverse terrestrial vertebrate genus: Diversification of direct-developing frogs (Craugastoridae: Pristimantis) across the Neotropics

The geology of the northern Andean region has driven the evolutionary history of Neotropical fauna through the creation of barriers and connections that have resulted in speciation and dispersal events, respectively. One of the most conspicuous groups of anuran fauna in the Andes and surrounding areas is the direct-developing species of the genus Pristimantis. We investigated the molecular phylogenetic placement of 12 species from the montane Andes of Colombia in a broader geographical context with a new genus-level phylogeny in order to identify the role of Andean orogeny over the last 40million years and the effect of elevational differences in diversification of Pristimantis. We examined the biogeographic history of the genus using ancestral range reconstruction by biogeographic regions and elevational ranges. We recognized the middle elevational band (between 1000 and 3000m) in the Northwestern Andes region of Colombia and Ecuador as a focal point for the origin and radiation of Pristimantis species. Additionally, we found several Andean migrations toward new habitats in Central Andes and Merida Andes for some species groups. We suggest that the paleogeological changes in the Northwestern Andes were the main promoter of speciation in Pristimantis, and may have served as a corridor for the dispersion of lowland species.

Higher speciation and lower extinction rates influence mammal diversity gradients in Asia.

BackgroundLittle is known about the patterns and correlates of mammal diversity gradients in Asia. In this study, we examine patterns of species distributions and phylogenetic diversity in Asia and investigate if the observed diversity patterns are associated with differences in diversification rates between the tropical and non-tropical regions. We used species distribution maps and phylogenetic trees to generate species and phylogenetic diversity measures for 1° × 1° cells across mainland Asia. We constructed lineage-through-time plots and estimated diversification shift-times to examine the temporal patterns of diversifications across orders. Finally, we tested if the observed gradients in Asia could be associated with geographical differences in diversification rates across the tropical and non-tropical biomes. We estimated speciation, extinction and dispersal rates across these two regions for mammals, both globally and for Asian mammals.ResultsOur results demonstrate strong latitudinal and longitudinal gradients of species and phylogenetic diversity with Southeast Asia and the Himalayas showing highest diversity. Importantly, our results demonstrate that differences in diversification (speciation, extinction and dispersal) rates between the tropical and the non-tropical biomes influence the observed diversity gradients globally and in Asia. For the first time, we demonstrate that Asian tropics act as both cradles and museums of mammalian diversity.ConclusionsTemporal and spatial variation in diversification rates across different lineages of mammals is an important correlate of species diversity gradients observed in Asia.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0289-1) contains supplementary material, which is available to authorized users.

Ecological Marginalization Facilitated Diversification in Conifers

Species selection occurs when species traits influence speciation or extinction. But it is often difficult to demonstrate a net effect of traits on diversification, for example due to balancing effects of extinction and speciation. We tested if, since conifers lost their former status of dominant land plants to angiosperm trees, their extant diversity shows a signature of traits conferring resistance to extinction. We compared extant species richness across conifer genera in relation to phenotypic and ecological traits predicted to affect speciation and/or extinction, in two geographic regions that experienced distinct regimes of extinction and species turnover (the northern vs. southern hemispheres). Species richness had low phylogenetic signal across conifer genera, and species-rich genera were not older than species-poor ones, indicating intrinsic differences in diversification. High-altitude genera, which are ecologically more dissimilar to angiosperm trees, were more species-rich, suggesting that distinctiveness from angiosperm competitors facilitated conifer diversification. This effect seems more due to increased speciation than to reduced extinction, because it was unique to northern hemisphere genera, where rates of extinction and species turnover have been higher than in the south. We found no strong evidence that resistance to extinction was the main mechanism of species selection in conifers. Instead, having a marginal ecological niche relative to angiosperm competitors facilitated diversification in conifers, likely due to increased speciation. Resistance to extinction may nonetheless have been important in certain ancient lineages, and we discuss suggestive results for traits that may explain the endurance of such lineages.

Host specificity shapes population structure of pinworm parasites in Caribbean reptiles

Host specificity is one of the potential factors affecting parasite diversification because gene flow may be facilitated or constrained by the number of host species that a parasite can exploit. We test this hypothesis using a costructure approach, comparing two sympatric pinworm parasites that differ in host specificity - Parapharyngodon cubensis and Spauligodon anolis - on the Puerto Rican Bank and St. Croix in the Caribbean. Spauligodon anolis specializes on Anolis lizards, whereas P.cubensis parasitizes Anolis lizards as well as many other species of lizards and snakes. We collected lizards from across the Puerto Rican Bank and St. Croix, sampled them for S.anolis and P.cubensis and generated nuclear and mitochondrial sequence data from the parasites. We used these data to show that P.cubensis is comprised of multiple cryptic species that exhibit limited population structure relative to S.anolis, which is consistent with our prediction based on their host specificity. We also provide evidence that the distribution of P.cubensis species is maintained by competitive exclusion, and in contrast to previous theoretical work, the parasites with the greatest number of host species also reach the highest prevalence rates. Overall, our results are consistent with the hypothesis that host specificity shapes parasite diversification, and suggest that even moderate differences in host specificity may contribute to substantial differences in diversification.

A PARAMETRIC METHOD FOR ASSESSING DIVERSIFICATION-RATE VARIATION IN PHYLOGENETIC TREES

Phylogenetic hypotheses are frequently used to examine variation in rates of diversification across the history of a group. Patterns of diversification-rate variation can be used to infer underlying ecological and evolutionary processes responsible for patterns of cladogenesis. Most existing methods examine rate variation through time. Methods for examining differences in diversification among groups are more limited. Here, we present a new method, parametric rate comparison (PRC), that explicitly compares diversification rates among lineages in a tree using a variety of standard statistical distributions. PRC can identify subclades of the tree where diversification rates are at variance with the remainder of the tree. A randomization test can be used to evaluate how often such variance would appear by chance alone. The method also allows for comparison of diversification rate among a priori defined groups. Further, the application of the PRC method is not restricted to monophyletic groups. We examined the performance of PRC using simulated data, which showed that PRC has acceptable false-positive rates and statistical power to detect rate variation. We apply the PRC method to the well-studied radiation of North American Plethodon salamanders, and support the inference that the large-bodied Plethodon glutinosus clade has a higher historical rate of diversification compared to other Plethodon salamanders.

Quantitative Traits and Diversification

Quantitative traits have long been hypothesized to affect speciation and extinction rates. For example, smaller body size or increased specialization may be associated with increased rates of diversification. Here, I present a phylogenetic likelihood-based method (quantitative state speciation and extinction [QuaSSE]) that can be used to test such hypotheses using extant character distributions. This approach assumes that diversification follows a birth-death process where speciation and extinction rates may vary with one or more traits that evolve under a diffusion model. Speciation and extinction rates may be arbitrary functions of the character state, allowing much flexibility in testing models of trait-dependent diversification. I test the approach using simulated phylogenies and show that a known relationship between speciation and a quantitative character could be recovered in up to 80% of the cases on large trees (500 species). Consistent with other approaches, detecting shifts in diversification due to differences in extinction rates was harder than when due to differences in speciation rates. Finally, I demonstrate the application of QuaSSE to investigate the correlation between body size and diversification in primates, concluding that clade-specific differences in diversification may be more important than size-dependent diversification in shaping the patterns of diversity within this group.

Asset Class Diversification and Delegation of Responsibilities between Central Banks and Sovereign Wealth Funds

This paper presents a model comparing the optimal degree of asset class diversification abroad by a central bank and a sovereign wealth fund. We show that if the central bank manages its foreign asset holdings in order to meet balance of payments needs, particularly in reducing the probability of sudden stops in foreign capital inflows, it will place a high weight on holding safer foreign assets. In contrast, if the sovereign wealth fund, acting on behalf of the Treasury, maximizes the expected utility of a representative domestic agent, it will opt for relatively greater holding of more risky foreign assets. We also show how the diversification differences between the strategies of the bank and SWF is affected by the government's delegation of responsibilities and by various parameters of the economy, such as the volatility of equity returns and the total amount of public foreign assets available for management.

Strong Diversification at the Treeline among Metallura Hummingbirds

The hummingbird genus Metallura comprises nine species. Six of them live at the treeline and replace each other sharply along the eastern slope of the tropical Andes (williami, baroni, odomae, theresiae, eupogon, and aeneocauda). Their ranges overlap for 3,000 km along the Andes with M. tyrianthina, which lives at lower elevations and is differentiated into several subspecies that show clinal variation. The genus also includes M. phoebe in semi- arid western Peru and M. iracunda in the Perija Mountains of the northern Andes. The group could be a good model to study relative differences in diversification between montane forest as such and the narrow transition zone toward the barren highlands. Analysis of nucleotide sequences from three different mitochondrial gene fragments (cytochrome b, ND2, and ND5) show that Metallura forms a monophyletic group whose sister taxon is the genus Chalcostig- ma. The treeline forms of Metallura, including the morphologically divergent M. phoebe, group in a clade sister to M. tyrianthina, confirming the idea that montane forest and treeline forms are sister taxa in a strict sense. Neighboring treeline species show greater morphological and genetic differentiation relative to neighboring montane forest forms. The split between mid- elevation and treeline forms is estimated to have occurred during the Pliocene, suggesting that much of the Metallura radiation took place during the Pleistocene. Received 6 March 1998, accepted 14 December 1998.

Detecting Shifts in Diversification Rates without Fossils

Studies of shifts in diversification rates and adaptive radiations are difficult when there are no fossils because past events cannot be inferred. The phylogenies of recent species, however, allow one to infer the patterns of past diversifications. I present a new method for estimating the diversification rate of a lineage, provided that a phylogeny of recent species, constructed, for instance, with molecular data, is available. This method was inspired by survival models and takes into account species that are not included in detailed phylogenetic data, provided that approximate dates of origin of these species are known. Likelihood ratio tests and Akaike Information Criterion make it possible to test for differences in diversification among lineages or groups of lineages and, thus, to evaluate adaptive radiation hypotheses. The present modeling approach can easily be extended to include temporal variations in diversification rates. A simulation study showed that the method is statistically consistent, avoiding Type I and Type II errors, and that it is robust to periodic or random fluctuations in the speciation rate. An example is presented with a composite phylogeny of primates.

Detecting Shifts in Diversification Rates without Fossils

Studies of shifts in diversification rates and adaptive radiations are difficult when there are no fossils because past events cannot be inferred. The phylogenies of recent species, however, allow one to infer the patterns of past diversifications. I present a new method for estimating the diversification rate of a lineage, provided that a phylogeny of recent species, constructed, for instance, with molecular data, is available. This method was inspired by survival models and takes into account species that are not included in detailed phylogenetic data, provided that approximate dates of origin of these species are known. Likelihood ratio tests and Akaike Information Criterion make it possible to test for differences in diversification among lineages or groups of lineages and, thus, to evaluate adaptive radiation hypotheses. The present modeling approach can easily be extended to include temporal variations in diversification rates. A simulation study showed that the method is statistically consistent, avoiding Type I and Type II errors, and that it is robust to periodic or random fluctuations in the speciation rate. An example is presented with a composite phylogeny of primates.

Evolution of host use in fruit flies of the genus Blepharoneura (Diptera: Tephritidae): cryptic species on sexually dimorphic host plants

Abstract Blepharoneura , a neotropical genus of tephritid fruit flies, includes newly discovered morphologically cryptic species that are highly host specific. Several species of fly, each specific to a different host tissue, can infest a single species of sexually dimorphic host. Using allozyme electrophoresis, we analysed morphologically similar populations of flies reared from various tissues of five species of host plant representing three subtribes of the Cucurbitaceae. In Venezuela, we found three sympatric species of Blepharoneura feeding on Gurania spinulosa Cogn. (one species on male flowers, another on female flowers, and a third on seeds). In Costa Rica, we found four non-interbreeding populations of Blepharoneura feeding on G. costaricensis Cogn.: two sympatric «lowland» populations (one feeding on male flowers and another on female flowers) and two sympatric «highland» populations (also specific to flowers of different sexes). Phylogenetic analyses revealed a species-rich clade of specialists associated with flowers of the Guraniinae, and a species-poor clade of generalists associated with seeds of the Guraniinae.Specialization on male vs. female flowers appears to have occurred repeatedly. Patterns of larval dispersal may underlie differences in diversification of seed vs. flower clades: larvae that feed on seeds are dispersed by bats; larvae feeding on flowers are not dispersed.