Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource

Omnivory of an Insular Lizard: Sources of Variation in the Diet of Podarcis lilfordi (Squamata, Lacertidae)

Through 17 years and from a sample of 7,790 faecal pellets and 26,346 prey items, we studied the diet of the Balearic lizard Podarcis lilfordi in Aire Island (Menorca, Balearic Islands, Spain). We analysed the diet in terms of prey frequencies, as well as by their volume and biomass contributions. The diet of the Balearic lizard was extremely variable through the years, months and areas under study. The dominance of small clumped prey, particularly ants, was confirmed. However, the main contribution by volume corresponded to beetles, with a relevant role for Diplopoda and terrestrial Isopoda during some months and at particular areas of the island. Several prey items were probably captured at the base of shrubs, under stones or inside rock crevices. Therefore, our estimations of electivity would only be reliable for epigeal and flying prey. The capacity of the Balearic lizard to include marine subsidies in its diet, such as coastal crustaceans, is noteworthy. Also, its consumption of carrion from carcasses of gulls and rabbits and leftovers from human visitors is remarkable. Juvenile conspecifics can also be a sporadic food resource, especially during the second half of summer, whereas the consumption of vegetal matter is constant for each whole year. The shifts of vegetal exploitation among areas of the island and months take place according to availability of different plant species at each area or during a given period. Thus, lizards are able to conduct a thorough monitoring of plant phenology, exploiting a large variety of plant species. Omnivory does not imply the indiscriminate inclusion of any edible food in its diet. Rather, the inclusion of several food items means the adoption of a wide range of foraging behaviours adapted to the exploitation of each food resource.

Island colonisation and the evolutionary rates of body size in insular neonate snakes

Island colonisation by animal populations is often associated with dramatic shifts in body size. However, little is known about the rates at which these evolutionary shifts occur, under what precise selective pressures and the putative role played by adaptive plasticity on driving such changes. Isolation time played a significant role in the evolution of body size in island Tiger snake populations, where adaptive phenotypic plasticity followed by genetic assimilation fine-tuned neonate body and head size (hence swallowing performance) to prey size. Here I show that in long isolated islands (>6000 years old) and mainland populations, neonate body mass and snout-vent length are tightly correlated with the average prey body mass available at each site. Regression line equations were used to calculate body size values to match prey size in four recently isolated populations of Tiger snakes. Rates of evolution in body mass and snout-vent length, calculated for seven island snake populations, were significantly correlated with isolation time. Finally, rates of evolution in body mass per generation were significantly correlated with levels of plasticity in head growth rates. This study shows that body size evolution occurs at a faster pace in recently isolated populations and suggests that the level of adaptive plasticity for swallowing abilities may correlate with rates of body mass evolution. I hypothesise that, in the early stages of colonisation, adaptive plasticity and directional selection may combine and generate accelerated evolution towards an 'optimal' phenotype.

Divergence across diet, time and populations rules out parallel evolution in the gut microbiomes of Trinidadian guppies

Diverse microbial consortia profoundly influence animal biology, necessitating an understanding of microbiome variation in studies of animal adaptation. Yet, little is known about such variability among fish, in spite of their importance in aquatic ecosystems. The Trinidadian guppy, Poecilia reticulata, is an intriguing candidate to test microbiome-related hypotheses on the drivers and consequences of animal adaptation, given the recent parallel origins of a similar ecotype across streams. To assess the relationships between the microbiome and host adaptation, we used 16S rRNA amplicon sequencing to characterize gut bacteria of two guppy ecotypes with known divergence in diet, life history, physiology and morphology collected from low-predation (LP) and high-predation (HP) habitats in four Trinidadian streams. Guts were populated by several recurring, core bacteria that are related to other fish associates and rarely detected in the environment. Although gut communities of lab-reared guppies differed from those in the wild, microbiome divergence between ecotypes from the same stream was evident under identical rearing conditions, suggesting host genetic divergence can affect associations with gut bacteria. In the field, gut communities varied over time, across streams and between ecotypes in a stream-specific manner. This latter finding, along with PICRUSt predictions of metagenome function, argues against strong parallelism of the gut microbiome in association with LP ecotype evolution. Thus, bacteria cannot be invoked in facilitating the heightened reliance of LP guppies on lower-quality diets. We argue that the macroevolutionary microbiome convergence seen across animals with similar diets may be a signature of secondary microbial shifts arising some time after host-driven adaptation.

Potential rapid evolution of foot morphology in Italian plethodontid salamanders (Hydromantes strinatii) following the colonization of an artificial cave

How organisms respond to environmental change is a long-standing question in evolutionary biology. Species invading novel habitats provide an opportunity to examine contemporary evolution in action and decipher the pace of evolutionary change over short timescales. Here, we characterized phenotypic evolution in the Italian plethodontid salamander, Hydromantes strinatii, following the recent colonization of an artificial cave by a forest floor population. When compared with a nearby and genetically related population in the natural forest floor and a nearby cave population, the artificial cave population displayed significant differences in overall foot shape, with more interdigital webbing relative to the other populations. Further, this population evolved significantly larger feet, which corresponded more closely to those found in other cave populations than to forest floor populations to which the cave population is closely related. Finally, we quantified the rate of evolution for both foot shape and foot area, and found that both traits displayed large and significant evolutionary rates, at levels corresponding to other classic cases of rapid evolution in vertebrates. Together, these findings reveal that the response to novel environmental pressures can be large and rapid and that the anatomical shifts observed in the artificial cave population of H. strinatii may represent a case of rapid evolution in response to novel environmental pressures.

Evolution of herbivory in Drosophilidae linked to loss of behaviors, antennal responses, odorant receptors, and ancestral diet

Herbivory is a key innovation in insects, yet has only evolved in one-third of living orders. The evolution of herbivory likely involves major behavioral changes mediated by remodeling of canonical chemosensory modules. Herbivorous flies in the genus Scaptomyza (Drosophilidae) are compelling species in which to study the genomic architecture linked to the transition to herbivory because they recently evolved from microbe-feeding ancestors and are closely related to Drosophila melanogaster. We found that Scaptomyza flava, a leaf-mining specialist on plants in the family (Brassicaceae), was not attracted to yeast volatiles in a four-field olfactometer assay, whereas D. melanogaster was strongly attracted to these volatiles. Yeast-associated volatiles, especially short-chain aliphatic esters, elicited strong antennal responses in D. melanogaster, but weak antennal responses in electroantennographic recordings from S. flava. We sequenced the genome of S. flava and characterized this species' odorant receptor repertoire. Orthologs of odorant receptors, which detect yeast volatiles in D. melanogaster and mediate critical host-choice behavior, were deleted or pseudogenized in the genome of S. flava. These genes were lost step-wise during the evolution of Scaptomyza. Additionally, Scaptomyza has experienced gene duplication and likely positive selection in paralogs of Or67b in D. melanogaster. Olfactory sensory neurons expressing Or67b are sensitive to green-leaf volatiles. Major trophic shifts in insects are associated with chemoreceptor gene loss as recently evolved ecologies shape sensory repertoires.

Reproductive patterns of European pond turtles differ between sites: a small scale scenario

From 1996 to 2002, we studied the body size, measures of reproductive strategy (relative clutch mass and delayed reproduction at sexual maturity), and reproductive output (clutch frequency and annual egg production) of female European Pond turtles,Emys orbicularis, at two sites separated by 12 km in central Mediterranean Tuscany (San Rossore and Camp Darby, central northern Italy). Females did not reproduce at the first appearance of external sexual characters, but reproduced at larger sizes, probably as older turtles. Among years, reproductive females were more common than were non-reproductive females, yet both groups had similar body sizes. Body size (carapace length and width, plastron length and width, shell height and body mass) varied between localities and among years. Body size differed between reproductive and non reproductive females in Camp Darby, but not in San Rossore females. Shell volume did not vary among years, nor between localities, nor between reproductive status. Reproductive females had higher body condition indices (BCI) than did non-reproductive females, while BCI did not differ between females laying one clutch and females laying multiple clutches. Clutch size did not vary among years. One clutch per year was much more frequent than multiple clutches, and multiple clutches were more frequent in Camp Darby than in San Rossore females, likely due to differences in population structures between sites.

Geographical variation in body size and sexual size dimorphism in an Australian lizard, Boulenger's Skink (Morethia boulengeri)

Ecogeographical rules help explain spatial and temporal patterns in intraspecific body size. However, many of these rules, when applied to ectothermic organisms such as reptiles, are controversial and require further investigation. To explore factors that influence body size in reptiles, we performed a heuristic study to examine body size variation in an Australian lizard, Boulenger's Skink Morethia boulengeri from agricultural landscapes in southern New South Wales, south-eastern Australia. We collected tissue and morphological data on 337 adult lizards across a broad elevation and climate gradient. We used a model-selection procedure to determine if environmental or ecological variables best explained body size variation. We explored the relationship between morphology and phylogenetic structure before modeling candidate variables from four broad domains: (1) geography (latitude, longitude and elevation), (2) climate (temperature and rainfall), (3) habitat (vegetation type, number of logs and ground cover attributes), and (4) management (land use and grazing history). Broad phylogenetic structure was evident, but on a scale larger than our study area. Lizards were sexually dimorphic, whereby females had longer snout-vent length than males, providing support for the fecundity selection hypothesis. Body size variation in M. boulengeri was correlated with temperature and rainfall, a pattern consistent with larger individuals occupying cooler and more productive parts of the landscape. Climate change forecasts, which predict warmer temperature and increased aridity, may result in reduced lizard biomass and decoupling of trophic interactions with potential implications for community organization and ecosystem function.

Geographical and temporal body size variation in a reptile: roles of sex, ecology, phylogeny and ecology structured in phylogeny

Geographical body size variation has long interested evolutionary biologists, and a range of mechanisms have been proposed to explain the observed patterns. It is considered to be more puzzling in ectotherms than in endotherms, and integrative approaches are necessary for testing non-exclusive alternative mechanisms. Using lacertid lizards as a model, we adopted an integrative approach, testing different hypotheses for both sexes while incorporating temporal, spatial, and phylogenetic autocorrelation at the individual level. We used data on the Spanish Sand Racer species group from a field survey to disentangle different sources of body size variation through environmental and individual genetic data, while accounting for temporal and spatial autocorrelation. A variation partitioning method was applied to separate independent and shared components of ecology and phylogeny, and estimated their significance. Then, we fed-back our models by controlling for relevant independent components. The pattern was consistent with the geographical Bergmann's cline and the experimental temperature-size rule: adults were larger at lower temperatures (and/or higher elevations). This result was confirmed with additional multi-year independent data-set derived from the literature. Variation partitioning showed no sex differences in phylogenetic inertia but showed sex differences in the independent component of ecology; primarily due to growth differences. Interestingly, only after controlling for independent components did primary productivity also emerge as an important predictor explaining size variation in both sexes. This study highlights the importance of integrating individual-based genetic information, relevant ecological parameters, and temporal and spatial autocorrelation in sex-specific models to detect potentially important hidden effects. Our individual-based approach devoted to extract and control for independent components was useful to reveal hidden effects linked with alternative non-exclusive hypothesis, such as those of primary productivity. Also, including measurement date allowed disentangling and controlling for short-term temporal autocorrelation reflecting sex-specific growth plasticity.

Transposons, environmental changes, and heritable induced phenotypic variability

The mechanisms of biological evolution have always been, and still are, the subject of intense debate and modeling. One of the main problems is how the genetic variability is produced and maintained in order to make the organisms adaptable to environmental changes and therefore capable of evolving. In recent years, it has been reported that, in flies and plants, mutations in Hsp90 gene are capable to induce, with a low frequency, many different developmental abnormalities depending on the genetic backgrounds. This has suggested that the reduction of Hsp90 amount makes different development pathways more sensitive to hidden genetic variability. This suggestion revitalized a classical debate around the original Waddington hypothesis of canalization and genetic assimilation making Hsp90 the prototype of morphological capacitor. Other data have also suggested a different mechanism that revitalizes another classic debate about the response of genome to physiological and environmental stress put forward by Barbara McClintock. That data demonstrated that Hsp90 is involved in repression of transposon activity by playing a significant role in piwi-interacting RNA (piRNAs)-dependent RNA interference (RNAi) silencing. The important implication is that the fixed phenotypic abnormalities observed in Hsp90 mutants are probably related to de novo induced mutations by transposon activation. In this case, Hsp90 could be considered as a mutator. In the present theoretical paper, we discuss several possible implications about environmental stress, transposon, and evolution offering also a support to the concept of evolvability.

Trophic divergence despite morphological convergence in a continental radiation of snakes

Ecological and phenotypic convergence is a potential outcome of adaptive radiation in response to ecological opportunity. However, a number of factors may limit convergence during evolutionary radiations, including interregional differences in biogeographic history and clade-specific constraints on form and function. Here, we demonstrate that a single clade of terrestrial snakes from Australia--the oxyuranine elapids--exhibits widespread morphological convergence with a phylogenetically diverse and distantly related assemblage of snakes from North America. Australian elapids have evolved nearly the full spectrum of phenotypic modalities that occurs among North American snakes. Much of the convergence appears to involve the recurrent evolution of stereotyped morphologies associated with foraging mode, locomotion and habitat use. By contrast, analysis of snake diets indicates striking divergence in feeding ecology between these faunas, partially reflecting regional differences in ecological allometry between Australia and North America. Widespread phenotypic convergence with the North American snake fauna coupled with divergence in feeding ecology are clear examples of how independent continental radiations may converge along some ecological axes yet differ profoundly along others.

Comparative digestive physiology

In vertebrates and invertebrates, morphological and functional features of gastrointestinal (GI) tracts generally reflect food chemistry, such as content of carbohydrates, proteins, fats, and material(s) refractory to rapid digestion (e.g., cellulose). The expression of digestive enzymes and nutrient transporters approximately matches the dietary load of their respective substrates, with relatively modest excess capacity. Mechanisms explaining differences in hydrolase activity between populations and species include gene copy number variations and single-nucleotide polymorphisms. Transcriptional and posttranscriptional adjustments mediate phenotypic changes in the expression of hydrolases and transporters in response to dietary signals. Many species respond to higher food intake by flexibly increasing digestive compartment size. Fermentative processes by symbiotic microorganisms are important for cellulose degradation but are relatively slow, so animals that rely on those processes typically possess special enlarged compartment(s) to maintain a microbiota and other GI structures that slow digesta flow. The taxon richness of the gut microbiota, usually identified by 16S rRNA gene sequencing, is typically an order of magnitude greater in vertebrates than invertebrates, and the interspecific variation in microbial composition is strongly influenced by diet. Many of the nutrient transporters are orthologous across different animal phyla, though functional details may vary (e.g., glucose and amino acid transport with K+ rather than Na+ as a counter ion). Paracellular absorption is important in many birds. Natural toxins are ubiquitous in foods and may influence key features such as digesta transit, enzymatic breakdown, microbial fermentation, and absorption.

Spatial phenotypic and genetic structure of threespine stickleback (Gasterosteus aculeatus) in a heterogeneous natural system, Lake Mývatn, Iceland

Eco-evolutionary responses of natural populations to spatial environmental variation strongly depend on the relative strength of environmental differences/natural selection and dispersal/gene flow. In absence of geographic barriers, as often is the case in lake ecosystems, gene flow is expected to constrain adaptive divergence between environments – favoring phenotypic plasticity or high trait variability. However, if divergent natural selection is sufficiently strong, adaptive divergence can occur in face of gene flow. The extent of divergence is most often studied between two contrasting environments, whereas potential for multimodal divergence is little explored. We investigated phenotypic (body size, defensive structures, and feeding morphology) and genetic (microsatellites) structure in threespine stickleback (Gasterosteus aculeatus) across five habitat types and two basins (North and South) within the geologically young and highly heterogeneous Lake Mývatn, North East Iceland. We found that (1) North basin stickleback were, on average, larger and had relatively longer spines than South basin stickleback, whereas (2) feeding morphology (gill raker number and gill raker gap width) differed among three of five habitat types, and (3) there was only subtle genetic differentiation across the lake. Overall, our results indicate predator and prey mediated phenotypic divergence across multiple habitats in the lake, in face of gene flow.

Ontogenetic development of intestinal length and relationships to diet in an Australasian fish family (Terapontidae)

BACKGROUND

One of the most widely accepted ecomorphological relationships in vertebrates is the negative correlation between intestinal length and proportion of animal prey in diet. While many fish groups exhibit this general pattern, other clades demonstrate minimal, and in some cases contrasting, associations between diet and intestinal length. Moreover, this relationship and its evolutionary derivation have received little attention from a phylogenetic perspective. This study documents the phylogenetic development of intestinal length variability, and resultant correlation with dietary habits, within a molecular phylogeny of 28 species of terapontid fishes. The Terapontidae (grunters), an ancestrally euryhaline-marine group, is the most trophically diverse of Australia's freshwater fish families, with widespread shifts away from animal-prey-dominated diets occurring since their invasion of fresh waters.

RESULTS

Description of ontogenetic development of intestinal complexity of terapontid fishes, in combination with ancestral character state reconstruction, demonstrated that complex intestinal looping (convolution) has evolved independently on multiple occasions within the family. This modification of ontogenetic development drives much of the associated interspecific variability in intestinal length evident in terapontids. Phylogenetically informed comparative analyses (phylogenetic independent contrasts) showed that the interspecific differences in intestinal length resulting from these ontogenetic developmental mechanisms explained ~65% of the variability in the proportion of animal material in terapontid diets.

CONCLUSIONS

The ontogenetic development of intestinal complexity appears to represent an important functional innovation underlying the extensive trophic differentiation seen in Australia's freshwater terapontids, specifically facilitating the pronounced shifts away from carnivorous (including invertebrates and vertebrates) diets evident across the family. The capacity to modify intestinal morphology and physiology may also be an important facilitator of trophic diversification during other phyletic radiations.

Evidence of adaptation from ancestral variation in young populations of beach mice

To understand how organisms adapt to novel habitats, which involves both demographic and selective events, we require knowledge of the evolutionary history of populations and also selected alleles. There are still few cases in which the precise mutations (and hence, defined alleles) that contribute to adaptive change have been identified in nature; one exception is the genetic basis of camouflaging pigmentation of oldfield mice (Peromyscus polionotus) that have colonized the sandy dunes of Florida's Gulf Coast. To quantify the genomic impact of colonization as well as the signature of selection, we resequenced 5000 1.5-kb noncoding loci as well as a 160-kb genomic region surrounding the melanocortin-1 receptor (Mc1r), a gene that contributes to pigmentation differences, in beach and mainland populations. Using a genome-wide phylogenetic approach, we recovered a single monophyletic group comprised of beach mice, consistent with a single colonization event of the Gulf Coast. We also found evidence of a severe founder event, estimated to have occurred less than 3000 years ago. In this demographic context, we show that all beach subspecies share a single derived light Mc1r allele, which was likely selected from standing genetic variation that originated in the mainland. Surprisingly, we were unable to identify a clear signature of selection in the Mc1r region, despite independent evidence that this locus contributes to adaptive coloration. Nonetheless, these data allow us to reconstruct and compare the evolutionary history of populations and alleles to better understand how adaptive evolution, following the colonization of a novel habitat, proceeds in nature.

Escherichia coli ATCC 8739 Adapts to the Presence of Sodium Chloride, Monosodium Glutamate, and Benzoic Acid after Extended Culture

Escherichia coli is commonly found in intestine of human, and any changes in their adaptation or evolution may affect the human body. The relationship between E. coli and food additives is less studied as compared to antibiotics. E. coli within our human gut are consistently interacting with the food additives; thus, it is important to investigate this relationship. In this paper, we observed the evolution of E. coli cultured in different concentration of food additives (sodium chloride, benzoic acid, and monosodium glutamate), singly or in combination, over 70 passages. Adaptability over time was estimated by generation time and cell density at stationary phase. Polymerase chain reaction (PCR)/restriction fragments length polymorphism (RFLP) using 3 primers and restriction endonucleases, each was used to characterize adaptation/evolution at genomic level. The amplification and digestion profiles were tabulated and analyzed by Nei-Li dissimilarity index. Our results demonstrate that E. coli in every treatment had adapted over 465 generations. The types of stress were discovered to be different even though different concentrations of same additives were used. However, RFLP shows a convergence of genetic distances, suggesting the presence of global stress response. In addition, monosodium glutamate may be a nutrient source and support acid resistance in E. coli.

Rapid changes in phenotype distribution during range expansion in a migratory bird

The capacity of species to track changing environmental conditions is a key component of population and range changes in response to environmental change. High levels of local adaptation may constrain expansion into new locations, while the relative fitness of dispersing individuals will influence subsequent population growth. However, opportunities to explore such processes are rare, particularly at scales relevant to species-based conservation strategies. Icelandic black-tailed godwits, Limosa limosa islandica, have expanded their range throughout Iceland over the last century. We show that current male morphology varies strongly in relation to the timing of colonization across Iceland, with small males being absent from recently occupied areas. Smaller males are also proportionately more abundant on habitats and sites with higher breeding success and relative abundance of females. This population-wide spatial structuring of male morphology is most likely to result from female preferences for small males and better-quality habitats increasing both small-male fitness and the dispersal probability of larger males into poorer-quality habitats. Such eco-evolutionary feedbacks may be a key driver of rates of population growth and range expansion and contraction.

Theories of Biodiversity Value

The discussion of the calculus of biodiversity values in Chap. 5 is the final requisite prop for the stage across which this chapter parades accounts of how biodiversity is supposed to be valuable. The selection of these accounts is generous and represents a great majority of discussions on this topic, though it is not all-inclusive. It does include, among other theories, the theory that biodiversity is valuable for “unspecified moral reasons” (Sect. 6.1), that it is valuable as a resource (Sect. 6.2), as a service provider (Sect. 6.3), as a sustainer of human life (Sect. 6.4), as a key to human health, both as pharmacopoeia and as an inoculation against infection (Sect. 6.5), as the progenitor of human biophilic tendencies (Sect. 6.6), as a generator of (more) value (Sect. 6.7), as font of knowledge (Sect. 6.8), as having option (and quasi-option) value (Sect. 6.9), as transformative (Sect. 6.10), as having experiential value (Sect. 6.11), and as an expression of the natural order (Sect. 6.12). The chapter concludes (Sect. 6.13) with some proposals that are not customarily offered as self-contained theories of biodiversity value, but that are often suggested as still heavily influencing evaluation. These include what conservation biologists call “viability” and “endangerment” as well as conservation “efficiency”. Section 6.13 concludes with an extended discussion of the significance for biodiversity’s value of the history of human impingements on the natural world.

Flexibility in locomotor-feeding integration during prey capture in varanid lizards: effects of prey size and velocity

Feeding movements are adjusted in response to food properties, and this flexibility is essential for omnivorous predators as food properties vary routinely. In most lizards, prey capture is no longer considered to solely rely on the movements of the feeding structures (jaws, hyolingual apparatus), but instead is understood to require the integration of the feeding system with the locomotor system (i.e., coordination of movements). Here, we investigate flexibility in the coordination pattern between jaw, neck and forelimb movements in omnivorous varanid lizards feeding on four prey types varying in length and mobility: grasshoppers, live newborn mice, adult mice and dead adult mice. We test for bivariate correlations between 3D locomotor and feeding kinematics, and compare the jaw-neck-forelimb coordination patterns across prey types. Our results reveal that locomotor-feeding integration is essential for the capture of evasive prey, and that different jaw-neck-forelimb coordination patterns are used to capture different prey types. Jaw-neck-forelimb coordination is indeed significantly altered by the length and speed of the prey, indicating that a similar coordination pattern can be finely tuned in response to prey stimuli. These results suggest feed-forward as well as feedback modulation of the control of locomotor-feeding integration. As varanids are considered to be specialized in the capture of evasive prey (although they retain their ability to feed on a wide variety of prey items), flexibility in locomotor-feeding integration in response to prey mobility is proposed to be a key component in their dietary specialization.

What invasive species reveal about the rate and form of contemporary phenotypic change in nature

Biological invasions are opportunities to gain insight into fundamental evolutionary questions, because reproductive isolation and sudden alterations in selection pressures are likely to lead to rapid evolutionary change. Here I investigate the role played by invasive species in revealing the rate and form of contemporary phenotypic change in wild populations by expanding a database of more than 5,500 rates of phenotypic change from 90 species of plants and animals. Invasive species are frequently used as model organisms and thus contribute disproportionately to available rates of phenotypic change. However, the preponderance of these rates is the consequence of extensive study in a small number of species. I found mixed evidence to support the hypothesis that phenotypic change is associated with time depending on the metric of choice (i.e., darwins or haldanes). Insights from both invasive and native species provide evidence for abrupt phenotypic change and suggest that the environment plays a potentially important role in driving trait change in wild populations, although the environmental influence on the observed trajectories remains unclear. Thus, future work should continue to seek an understanding of the mechanistic underpinnings--both genetic and environmental--of how phenotypic variation allows populations to adapt to rapidly changing global environments.

Finite element analysis of performance in the skulls of marmosets and tamarins

Reliance on plant exudates is a relatively rare dietary specialization among mammals. One well-studied example of closely related exudate feeders is the New World marmosets and tamarins. Whereas marmosets actively gouge tree bark with their incisors to stimulate the flow of sap, tamarins are opportunistic exudate feeders that do not gouge bark. Several studies of the dentaries and jaw adductors indicate that marmosets exhibit specializations for increased gape at the expense of bite force. Few studies, however, have looked to the cranium of marmosets for evidence of functional specializations. Using 3D finite element models of the marmoset Callithrix jacchus and the tamarin Saguinus fuscicollis, we investigated the performance of the cranium under loading regimes that mimicked unilateral molar biting and bark-gouging. We investigated three measures of performance: the efficiency with which muscle force is transferred to bite force, the extent to which the models are stressed (a predictor of failure), and the work expended by muscles as they deform the skull (total strain energy). We found that during molar biting the two models exhibited similar levels of performance, though the Saguinus model had slightly higher mechanical efficiency, a slightly lower state of stress, and expended more energy on deformation. In contrast, under the bark-gouging load, Callithrix exhibited much higher mechanical efficiency than Saguinas, but did so at the expense of more work and higher levels of von Mises stress. This analysis illustrates that differences in the shapes of the skulls of Callithrix and Saguinus confer differences in performance. Whether these aspects of performance are targets of selection awaits broader comparative analyses.

Character displacement and the origins of diversity

In The Origin of Species, Darwin proposed his principle of divergence of character (a process now termed "character displacement") to explain how new species arise and why they differ from each other phenotypically. Darwin maintained that the origin of species and the evolution of differences between them is ultimately caused by divergent selection acting to minimize competitive interactions between initially similar individuals, populations, and species. Here, we examine the empirical support for the various claims that constitute Darwin's principle, specifically that (1) competition promotes divergent trait evolution, (2) the strength of competitively mediated divergent selection increases with increasing phenotypic similarity between competitors, (3) divergence can occur within species, and (4) competitively mediated divergence can trigger speciation. We also explore aspects that Darwin failed to consider. In particular, we describe how (1) divergence can arise from selection acting to lessen reproductive interactions, (2) divergence is fueled by the intersection of character displacement and sexual selection, and (3) phenotypic plasticity may play a key role in promoting character displacement. Generally, character displacement is well supported empirically, and it remains a vital explanation for how new species arise and diversify.

Genetic divergence in morphology-performance mapping between Misty Lake and inlet stickleback

Different environments should select for different aspects of organismal performance, which should lead to correlated divergence in morphological traits that influence performance. The result should be genetic divergence in aspects of performance, morphology and associations ('maps') between morphology and performance. Testing this hypothesis requires quantifying performance and morphology in multiple populations after controlling for environmental differences, but this is rarely attempted. We used a common-garden experiment to examine morphology and several aspects of swimming performance within and between the lake and inlet populations of threespine stickleback (Gasterosteus aculeatus) from the Misty system, Vancouver Island, Canada. Controlling for body size, lake stickleback had shallower bodies, larger caudal fins and smaller pelvic girdles. With or without morphological covariates, lake stickleback showed greater performance in both sustained and burst swimming. In contrast, inlet stickleback showed greater manoeuverability than did lake stickleback in some analyses. Morphology-performance relationships were decoupled when considering variation within vs. between populations. Moreover, morphology-performance mapping differed between the two populations. Based on these observations, we advance a hypothesis for why populations adapting to different environments should show adaptive genetic divergence in morphology-performance mapping.