Feeding ecology has a stronger evolutionary influence on functional morphology than on body mass in mammals

Ecological specialization is a central driver of adaptive evolution. However, selective pressures may uniquely affect different ecomorphological traits (e.g., size and shape), complicating efforts to investigate the role of ecology in generating phenotypic diversity. Comparative studies can help remedy this issue by identifying specific relationships between ecologies and morphologies, thus elucidating functionally relevant traits. Jaw shape is a dietary correlate that offers considerable insight on mammalian evolution, but few studies have examined the influence of diet on jaw morphology across mammals. To this end, I apply phylogenetic comparative methods to mandibular measurements and dietary data for a diverse sample of mammals. Especially powerful predictors of diet are metrics that capture either the size of the angular process, which increases with greater herbivory, or the length of the posterior portion of the jaw, which decreases with greater herbivory. The size of the angular process likely reflects sizes of attached muscles that produce jaw movements needed to grind plant material. Further, I examine the impact of feeding ecology on body mass, an oft-used ecological surrogate in macroevolutionary studies. Although body mass commonly increases with evolutionary shifts to herbivory, it is outperformed by functional jaw morphology as a predictor of diet. Body mass is influenced by numerous factors beyond diet, and it may be evolutionarily labile relative to functional morphologies. This suggests that ecological diversification events may initially facilitate body mass diversification at smaller taxonomic and temporal scales, but sustained selective pressures will subsequently drive greater trait partitioning in functional morphologies.     

Adaptive processes drive ecomorphological convergent evolution in antwrens (Thamnophilidae)

Phylogenetic niche conservatism (PNC) and convergence are contrasting evolutionary patterns that describe phenotypic similarity across independent lineages. Assessing whether and how adaptive processes give origin to these patterns represent a fundamental step toward understanding phenotypic evolution. Phylogenetic model‐based approaches offer the opportunity not only to distinguish between PNC and convergence, but also to determine the extent that adaptive processes explain phenotypic similarity. The Myrmotherula complex in the Neotropical family Thamnophilidae is a polyphyletic group of sexually dimorphic small insectivorous forest birds that are relatively homogeneous in size and shape. Here, we integrate a comprehensive species‐level molecular phylogeny of the Myrmotherula complex with morphometric and ecological data within a comparative framework to test whether phenotypic similarity is described by a pattern of PNC or convergence, and to identify evolutionary mechanisms underlying body size and shape evolution. We show that antwrens in the Myrmotherula complex represent distantly related clades that exhibit adaptive convergent evolution in body size and divergent evolution in body shape. Phenotypic similarity in the group is primarily driven by their tendency to converge toward smaller body sizes. Differences in body size and shape across lineages are associated to ecological and behavioral factors.     

PHYLOGENETIC,ECOLOGICAL, AND ALLOMETRIC CORRELATES OF CRANIAL SHAPE IN MALAGASY LEMURIFORMS

Adaptive radiations provide important insights into many aspects of evolution, including the relationship between ecology and morphological diversification as well as between ecology and speciation. Many such radiations include divergence along a dietary axis, although other ecological variables may also drive diversification, including differences in diel activity patterns. This study examines the role of two key ecological variables, diet and activity patterns, in shaping the radiation of a diverse clade of primates, the Malagasy lemurs. When phylogeny was ignored, activity pattern and several dietary variables predicted a significant proportion of cranial shape variation. However, when phylogeny was taken into account, only typical diet accounted for a significant proportion of shape variation. One possible explanation for this discrepancy is that this radiation was characterized by a relatively small number of dietary shifts (and possibly changes in body size) that occurred in conjunction with the divergence of major clades. This pattern may be difficult to detect with the phylogenetic comparative methods used here, but may characterize not just lemurs but other mammals.     

Developing flow–ecology relationships: Implications of nonlinear biological responses for water management

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Adaptive landscape and functional diversity of Neotropical cichlids: implications for the ecology and evolution of Cichlinae (Cichlidae; Cichliformes)

Morphological, lineage and ecological diversity can vary substantially even among closely related lineages. Factors that influence morphological diversification, especially in functionally relevant traits, can help to explain the modern distribution of disparity across phylogenies and communities. Multivariate axes of feeding functional morphology from 75 species of Neotropical cichlid and a stepwise‐AIC algorithm were used to estimate the adaptive landscape of functional morphospace in Cichlinae. Adaptive landscape complexity and convergence, as well as the functional diversity of Cichlinae, were compared with expectations under null evolutionary models. Neotropical cichlid feeding function varied primarily between traits associated with ram feeding vs. suction feeding/biting and secondarily with oral jaw muscle size and pharyngeal crushing capacity. The number of changes in selective regimes and the amount of convergence between lineages was higher than expected under a null model of evolution, but convergence was not higher than expected under a similarly complex adaptive landscape. Functional disparity was compatible with an adaptive landscape model, whereas the distribution of evolutionary change through morphospace corresponded with a process of evolution towards a single adaptive peak. The continentally distributed Neotropical cichlids have evolved relatively rapidly towards a number of adaptive peaks in functional trait space. Selection in Cichlinae functional morphospace is more complex than expected under null evolutionary models. The complexity of selective constraints in feeding morphology has likely been a significant contributor to the diversity of feeding ecology in this clade.     

Range edges in heterogeneous landscapes: Integrating geographic scale and climate complexity into range dynamics

The impacts of climate change have re‐energized interest in understanding the role of climate in setting species geographic range edges. Despite the strong focus on species' distributions in ecology and evolution, defining a species range edge is theoretically and empirically difficult. The challenge of determining a range edge and its relationship to climate is in part driven by the nested nature of geography and the multidimensionality of climate, which together generate complex patterns of both climate and biotic distributions across landscapes. Because range‐limiting processes occur in both geographic and climate space, the relationship between these two spaces plays a critical role in setting range limits. With both conceptual and empirical support, we argue that three factors—climate heterogeneity, collinearity among climate variables, and spatial scale—interact to shape the spatial structure of range edges along climate gradients, and we discuss several ways that these factors influence the stability of species range edges with a changing climate. We demonstrate that geographic and climate edges are often not concordant across species ranges. Furthermore, high climate heterogeneity and low climate collinearity across landscapes increase the spectrum of possible relationships between geographic and climatic space, suggesting that geographic range edges and climatic niche limits correspond less frequently than we may expect. More empirical explorations of how the complexity of real landscapes shapes the ecological and evolutionary processes that determine species range edges will advance the development of range limit theory and its applications to biodiversity conservation in the context of changing climate.     

Allometric escape from acoustic constraints is rare for frog calls

Allometric constraint is a product of natural selection and physical laws, particularly with respect to body size and traits constrained by properties thereof, such as metabolism, longevity, and vocal frequency. Allometric relationships are often conserved across lineages, indicating that physical constraints dictate scaling patterns in deep time, despite substantial genetic and ecological divergence among organisms. In particular, acoustic allometry (sound frequency ~ body size) is conserved across frogs, in defiance of massive variation in both body size and frequency. Here, we ask how many instances of allometric escape have occurred across the frog tree of life using a Bayesian framework that estimates the location, number, and magnitude of shifts in the adaptive landscape of acoustic allometry. Moreover, we test whether ecology in terms of calling site could affect these relationships. We find that calling site has a major influence on acoustic allometry. Despite this, we identify only four major instances of allometric escape, potentially deriving from ecomorphological adaptations to new signal modalities. In these instances of allometric escape, the optima and strength of the scaling relationship are different than expected for most other frog species, representing new adaptive regimes of body size ~ call frequency. Allometric constraints on frog calls are highly conserved and have rarely allowed escape, despite frequent invasions of new adaptive regimes and dramatic ecomorphological divergence. Our results highlight the rare instances in which natural and sexual selection combined can overcome physical constraints on sound production.     

Ecomorphological convergence in planktivorous surgeonfishes

Morphological convergence plays a central role in the study of evolution. Often induced by shared ecological specialization, homoplasy hints at underlying selective pressures and adaptive constraints that deterministically shape the diversification of life. Although midwater zooplanktivory has arisen in adult surgeonfishes (family Acanthuridae) at least four independent times, it represents a clearly specialized state, requiring the capacity to swiftly swim in midwater locating and sucking small prey items. Whereas this diet has commonly been associated with specific functional adaptations in fishes, acanthurids present an interesting case study as all nonplanktivorous species feed by grazing on benthic algae and detritus, requiring a vastly different functional morphology that emphasizes biting behaviours. We examined the feeding morphology in 30 acanthurid species and, combined with a pre‐existing phylogenetic tree, compared the fit of evolutionary models across two diet regimes: zooplanktivores and nonzooplanktivorous grazers. Accounting for phylogenetic relationships, the best‐fitting model indicates that zooplanktivorous species are converging on a separate adaptive peak from their grazing relatives. Driving this bimodal landscape, zooplanktivorous acanthurids tend to develop a slender body, reduced facial features, smaller teeth and weakened jaw adductor muscles. However, despite these phenotypic changes, model fitting suggests that lineages have not yet reached the adaptive peak associated with plankton feeding even though some transitions appear to be over 10 million years old. These findings demonstrate that the selective demands of pelagic feeding promote repeated – albeit very gradual – ecomorphological convergence within surgeonfishes, while allowing local divergences between closely related species, contributing to the overall diversity of the clade.     

Functional performance of turtle humerus shape across an ecological adaptive landscape

The concept of the adaptive landscape has been invaluable to evolutionary biologists for visualizing the dynamics of selection and adaptation, and is increasingly being used to study morpho‐functional data. Here, we construct adaptive landscapes to explore functional trade‐offs associated with variation in humerus morphology among turtles adapted to three different locomotor environments: marine, semiaquatic, and terrestrial. Humerus shape from 40 species of cryptodire turtles was quantified using a pseudolandmark approach. Hypothetical shapes were extracted in a grid across morphospace and four functional traits (strength, stride length, mechanical advantage, and hydrodynamics) measured on those shapes. Quantitative trait modeling was used to construct adaptive landscapes that optimize the functional traits for each of the three locomotor ecologies. Our data show that turtles living in different environments have statistically different humeral shapes. The optimum adaptive landscape for each ecology is defined by a different combination of performance trade‐offs, with turtle species clustering around their respective adaptive peak. Further, species adhere to pareto fronts between marine–semiaquatic and semiaquatic–terrestrial optima, but not between marine–terrestrial. Our study demonstrates the utility of adaptive landscapes in informing the link between form, function, and ecological adaptation, and establishes a framework for reconstructing turtle ecological evolution using isolated humeri from the fossil record.     

Adaptation and habitat selection in the eco-evolutionary process

The struggle for existence occurs through the vital rates of population growth. This basic fact demonstrates the tight connection between ecology and evolution that defines the emerging field of eco-evolutionary dynamics. An effective synthesis of the interdependencies between ecology and evolution is grounded in six principles. The mechanics of evolution specifies the origin and rules governing traits and evolutionary strategies. Traits and evolutionary strategies achieve their selective value through their functional relationships with fitness. Function depends on the underlying structure of variation and the temporal, spatial and organizational scales of evolution. An understanding of how changes in traits and strategies occur requires conjoining ecological and evolutionary dynamics. Adaptation merges these five pillars to achieve a comprehensive understanding of ecological and evolutionary change. I demonstrate the value of this world-view with reference to the theory and practice of habitat selection. The theory allows us to assess evolutionarily stable strategies and states of habitat selection, and to draw the adaptive landscapes for habitat-selecting species. The landscapes can then be used to forecast future evolution under a variety of climate change and other scenarios.     

Ecology of a shell convergence between subfamilies of polygyrid land snails

The shell convergence between Neohelix dentifera and Inflectarius ferrissi is an important key to understanding the unusual evolutionary pattern of polygyrid land snails in eastern North America. This group is characterized by conchologically distinct shell-static clades that have radiated ecologically to the extent that shell shape and habitat are not detectably correlated at the species level; furthermore, entire shell-static clades converge conchologically between anatomically distinct lineages (triodopsines and polygyrines). The triodopsine N. dentifera and the polygyrine I. ferrissi both represent unique, drastic shifts from their shell-static clades, and thus are possible founders of new shell-static clades. This study shows that, within a context of high intraspecific variance, N. dentifera and I. ferrissi exhibit ecological parallelism or convergence in addition to conchological convergence. In coarse-grained ecology, both are more associated with acidic, anionic, deep, low- density soils in talus in high-altitude, exposed non-oak-hickory forests than are their close relatives, even though the two are not very close to each other. In fine-grained ecology, N. dentifera converges on I. ferrissi by increased rock-association relative to outgroups, and I. ferrissi likewise converges on N. dentifera by decreased log association. Thus the shell shape unique to these two species appears to be adaptive, whereas their intraspecific ecological variances appear high enough to permit ecological radiation.     

Convergence, divergence, and homogenization in the ecological structure of emydid turtle communities: the effects of phylogeny and dispersal

Studies that have explored the origins of patterns of community structure from a phylogenetic perspective have generally found either convergence (similarity) in community structure between regions through adaptive evolution or lack of convergence (dissimilarity) due to phylogenetic conservatism in the divergent ecological characteristics of lineages inhabiting different regions. We used a phylogenetic approach to document a third pattern in the structure of emydid turtle communities. Emydid communities in southeastern North America tend to have a higher proportion of aquatic species than those in the northeast. This pattern reflects phylogenetic conservatism in the ecology and biogeography of two basal emydid clades, limiting convergence in community structure between these regions. However, differences in community structure between northeastern and southeastern North America have also been homogenized considerably by the dispersal of species with phylogenetically conserved ecological characteristics between regions. This pattern of ecologically conservative dispersal may be important in many continental and oceanic systems.     

Association between integration structure and functional evolution in the opercular four‐bar apparatus of the threespine stickleback,Gasterosteus aculeatus (Pisces: Gasterosteidae)

Phenotypes may evolve to become integrated in response to functional demands. Once evolved, integrated phenotypes, often modular, can also influence the trajectory of subsequent responses to selection. Clearly, connecting modularity and functionally adaptive evolution has been challenging. The teleost skull and jaw structures are useful for understanding this connection because of the key roles that these structures play in feeding in novel environments with different prey resources. In the present study, we examined such a structure in the threespine stickleback: the opercular four‐bar lever that functions in jaw opening. Comparing oceanic and two fresh‐water populations, we find marked phenotypic divergence in the skull opercular region, and the major axes of morphological and functional variation of the lever are found to be highly correlated. All three populations share the same global skull integration structure, and a conserved, strongly‐supported modular organization is evident in the region encompassing the lever. Importantly, a boundary between two modules that subdivides the lever apparatus corresponds to the region of most prominent morphological evolution. The matched modular phenotypic and functional architecture of head and jaw structures of stickleback therefore may be important for facilitating their rapid adaptive transitions between highly divergent habitats. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 375–390.     

Skull shape variation in extant and extinct Testudinata and its relation to habitat and feeding ecology

Turtles (Testudinata) are a diverse group of reptiles that conquered a broad set of habitats and feeding ecologies over the course of their well‐documented evolutionary history. We here investigate the cranial shape of 171 representatives of the turtle lineage and the relationship of shape to different habitat and diet preferences using two‐dimensional geometric morphometrics. The skull shape of extant turtles correlates with both ecological proxies, but is more affected by habitat than diet. However, the application of these correlations to extinct turtles produces mostly flawed results, as least when compared to external data such as sedimentary environment, highlighting that the morphospace held by extant turtles is not necessarily the optimal location in tree space for a particular ecology. The inability of this study to correctly predict the ecology of extinct turtles is likely related to the fact that the shape of turtle skulls is dominated by the emarginations and jaw closure mechanisms, two shape features unrelated to habitat or feeding ecology. This indicates that various specializations that are apparent in the skull only contribute little to overall shape.     

Evolution in ecological field experiments: implications for effect size

Rapid evolution in response to strong selection, much of which is human-induced, has been indisputably documented. In this perspective, we suggest that adaptation may influence the effect size of treatments in ecological field experiments and alter our predictions of future dynamics in ecological systems. Field experiments often impose very strong and consistent selection over multiple generations. Focal populations may adapt to these treatments and, in the process, increase or decrease the magnitude of the treatment effect through time. We argue that how effect size changes through time will depend on the evolutionary history of the experimental population, the type of experimental manipulation, and the traits involved in adaptive responses. While no field study has conclusively demonstrated evolution in response to treatments with concomitant changes in ecological effect size, we present several examples that provide strong circumstantial evidence that such effects occur. We conclude with a consideration of the differences between plastic and genetic responses to treatments and discuss future research directions linking adaptation to ecological effect size.     

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.     

Diet,bite force and skull morphology in the generalist rodent morphotype

For many vertebrate species, bite force plays an important functional role. Ecological characteristics of a species' niche, such as diet, are often associated with bite force. Previous evidence suggests a biomechanical trade‐off between rodents specialized for gnawing, which feed mainly on seeds, and those specialized for chewing, which feed mainly on green vegetation. We tested the hypothesis that gnawers are stronger biters than chewers. We estimated bite force and measured skull and mandible shape and size in 63 genera of a major rodent radiation (the myomorph sigmodontines). Analysis of the influence of diet on bite force and morphology was made in a comparative framework. We then used phylogenetic path analysis to uncover the most probable causal relationships linking diet and bite force. Both granivores (gnawers) and herbivores (chewers) have a similar high bite force, leading us to reject the initial hypothesis. Path analysis reveals that bite force is more likely influenced by diet than the reverse causality. The absence of a trade‐off between herbivores and granivores may be associated with the generalist nature of the myomorph condition seen in sigmodontine rodents. Both gnawing and chewing sigmodontines exhibit similar, intermediate phenotypes, at least compared to extreme gnawers (squirrels) and chewers (chinchillas). Only insectivorous rodents appear to be moving towards a different direction in the shape space, through some notable changes in morphology. In terms of diet, natural selection alters bite force through changes in size and shape, indicating that organisms adjust their bite force in tandem with changes in food items.     

Ancient and contingent body shape diversification in a hyperdiverse continental fish radiation

The characiform fishes of the Neotropics and Africa radiated remarkably in ecomorphology, but the macroevolutionary processes responsible for their biodiversity remain unexplored, and the degree to which their continental diversification parallels classic adaptive radiations remains untested. We reconstruct their diversification using a new fossil‐calibrated molecular phylogeny, dietary information, and geometric morphometrics. Though body shape diversified early in a manner consistent with an ancient continental adaptive radiation, trophic shifts did not always coincide with shape changes. With the notable exception of piscivores, lineages that converged in diet did not converge closely in body shape. Shifts in habitat or other variables likely influenced body shape evolution in addition to changes in diet, and the clade's history departs from many classic adaptive radiations in lakes or on islands, in which trophic convergence drives morphological convergence. The contrast between the Neotropical radiation's exhaustive exploration of morphospace and the more restrained diversification in Africa suggests a major role for contingency in characiform evolution, with the presence of cypriniform competitors in the Old World, but not the New, providing one possible explanation. Our results depict the clearest ecomorphological reconstruction to date for Characiformes and set the stage for studies further elucidating the processes underlying its diversification.     

Ecological diversification associated with the benthic‐to‐pelagic transition by North American minnows

Ecological opportunity is often regarded as a key factor that explains why diversity is unevenly distributed across life. Colonization of novel environments or adaptive zones may promote diversification. North American minnows exhibit an ancestral benthic‐to‐pelagic habitat shift that coincided with a burst in diversification. Here, we evaluate the phenotypic and ecological implications of this habitat shift by assessing craniofacial and dietary traits among 34 species and testing for morphology–diet covariation, convergence and adaptive optima. There were several instances of morphology–diet covariation such as correlations between mouth angle and the consumption of terrestrial insects and between relative gut length and the consumption of algae. After accounting for size and phylogenetic nonindependence, benthic species had longer heads, longer snouts, eyes positioned higher on their head, smaller mouth angles and longer digestive tracts than pelagic minnows. Benthic minnows also consumed more algae but less terrestrial insects, by volume, than pelagic minnows. Lastly, there were three distinct evolutionary regimes and more convergence in morphology and dietary characteristics than expected under a Brownian motion model of evolution. These findings indicate that colonization of the pelagic zone by minnows involved myriad phenotypic and dietary changes associated with exploitation of terrestrial subsidies. Thus, minnows exhibit phenotype–dietary covariation, an expansion of ecological roles and a burst in diversification rates in response to the ecological opportunity afforded by the colonization of a novel habitat.