Disparity and Evolutionary Rate Do Not Explain Diversity Patterns in Muroid Rodents (Rodentia: Muroidea)

Quaternary rodents of South Africa: A companion guide for cranio-dental identification

Rodentia is the most species-rich order among mammals. The Republic of South Africa harbours a high rodent diversity whose taxonomy and phylogeny have been extensively studied using genetic tools. Such advances have led to the establishment of new faunal lists for the country. Because rodents are frequently recovered from archaeological cave site material and owl pellets, and constitute prime material for studying both past and present environmental conditions, it is necessary to characterize their osteological remains. The skull and teeth are the most useful diagnostic skeletal elements preserved in modern and fossil accumulations. This key provides updated craniodental criteria for identifying rodent genera found in Quaternary deposits, and modern material from the Republic of South Africa, thus facilitating research on past and present rodent diversity.

A geometric morphometric analysis of geographic variation in the Cape Short-eared gerbil, Desmodillus auricularis (Rodentia: Gerbillinae)

The genus Desmodillus is monospecific, consisting of only the Cape short-eared gerbil (Desmodillus auricularis). Despite being widely distributed across southern Africa, previous studies did not find evidence of intraspecific phenotypic geographic differentiation. The objectives of this study is to use geometric morphometrics to investigate if and how the skull of D. auricularis varies spatially. It examines the covariation of skull morphology with broad spatial (latitude and longitude) and climatic variables, based on a sample of 580 specimens from southern Africa (Botswana, Namibia, and South Africa). The results did not support the differentiation of D. auricularis populations into distinct geographically isolated phenotypic groups. However, there is strong evidence for clinal variation in skull morphology; the most prominent pattern being a decrease in size from the west (closest to the South Atlantic coast) to the east (towards the continent’s interior). Shape variation was not localized in any skull region and seem to be driven mostly by size (allometry), although it also covaried significantly with latitude and longitude. Statistically significant skull shape sexual dimorphism was also detected, with males having larger crania than females. Spatial clinal variation in skull morphology was mostly associated with differences in the aridity of the habitats relative to their distance from the coast as evidenced by precipitation-related bioclimatic variables—annual precipitation (BIO12), precipitation of driest month (BIO14), and precipitation of driest quarter (BIO17)—covarying the most with skull morphology. This could be driven by either the climate influencing local resources available to populations or by the climate directly instigating phenotypic climatic adaptations.

Evolvability and Macroevolution: Overview and Synthesis

Evolvability is best addressed from a multi-level, macroevolutionary perspective through a comparative approach that tests for among-clade differences in phenotypic diversification in response to an opportunity, such as encountered after a mass extinction, entering a new adaptive zone, or entering a new geographic area. Analyzing the dynamics of clades under similar environmental conditions can (partially) factor out shared external drivers to recognize intrinsic differences in evolvability, aiming for a macroevolutionary analog of a common-garden experiment. Analyses will be most powerful when integrating neontological and paleontological data: determining differences among extant populations that can be hypothesized to generate large-scale, long-term contrasts in evolvability among clades; or observing large-scale differences among clade histories that can by hypothesized to reflect contrasts in genetics and development observed directly in extant populations. However, many comparative analyses can be informative on their own, as explored in this overview. Differences in clade-level evolvability can be visualized in diversity-disparity plots, which can quantify positive and negative departures of phenotypic productivity from stochastic expectations scaled to taxonomic diversification. Factors that evidently can promote evolvability include modularity—when selection aligns with modular structure or with morphological integration patterns; pronounced ontogenetic changes in morphology, as in allometry or multiphase life cycles; genome size; and a variety of evolutionary novelties, which can also be evaluated using macroevolutionary lags between the acquisition of a trait and phenotypic diversification, and dead-clade-walking patterns that may signal a loss of evolvability when extrinsic factors can be excluded. High speciation rates may indirectly foster phenotypic evolvability, and vice versa. Mechanisms are controversial, but clade evolvability may be higher in the Cambrian, and possibly early in the history of clades at other times; in the tropics; and, for marine organisms, in shallow-water disturbed habitats.

Geometric differences between the crania of Australian hopping mice (Notomys, Murinae, Rodentia)

Half of the ten Australian hopping mice (Notomys) species have become extinct following the European colonisation of Australia, and most of the rest are threatened. This makes the study of their present diversity paramount. Although recent molecular phylogenies improved our understanding of the relationships among the species, detailed interspecific phenotypic comparisons are still lacking. This is the aim of the present study. Geometric morphometric methods were used to compare the crania of all five extant species (N. alexis, N. aquillo, N. cervinus, N. fuscus, and N. mitchellii) along with the extinct N. longicaudatus. Although previous work (based on traditional approaches) find intragenerically conserved crania, the present study discovers significant differences in cranial size and shape among Notomys species, with the ventral view being more distinct than the dorsal view. There was no evidence of sexual dimorphism in cranial size nor shape, and only a weak allometric effect. Most aspects of cranial shape differed among the species. The extant species pair that differed in cranial shape the most was N. aquilo – N. cervinus, differing in the foramen magnum, tympanic bulla, orbit, incisive foramen, and rostrum, along with cranial width, potentially a consequence of N. cervinus’ phylogenetic position, and N. aquilo’s s ecological uniqueness.

Analyzing Disparity and Rates of Morphological Evolution with Model-Based Phylogenetic Comparative Methods

Understanding variation in rates of evolution and morphological disparity is a goal of macroevolutionary research. In a phylogenetic comparative methods framework, we present three explicit models for linking the rate of evolution of a trait to the state of another evolving trait. This allows testing hypotheses about causal influences on rates of phenotypic evolution with phylogenetic comparative data. We develop a statistical framework for fitting the models with generalized least-squares regression and use this to discuss issues and limitations in the study of rates of evolution more generally. We show that the power to detect effects on rates of evolution is low in that even strong causal effects are unlikely to explain more than a few percent of observed variance in disparity. We illustrate the models and issues by testing if rates of beak-shape evolution in birds are influenced by brain size, as may be predicted from a Baldwin effect in which presumptively more behaviorally flexible large-brained species generate more novel selection on themselves leading to higher rates of evolution. From an analysis of morphometric data for 645 species, we find evidence that both macro- and microevolution of the beak are faster in birds with larger brains, but with the caveat that there are no consistent effects of relative brain size.[Baldwin effect; beak shape; behavioral drive; bird; brain size; disparity; phylogenetic comparative method; rate of evolution.]

Commonalities and evolutionary divergences of mandible shape ontogenies in rodents

In mammals, significant changes take place during postnatal growth, linked to changes in diet (from sucking to gnawing). During this period, mandible development is highly interconnected with muscle growth and the epigenetic interactions between muscle and bone control the spatialization of bone formation and remodelling in response to biomechanical strain. This mechanism contributes to postnatal developmental plasticity and may have influenced the course of evolutionary divergences between species and clades. We sought to model postnatal changes at a macroevolutionary scale by analysing ontogenetic trajectories of mandible shape across 16 species belonging mainly to two suborders of Rodents, Myomorpha and Hystricomorpha, which differ in muscle attachments, tooth growth and life-history traits. Myomorpha species present a much stronger magnitude of changes over a shorter growth period. Among Hystricomorpha, part of the observed adult shape is set up prenatally, and most postnatal trajectories are genus-specific, which agrees with nonlinear developmental trajectories over longer gestational periods. Beside divergence at large scale, we find some collinearities between evolutionary and developmental trajectories. A common developmental trend was also observed, leading to enlargement of the masseter fossa during postnatal growth. The tooth growth, especially hypselodonty, seems to be a major driver of divergences of postnatal trajectories. These muscle- and tooth-related effects on postnatal trajectories suggest opportunities for developmental plasticity in the evolution of the mandible shape, opportunities that may have differed across Rodent clades.

Desmodilliscus braueri crania compared to Pachyuromys duprasi (Desmodilliscini, Gerbillinae, Rodentia)

Molecular phylogenies support the monophyly of Desmodilliscus braueri and Pachyuromys duprasi as a clade basally split from all other gerbillines. While this monophyly supports their placement in a single tribe (Desmodilliscini), no morphological synapomorphies exist among desmodilliscines. This study compares the scale-independent cranial shapes of these two species using geometric morphometrics to determine how they differ and/or converge. Tribal synapomorphies, should they exist, may appear as interspecifically invariable cranial regions. No such invariable cranial regions were detected. The two species significantly differed in cranial size and shape. A small part of shape variation was allometric, with a weak unique allometric effect. No sexual size nor shape dimorphism was found. The sister taxa greatly differed in almost all cranial features, with Pachyuromys (when compared to Desmodilliscus) having a larger-sized cranium, with a larger bulla and suprameatal triangle, a more posteriorly placed palatine foramina, and more anteriorly shifted (and reduced) rostral cranial structures, due to being crowded by the hypertrophied bulla. Cranial variation patterns are consistent with the literature. The extreme morphological divergence among these species is explained by the distant divergence time and ecological differences. Absence of cranial shape synapomorphies does not preclude synapomorphies in other craniodental morphological features (e.g., detailed morphology of the dentition and cranial foramina) or in other morphological structures, such as the postcranial skeleton.

The Relationship between Genus/Species Richness and Morphological Diversity among Subfamilies of Jewel Beetles

Simple Summary

Morphological diversity and species richness provide insights into biodiversity and have been studied extensively in recent years. Most researchers have found a positive correlation between these factors in many groups at the local community scale; however, this documented relationship has not always been consistent because of diverse niches and the status of an organism in a given ecosystem. Here we propose a new paradigm for the analysis of higher taxa biodiversity based on a cosmopolitan dataset to further investigate this contradiction. The morphological diversity of 1106 buprestid species from around the world was quantified based on the contours of the pronotum and elytron in dorsal view using a geometric morphometric approach. We found a positive correlation between morphological diversity and genus richness while no significance was found in the species-level test. Furthermore, the correlation between morphological diversity and genus richness is higher than it is in the species-level test. Our results demonstrate the superiority of higher taxa in biodiversity, and that the geometric morphometric approach could quite accurately reveal diversity patterns of the family Buprestidae. These conclusions complement the crucial aspect in several disciplines, including biodiversity, phylogeny and evolutionary strategy.

Abstract

A positive correlation between the species richness and morphological diversity of some organisms has been found in almost all studies at the local community scale. However, this documented relationship has not always been consistent because of diverse niches and the status of an organism in an ecosystem. Global taxon sampling, new morphological approaches, and consideration of more taxonomic categories other than species level are possible methods to further investigate this contradiction. In this study, we proposed a new paradigm for higher taxa biodiversity analysis based on a cosmopolitan dataset. A total of 1106 species from around the world representing all subfamilies and 33% genera of Buprestidae (jewel beetles) were selected to test the correlation between morphological diversity (MD) and genus/species richness (GR/SR) among subfamilies. The MD was quantified by the contours of the pronotum and elytron in dorsal view based on a geometric morphometric approach. The positive correlation between MD and GR was found in all test combinations, but was irrelevant in the species-level test. Interestingly, the correlation between MD and GR was higher than MD and SR in both pronotum and elytron measurements. Additionally, the MD of the pronotum is obviously higher than the MD of the elytron. Our results demonstrate that the geometric morphometric approach could quite accurately reveal diversity patterns of the family Buprestidae. Future studies on different groups, using more characters, more analyses and detailed biological interpretations, are required to fully understand the relationship between MD and SR.

Evolutionary trends of the conserved neurocranium shape in angel sharks (Squatiniformes, Elasmobranchii)

Elasmobranchii (i.e., sharks, skates, and rays) forms one of the most diverse groups of marine predators. With a fossil record extending back into the Devonian, several modifications in their body plan illustrate their body shape diversity through time. The angel sharks, whose fossil record dates back to the Late Jurassic, some 160 Ma, have a dorsoventrally flattened body, similar to skates and rays. Fossil skeletons of this group show that the overall morphology was well established earlier in its history. By examining the skull shape of well-preserved fossil material compared to extant angel sharks using geometric morphometric methods, within a phylogenetic framework, we were able to determine the conservative skull shape among angel sharks with a high degree of integration. The morphospace occupation of extant angel sharks is rather restricted, with extensive overlap. Most of the differences in skull shape are related to their geographic distribution patterns. We found higher levels of disparity in extinct forms, but lower ones in extant species. Since angel sharks display a highly specialized prey capture behaviour, we suggest that the morphological integration and biogeographic processes are the main drivers of their diversity, which might limit their capacity to display higher disparities since their origin.

A fly in a tube: Macroevolutionary expectations for integrated phenotypes

Phenotypic integration and modularity are ubiquitous features of complex organisms, describing the magnitude and pattern of relationships among biological traits. A key prediction is that these relationships, reflecting genetic, developmental, and functional interactions, shape evolutionary processes by governing evolvability and constraint. Over the last 60 years, a rich literature of research has quantified patterns of integration and modularity across a variety of clades and systems. Only recently has it become possible to contextualize these findings in a phylogenetic framework to understand how trait integration interacts with evolutionary tempo and mode. Here, we review the state of macroevolutionary studies of integration and modularity, synthesizing empirical and theoretical work into a conceptual framework for predicting the effects of integration on evolutionary rate and disparity: a fly in a tube. While magnitude of integration is expected to influence the potential for phenotypic variation to be produced and maintained, thus defining the shape and size of a tube in morphospace, evolutionary rate, or the speed at which a fly moves around the tube, is not necessarily controlled by trait interactions. Finally, we demonstrate this reduced disparity relative to the Brownian expectation for a given rate of evolution with an empirical example: the avian cranium.