Increased energy promotes size-based niche availability in marine mollusks

Variation in chemical energy, that is food availability, is posited to cause variation in body size. However, examinations of the relationship are rare and primarily limited to amniotes and zooplankton. Moreover, the relationship between body size and chemical energy may be impacted by phylogenetic history, clade-specific ecology, and heterogeneity of chemical energy in space and time. Considerable work remains to both document patterns in body size over gradients in food availability and understanding the processes potentially generating them. Here, we examine the functional relationship between body size and chemical energy availability over a broad assortment of marine mollusks varying in habitat and mobility. We demonstrate that chemical energy availability is likely driving body size patterns across habitats. We find that lower food availability decreases size-based niche availability by setting hard constraints on maximum size and potentially on minimum size depending on clade-specific ecology. Conversely, higher food availability promotes greater niche availability and potentially promotes evolutionary innovation with regard to size. We posit based on these findings and previous work that increases in chemical energy are important to the diversification of Metazoans through size-mediated niche processes.     

Rates of morphological evolution are correlated with species richness in salamanders

The tempo and mode of species diversification and phenotypic evolution vary widely across the tree of life, yet the relationship between these processes is poorly known. Previous tests of the relationship between rates of phenotypic evolution and rates of species diversification have assumed that species richness increases continuously through time. If this assumption is violated, simple phylogenetic estimates of net diversification rate may bear no relationship to processes that influence the distribution of species richness among clades. Here, we demonstrate that the variation in species richness among plethodontid salamander clades is unlikely to have resulted from simple time-dependent processes, leading to fundamentally different conclusions about the relationship between rates of phenotypic evolution and species diversification. Morphological evolutionary rates of both size and shape evolution are correlated with clade species richness, but are uncorrelated with simple estimators of net diversification that assume constancy of rates through time. This coupling between species diversification and phenotypic evolution is consistent with the hypothesis that clades with high rates of morphological trait evolution may diversify more than clades with low rates. Our results indicate that assumptions about underlying processes of diversity regulation have important consequences for interpreting macroevolutionary patterns.     

Body size estimation and evolution in metriorhynchid crocodylomorphs: implications for species diversification and niche partitioning

Metriorhynchids were a peculiar group of fully marine Mesozoic crocodylomorphs, some of which reached large body size and were probably apex predators. The estimation of their total body length in the past has proven problematic. Rigorous size estimation was provided using five complete metriorhynchid specimens, by means of regression equations derived from basicranial and femoral length against total body length. The use of the Alligator femoral regression equation as a proxy to estimate metriorhynchid total body length led to a slight underestimation, whereas cranial regression equations of extant genera resulted in an overestimation of body length. Therefore, the scaling of crania and femora to total body length of metriorhynchids is noticeably different from that of extant crocodylians, indicating that extant crocodylians are not ideal proxies for size reconstruction of extinct taxa that deviate from their semi‐aquatic morphotype. The lack of a correlation between maximum, minimum, or the range of generic body lengths with species richness demonstrates that species diversification is driven by factors other than just variation in body size. Maximum likelihood modelling also found no evidence for directionality in body size evolution. However, niche partitioning in Metriorhynchidae is mediated not only by craniodental differentiation, as shown by previous studies, but also by body size variation. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163 , 1199–1216.     

REEXAMINING THE RELATIONSHIP BETWEEN BODY SIZE AND TONAL SIGNALS FREQUENCY IN WHALES: A COMPARATIVE APPROACH USING A NOVEL PHYLOGENY

A negative relationship between cetacean body size and tonal sound minimum and maximum frequencies has been demonstrated in several studies using standard statistical approaches where species are considered independent data points. Such studies, however, fail to account for known dependencies among related species—shared similarity due to common ancestry. Here we test these hypotheses by generating the most complete species level cetacean phylogeny to date, which we then use to reconstruct the evolutionary history of body size and standard tonal sounds parameters (minimum, maximum, and center frequency). Our results show that when phylogenetic relationships are considered the correlation between body size (length or mass) and minimum frequency is corroborated with approximately 27% of the variation in tonal sound frequency being explained by body size compared to 86% to 93% explained when phylogenetic relationships are not considered. Central frequency also correlates with body size in toothed whales, but for other tonal sound frequency parameters, including maximum frequency, this hypothesized correlation disappears. Therefore, constraints imposed by body size seem to have played a role in the evolution of minimum frequency but alternative hypotheses are required to explain variation in maximum frequency.     

Macroecological explanations for differences in species richness gradients: a canonical analysis of South American birds

Aim To evaluate how spatial variation of species richness in different bird orders responds to environmental gradients and determine which order level trait best predicts these relationships. Location South America. Methods A canonical correlation analysis was performed between the species richness in each of 17 bird orders and eight environmental variables in 374, 220 × 220 km cells. Loadings associated with the first two canonical variables were regressed against six order‐level predictors, including diversification level (number of species in each order), body size, median geographical range size and characteristics included in the model to control Type I error rates (the phylogenetic relationship among orders and levels of local‐scale spatial autocorrelation). Results Richness patterns of 14 bird orders were highly correlated with the first canonical axis, indicating that most orders respond similarly to energy‐water gradients (primarily actual evapotranspiration, minimum temperature and potential evapotranspiration). In contrast, species richness within Trochiliformes, Apodiformes and Galliformes were also correlated with the second canonical variable, representing measures of mesoscale climatic variation (range in elevation within cells, minimum temperature, and the interaction term between them) and landcover (habitat diversity). We also found that total diversification within orders was the best predictor of the loadings associated with the first canonical axis, whereas body size of each order best predicted loadings on the second axis. Conclusion Our results broadly support climatic‐related hypotheses as explanations for spatial variation in species richness of different orders. However, both historical (order‐specific variation in speciation rates) and ecological (dispersal of species that evolved by independent processes into areas amenable to birds) processes can explain the relationship between order level traits, such as body size and diversification level, and magnitude of response to current environment, furnishing then guidelines for a further and deeper understanding of broad‐scale diversity gradients.     

Fine-scale phenotypic change across a species transition zone in the genus neotoma: disentangling independent evolution from phylogenetic history

Zones of secondary contact between closely related species provide a rare opportunity to examine evidence of evolutionary processes that reinforce species boundaries and/or promote diversification. Here, we report on genetic and morphological variation in two sister species of woodrats, Neotoma fuscipes and N. macrotis, across a 30-km transition zone in the Sierra Nevada of California. We assessed whether these lineages readily hybridize, and whether their morphology suggests ecological interactions favoring phenotypic diversification. We combined measurements of body size and 11 craniodental traits from nine populations with genetic data to examine patterns of variation within and between species. We used phylogenetic autocorrelation methods to estimate the degree to which phenotypic variation in our dataset arose from independent evolution within populations versus phylogenetic history. Although no current sympatry or hybridization was evident, craniodental morphology diverged in both lineages near their distributional limits, whereas body size converged. The shift in craniodental morphology arose independently within populations whereas body size retained a strong phylogenetic signal, yet both patterns are consistent with expectations of phenotypic change based on different models of resource competition. Our findings demonstrate the importance of examining a suite of morphological traits across contact zones to provide a more complete picture of potential ecological interactions: competition may drive both diversification and convergence in different phenotypic traits.     

Loudness of birdsong is related to the body size,syntax and phonology of passerine species

Songs of passerines are generally complex, long‐range acoustic signals, and are highly diverse across species. This diversity must nevertheless be shaped by the capabilities of the avian vocal physiology. For example, within species, loudness has been shown to trade‐off with aspects of song complexity. Here, I ask if such trade‐offs with loudness influenced the evolutionary diversification of song among passerines. Comparing perceived song loudness across > 140 European and North American species showed that loudness is positively related to body size and to singing with simple trilled syntax, and negatively related to aspects of syllable complexity. Syntax and syllable phonology together explained more variation than body size did, indicating that the acoustic design of songs is an important factor determining loudness. These results show for the first time that loudness covaries with, and possibly limits, song complexity across species, suggesting that a trade‐off with loudness shaped the evolutionary diversification of passerine song.     

Environmental correlates of body size distributions of European springtails (Hexapoda: Collembola)

Aim Species–body size distributions (SBDs) are plots of species richness across body size classes. They have been linked to energetic constraints, speciation–extinction dynamics and to evolutionary trends. However, little is known about the spatial variation of size distributions. Here we study SBDs of European springtails (Collembola) at a continental scale and test whether minimum, average and maximum body size and the shapes of size distributions change across latitudinal and longitudinal gradients and whether SBDs of islands and mainlands differ. We also test whether the island rule and the positive body size–range size relationship of vertebrates also holds for Collembola. Location Europe. Methods We use a unique data set on the spatial distributions of 2102 species of European springtails across 52 countries and larger islands together with associated data on body size, area, climate variables, longitude and latitude. Differences in the central moments of SBDs are inferred from simultaneous spatial autoregression models. Results The SBD of the European Collembola and its largest suborder Entomobryomorpha is unimodal and symmetrical. Average, minimum and maximum body weight and the skewness of the mainland/island SBDs peaked at intermediate latitudes. We could not find simple latitudinal gradients in minimum and maximum body weight. Average and maximum body size increased with country/island area in accordance with the island rule in vertebrates, while minimum body size did not significantly differ between islands and mainlands. Finally, we found a weak but statistically significant positive correlation of range size and body size. Main conclusions We provide evidence for differences in body size distributions between islands and mainlands that are in part in line with the island rule in invertebrates. We also find evidence for an interspecific body size–range size relationship similar to that of vertebrates although the vertebrate pattern is much stronger than the springtail pattern. Our results on latitudinal gradients of maximum and average body size imply the need to account for species richness and area effects in the study of latitudinal gradients in body size. We recommend implementing sample size and area effects in the study of body size distributions on islands and mainlands.     

Convergent evolution across the Australian continent: ecotype diversification drives morphological convergence in two distantly related clades of Australian frogs

Animals from different clades but subject to similar environments often evolve similar body shapes and physiological adaptations due to convergent evolution, but this has been rarely tested at the transcontinental level and across entire classes of animal. Australia's biome diversity, isolation and aridification history provide excellent opportunities for comparative analyses on broad‐scale macroevolutionary patterns. We collected morphological and environmental data on eighty‐four (98%) Australian hylid frog species and categorized them into ecotypes. Using a phylogenetic framework, we tested the hypothesis that frogs from the same ecotype display similar body shape patterns: (i) across all the Australian hylids, and (ii) through comparison with a similar previous study on 127 (97%) Australian myobatrachid species. Body size and shape variation did not follow a strong phylogenetic pattern and was not tightly correlated with environment, but there was a stronger association between morphotype and ecotype. Both arboreal and aquatic frogs had long limbs, whereas limbs of fossorial species were shorter. Other terrestrial species were convergent on the more typical frog body shape. We quantified the strength of morphological convergence at two levels: (i) between fossorial myobatrachid and hylid frogs, and (ii) in each ecomorph within the hylids. We found strong convergence within ecotypes, especially in fossorial species. Ecotypes were also reflected in physiological adaptations: both arboreal and cocooned fossorial frogs tend to have higher rates of evaporative water loss. Our results illustrate how adaptation to different ecological niches plays a crucial role in morphological evolution, boosting phenotypic diversity within a clade. Despite phylogenetic conservatism, morphological adaptation to repeatedly emerging new environments can erase the signature of ancestral morphotypes, resulting in phenotypic diversification and convergence both within and between diverse clades.     

Patterns of diversity, altitudinal range and body size among freshwater fishes in the Yangtze River basin, China

Aim To document patterns in diversity, altitudinal range and body size of freshwater fishes along an elevational gradient in the Yangtze River basin. Location The Yangtze River basin, China. Methods We used published data to compile the distribution, altitudinal range and body size of freshwater fishes. Correlation, regression, clustering and graphical analyses were used to explore patterns in diversity, altitudinal range and body size of freshwater fishes in 100‐m elevation zones from 0 to 5200 m. Results Species richness patterns across the elevational gradient for total, non‐endemic and endemic fishes were different. The ratio of endemics to total richness peaked at mid elevation. Land area on a 500‐m interval scale explained a significant amount of the variation in species richness. Species density displayed two peaks at mid‐elevation zones. The cluster analysis revealed five distinct assemblages across the elevation gradient. The relationship between elevational range size and the midpoint of the elevational range revealed a triangular distribution. The frequency distribution of log maximum standard length data displayed an atypical right‐skewed pattern. Intermediate body sizes occurred across the greatest range of elevation while small and large body sizes possessed only small elevational amplitudes. The size‐elevation relationship between the two major families revealed a very strong pattern of body size constraint among the Cobitidae with no corresponding elevational constraint and a lot of body size and elevational diversification among the Cyprinidae. Main conclusion The data failed to support either Rapoport's rule or Bergmann's rule.     

Size diversity and species diversity relationships in fish assemblages of Western Palearctic lakes

Body size, coupled with abundance and taxonomy, may help to understand the mechanisms shaping community structure. Since the body size of fish is closely related to their trophic niche, size diversity (based on individual body size) of fish communities may capture intraspecific variations in fish trophic niches that are not detected by species diversity. Thus, the relationship between size diversity and species diversity may help to integrate variation at both intraspecific and interspecific levels. We studied the relationship between species diversity and size diversity as a measure of the degree of overlap in size among species and thereby the potential overlap in niches in a community. We hypothesized that the relationship between size diversity and species would be different across the European continent due to different levels of size overlap in fish communities. The data were derived from samplings of fish communities using standardised benthic gill nets in 363 lakes. At the continental scale, size diversity increased with species diversity; at the ecoregion scale, the slope of the relation changed across the continent, with the greatest mismatch occurring in northern Europe where communities comprised only one or a few species, but each of which exhibited a great range in size. There was an increase in slope towards the south with significant relations for four out of six ecoregions. The steeper size diversity‐species diversity slope at lower latitudes is attributable to a lower overlap in fish size and thus likely to finer niche separation. Our results also suggest that size diversity is not a strong surrogate for species diversity in European lake fish communities. Thus, particularly in fish communities composed of few species, measuring size diversity may help to detect potential functional variation which may be neglected by measuring species diversity alone.     

Morphological disparity opposes latitudinal diversity gradient in lacertid lizards

While global variation in taxonomic diversity is strongly linked to latitude, the extent to which morphological disparity follows geographical gradients is less well known. We estimated patterns of lineage diversification, morphological disparity and rates of phenotypic evolution in the Old World lizard family Lacertidae, which displays a nearly inverse latitudinal diversity gradient with decreasing species richness towards the tropics. We found that lacertids exhibit relatively constant rates of lineage accumulation over time, although the majority of morphological variation appears to have originated during recent divergence events, resulting in increased partitioning of disparity within subclades. Among subclades, tropical arboreal taxa exhibited the fastest rates of shape change while temperate European taxa were the slowest, resulting in an inverse relationship between latitudinal diversity and rates of phenotypic evolution. This pattern demonstrates a compelling counterexample to the ecological opportunity theory of diversification, suggesting an uncoupling of the processes generating species diversity and morphological differentiation across spatial scales.     

What determines species richness of parasitic organisms? A meta‐analysis across animal,plant and fungal hosts

Although a small set of external factors account for much of the spatial variation in plant and animal diversity, the search continues for general drivers of variation in parasite species richness among host species. Qualitative reviews of existing evidence suggest idiosyncrasies and inconsistent predictive power for all proposed determinants of parasite richness. Here, we provide the first quantitative synthesis of the evidence using a meta‐analysis of 62 original studies testing the relationship between parasite richness across animal, plant and fungal hosts, and each of its four most widely used presumed predictors: host body size, host geographical range size, host population density, and latitude. We uncover three universal predictors of parasite richness across host species, namely host body size, geographical range size and population density, applicable regardless of the taxa considered and independently of most aspects of study design. A proper match in the primary studies between the focal predictor and both the spatial scale of study and the level at which parasite species richness was quantified (i.e. within host populations or tallied across a host species' entire range) also affected the magnitude of effect sizes. By contrast, except for a couple of indicative trends in subsets of the full dataset, there was no strong evidence for an effect of latitude on parasite species richness; where found, this effect ran counter to the general latitude gradient in diversity, with parasite species richness tending to be higher further from the equator. Finally, the meta‐analysis also revealed a negative relationship between the magnitude of effect sizes and the year of publication of original studies (i.e. a time‐lag bias). This temporal bias may be due to the increasing use of phylogenetic correction in comparative analyses of parasite richness over time, as this correction yields more conservative effect sizes. Overall, these findings point to common underlying processes of parasite diversification fundamentally different from those controlling the diversity of free‐living organisms.     

Body size as a primary determinant of ecomorphological diversification and the evolution of mimicry in the lampropeltinine snakes (Serpentes: Colubridae)

Evolutionary correlations between functionally related character suites are expected as a consequence of coadaptation due to physiological relationships between traits. However, significant correlations may also exist between putatively unrelated characters due to shared relationships between those traits and underlying variables, such as body size. Although such patterns are often dismissed as simple body size scaling, this presumption may overlook important evolutionary patterns of diversification. If body size is the primary determinant of potential diversity in multiple unrelated characters, the observed differentiation of species may be governed by variability in body size, and any biotic or abiotic constraints on the diversification thereof. Here, we demonstrate that traits related to both predatory specialization (gape and diet preference) and predatory avoidance (the development of Batesian mimicry) are phylogenetically correlated in the North American snake tribe Lampropeltini. This is apparently due to shared relationships between those traits and adult body size, suggesting that size is the primary determinant of ecomorphological differentiation in the lampropeltinines. Diversification in body size is apparently not linked to climatic or environmental factors, and may have been driven by interspecific interactions such as competition. Additionally, we find the presence of a ‘key zone’ for the development of both rattle‐ and coral snake mimicry; only small snakes feeding primarily on ectothermic prey develop mimetic colour patterns, in or near the range of venomous model species.     

Maximum body size in a radiating clade as a function of time

The behavior of the maximum body size (body length) in an evolving clade is exemplified by the evolutionary histories of Bivalvia, Cetacea, and Camerata (Crinoidea). Changes of the maximum size with time track closely diversification history: when a clade diversifies exponentially, the maximum size also increases exponentially, and when the number of species changes irregularly (at varying rates), the maximum size also changes in that manner. However, within any given clade, the maximum body size changes at lower rates than diversity does. The observed shifts in maximum body size approximate the rate of diversification per unit of time to the power of about 0.5.     

Genome size variation and species diversity in salamanders

Salamanders (Urodela) have among the largest vertebrate genomes, ranging in size from 10 to 120 pg. Although changes in genome size often occur randomly and in the absence of selection pressure, nonrandom patterns of genome size variation are evident among specific vertebrate lineages. Several reports suggest a relationship between species richness and genome size, but the exact nature of that relationship remains unclear both within and across different taxonomic groups. Here, we report (a) a negative relationship between haploid genome size (C‐value) and species richness at the family taxonomic level in salamander clades; (b) a correlation of C‐value and species richness with clade crown age but not with diversification rates; (c) strong associations between C‐value and both geographic area and climatic‐niche rate. Finally, we report a relationship between C‐value diversity and species diversity at both the family‐ and genus‐level clades in urodeles.     

Interspecific geographic range size–body size relationship and the diversification dynamics of Neotropical furnariid birds

Among the earliest macroecological patterns documented, is the range and body size relationship, characterized by a minimum geographic range size imposed by the species’ body size. This boundary for the geographic range size increases linearly with body size and has been proposed to have implications in lineages evolution and conservation. Nevertheless, the macroevolutionary processes involved in the origin of this boundary and its consequences on lineage diversification have been poorly explored. We evaluate the macroevolutionary consequences of the difference (hereafter the distance) between the observed and the minimum range sizes required by the species’ body size, to untangle its role on the diversification of a Neotropical species‐rich bird clade using trait‐dependent diversification models. We show that speciation rate is a positive hump‐shaped function of the distance to the lower boundary. The species with highest and lowest distances to minimum range size had lower speciation rates, while species close to medium distances values had the highest speciation rates. Further, our results suggest that the distance to the minimum range size is a macroevolutionary constraint that affects the diversification process responsible for the origin of this macroecological pattern in a more complex way than previously envisioned.     

Cope's rule and the evolution of body size in Pinnipedimorpha (Mammalia: Carnivora)

Cope's rule describes the evolutionary trend for animal lineages to increase in body size over time. In this study, we tested the validity of Cope's rule for a marine mammal clade, the Pinnipedimorpha, which includes the extinct Desmatophocidae, and extant Phocidae (earless seals), Otariidae (fur seals and sea lions), and Odobenidae (walruses). We tested for the presence of Cope's rule by compiling a large dataset of body size data for extant and fossil pinnipeds and then examined how body size evolved through time. We found that there was a positive relationship between geologic age and body size. However, this trend is the result of differences between early assemblages of small-bodied pinnipeds (Oligocene to early Miocene) and later assemblages (middle Miocene to Pliocene) for which species exhibited greater size diversity. No significant differences were found between the number of increases or decreases in body size within Pinnipedimorpha or within specific pinniped clades. This suggests that the pinniped body size increase was driven by passive diversification into vacant niche space, with the common ancestor of Pinnipedimorpha occurring near the minimum adult body size possible for a marine mammal. Based upon the above results, the evolutionary history of pinnipeds does not follow Cope's rule.     

Ambient temperature correlates with geographic variation in body size of least horseshoe bats

Geographic variation in body size is common within many animal species. The causes of this pattern, however, remain largely unexplored in most vertebrate groups. Bats are widely distributed globally owing to their ability of powered flight. Most bat species encounter a variety of climatic conditions across their distribution range, making them an ideal taxon for the study of ecogeographic patterns in body size. Here, we used adult least horseshoe bats, Rhinolophus pusillus, to test whether geographic variation in body size was determined by heat conservation, heat dissipation, climatic seasonality, or primary productivity. We measured body mass and head-body length for 246 adult bats from 12 allopatric colonies in China. We quantified the ecological conditions inhabited by each colony, including mean maximum temperature of the warmest month, mean minimum temperature of the coldest month, temperature seasonality, precipitation seasonality, and annual net primary productivity (ANPP). Body mass and head-body length, 2 of the most reliable indicators of body size, exhibited marked differences between colonies. After controlling for spatial autocorrelation, the mean minimum temperature of the coldest month explained most of the variation in body size among colonies, regardless of sex. The mean maximum temperature, climatic seasonality, and ANPP had limited power in predicting body size of males or females in comparison with mean minimum temperature. These results support the heat conservation hypothesis and suggest adaptive responses of body size to cold climates in cave-dwelling bats.     

Abundance, diversity and body size: patterns from a grassland arthropod community

1. The empirical relationships among body size, species richness and number of individuals may give insight into the factors controlling species diversity and the relative abundances of species. To determine these relationships, we sampled the arthropods of grasslands and savannahs at Cedar Creek, MN using sweep nets (90 525 individuals of 1225 species) and pitfall traps (12 721 individuals of 92 species). Specimens were identified, enumerated and measured to determine body size.
2. Both overall and within abundant taxonomic orders, species richness and numbers of individuals peaked at body sizes intermediate for each group. Evolution could create unimodal diversity patterns by random diversification around an ancestral body size or from size-dependent fitness differences. Local processes such as competition or predation could also create unimodal diversity distributions.
3. The average body size of a species depended significantly on its taxonomic order, but on contemporary trophic role only within the context of taxonomic order.
4. Species richness ( S _i) within size classes was related to the number of individuals ( I _i) as S _i =  I i^0·5. This relationship held across a 100 000-fold range of body sizes. Within size classes, abundance distributions of size classes were all similar power functions. A general rule of resource division, together with similar minimum population sizes, is sufficient to generate the relationship between species richness and number of individuals.
5. Smaller bodied species had slightly shallower abundance distributions and may, in general, persist at lower densities than larger species.
6. Our results suggest there may be fewer undescribed small arthropod species than previously thought and that most undescribed species will be smaller than arthropods.