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Mechanisms of Large-Scale Evolutionary Trends

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Abstract

Large-scale evolutionary trends may result from driving forces or from passive diffusion in bounded spaces. Such trends are persistent directional changes in higher taxa spanning significant periods of geological time; examples include the frequently cited long-term trends in size, complexity, and fitness in life as a whole, as well as trends in lesser supraspecific taxa and trends in space. In a driven trend, the distribution mean increases on account of a force that acts on lineages throughout the space in which diversification occurs. In a passive system, no pervasive force of bias exists, but the mean increases because change in one direction is blocked by a boundary, or other inhomogeneity, in some limited region of the space. Two tests have been used to distinguish these trend mechanisms: 1) the test based on the behaviour of the minimum; and 2) the ancestor-descendant test, based on comparisons in a random sample of ancestor-descendant pairs that lie far from any possible lower bound. For skewed distributions, a third test is introduced here: 3) the subclade test, based on the mean skewness of a sample of subclades drawn from the trial of a terminal distribution. With certain restrictions, a system is driven if the minimum increases, if increases significantly outnumber decreases among ancestor-descendant pairs, and if the mean skew of subclades is significantly positive. A passive mechanism is more difficult to demonstrate but is the more likely mechanism if decreases outnumber increases and if the mean skew of subclades is negative. The subclade test requires no detailed phylogeny or paleontological time series, but only terminal (eg modern) distributions. -from Author
... The body-size distribution (BSD) of species in a guild, clade, or assemblage reflects their niche requirements and is the macroecological and macroevolutionary outcome of the many drivers that constrain body size (Calder 1984;Roy et al. 2000). The "null" shape of the BSD fits to a lognormal distribution (McKinney 1990;McShea 1994;Marquet and Taper 1998;Maurer 1998Maurer , 2003Kozłowski and Gawelczyk 2002;Maurer and Marquet 2013), in which the minimum and maximum sizes are constrained by energetic requirements, biotic interactions, and environmental drivers (Brown et al. 1978(Brown et al. , 1993Schmidt-Nielsen 1984;Marquet and Taper 1998;Allen et al. 2006;Maurer and Marquet 2013;Smith et al. 2016) and the height (or modal size) is the most common bin size used by different species or individuals (Maurer and Marquet 2013). BSDs have been described for several marine assemblages (Warwick 1984;Kendall et al. 1997;Roy et al. 2000;Hildrew et al. 2007;Baltanas and Danielopol 2013;Gearty et al. 2018) and tend to change in response to nutrient availability, the type of ecosystem, and the level of disturbance (Warwick 1984;McClain et al. 2012). ...
... Theoretical models would suggest that BSD would emerge throughout a cladogenetic diffusion model of species body size (McKinney 1990(McKinney , 1997McShea 1994;Clauset and Erwin 2008), in which small-bodied species are expected to be common relative to larger-bodied species (Stanley 1973(Stanley , 1979Hayami 1978;Gould 1988;McShea 1994;Monroe and Bokma 2013) (Fig. 1). However, the size diffusion process can be punctuated by episodes of biotic crisis (Gould 1988;McKinney 1990), and two possible BSD shapes may result depending on the selective nature of the disturbance (Fig. 1). ...
... Theoretical models would suggest that BSD would emerge throughout a cladogenetic diffusion model of species body size (McKinney 1990(McKinney , 1997McShea 1994;Clauset and Erwin 2008), in which small-bodied species are expected to be common relative to larger-bodied species (Stanley 1973(Stanley , 1979Hayami 1978;Gould 1988;McShea 1994;Monroe and Bokma 2013) (Fig. 1). However, the size diffusion process can be punctuated by episodes of biotic crisis (Gould 1988;McKinney 1990), and two possible BSD shapes may result depending on the selective nature of the disturbance (Fig. 1). ...
Article
The synergic relationship between physiology, ecology, and evolutionary process makes the body-size distribution (BSD) an essential component of the community ecology. Body size is highly susceptible to environmental change, and extreme upheavals, such as during a mass extinction event, could exert drastic changes on a taxon's BSD. It has been hypothesized that the Late Triassic mass extinction event (LTE) was triggered by intense global warming, linked to massive volcanic activity associated with the Central Atlantic Magmatic Province. We test the effects of the LTE on the BSD of fossil bivalve assemblages from three study sites spanning the Triassic/Jurassic boundary in the United Kingdom. Our results show that the effects of the LTE were rapid and synchronous across sites, and the BSDs of the bivalves record drastic changes associated with species turnover. No phylogenetic signal of size selectivity was recorded, although semi-infaunal species were apparently most susceptible to change. Each size class had the same likelihood of extinction during the LTE, which resulted in a platykurtic BSD with negative skew. The immediate postextinction assemblage exhibits a leptokurtic BSD, although with negative skew, wherein surviving species and newly appearing small-sized colonizers exhibit body sizes near the modal size. Recovery was relatively rapid (~100 kyr), and larger bivalves began to appear during the pre-Planorbis Zone, despite recurrent dysoxic/anoxic conditions. This study demonstrates how a mass extinction acts across the size spectrum in bivalves and shows how BSDs emerge from evolutionary and ecological processes.
... According to this vision, Cope's rule is simply produced by an increase in size variance through time (Gould, 1988b). Based on these considerations, McShea (1994McShea ( , p. 1747 distinguished between passive and driven ETs, stating that, "in a driven trend, the distribution mean increases on account of a force (which may manifest itself as a bias in the direction of change) that acts on lineages throughout the trait space in which diversification occurs. In a passive system, no pervasive force or bias exists, but the mean increases because change in one direction is blocked by a boundary, or other inhomogeneity, in some limited region of the trait space". ...
... Regardless of the adopted version of landscape, these graphs are particularly indicated for the visual detection of ETs, since their conformation is likely to reflect the acting evolutionary pattern. For instance, in the absence of boundaries (i.e., driven ETs sensu McShea, 1994), favoured phenotypes are distributed along a linear line whenever directionality is common to the entire sample. The resulting evolutionary landscape is a rectilinear ridge (Fig. 3A). ...
... By contrast, non-directional evolution, as in the case of BM evolution, is likely to generate a flat surface (Fig. 3E). In the presence of boundaries (i.e., passive ETs sensu McShea, 1994), their position in the evolutionary landscape contributes to determining the resulting evolutionary pattern. ...
Article
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Evolutionary trends (ETs) are traditionally defined as substantial changes in the state of traits through time produced by a persistent condition of directional evolution. ETs might also include directional responses to ecological, climatic or biological gradients and represent the primary evolutionary pattern at high taxonomic levels and over long-time scales. The absence of a well-supported operative definition of ETs blurred the definition of conceptual differences between ETs and other key concepts in evolution such as convergence, parallel evolution, and divergence. Also, it prevented the formulation of modern guidelines for studying ETs and evolutionary dynamics related to them. In phenotypic evolution, the theory of morphodynamics states that the interplay between evolutionary factors such as phylogeny, evo-devo constraints, environment, and biological function determines morphological evolution. After introducing a new operative definition, here we provide a morphodynamics-based framework for studying phenotypic ETs, discussing how understanding the impact of these factors on ETs improves the explanation of links between biological patterns and processes underpinning directional evolution. We envisage that adopting a quantitative, pattern-based, and multifactorial approach will pave the way to new potential applications for this field of evolutionary biology. In this framework, by exploiting the catalysing effect of climate change on evolution, research on ETs induced by global change might represent an ideal arena for validating hypotheses about the predictability of evolution.
... Two different pathways have been identified for this macroevolutionary increase in body size. Firstly, directional increase of mean, maximum and minimum body size towards larger values, where an increase in body size is achieved by a driven trend (McShea 1994;Turner 2009;Hunt and Rabosky 2014). Alternatively, mean and maximum body size increase can be achieved through passive diffusion within a clade when minimum body size remains unchanged (Stanley 1973;Gould 1988;Jablonski 1997). ...
... A number of previous studies have argued for the importance of testing Cope's rule within monophyletic groups, where ancestor-descendent pairs can be established (McShea 1994(McShea , 2000Jablonski 1997;Alroy 1998Alroy , 2000Wang 2005). Evidence from two different lines of arguments suggests that application of a phylogenetic approach may not produce a body size trend different than the one observed. ...
Article
Although empirical testing of Cope's rule, the tendency for size to increase over time, has received significant attention in the last few decades, there is no consensus about the applicability of this rule across taxonomic levels. In the present study, we investigate the distribution of body size of Trigoniida bivalves, at order-, family-, genus- and species-level, through the Middle Jurassic and Early Cretaceous of the Kutch region in India. Our data suggest that the body size of Trigoniida bivalves did not vary significantly in the Middle–Late Jurassic, followed by an increase after the Jurassic–Cretaceous mass extinction boundary and a reduction in the late Early Cretaceous. Changes in relative sea-level and associated sedimentary facies composition generally exhibit poor correlation with the overall stasis, or no net body size change, displayed by Trigoniida bivalves. Body-size analysis across taxonomic hierarchy reveals that order-level trends are not a simple aggregation of trends at lower taxon levels. An important observation of our study is the body-size increase immediately in the aftermath of the Jurassic– Cretaceous mass extinction, a deviation from the general observation that size reduction occurs in post-extinction communities. We argue that this increase may be result of both ecological competition and evolutionary faunal turnover.
... Another broad hypothesis summarizing body size patterns is Cope's rule, stating that species tend to increase in size over evolutionary time. Explanations for Cope's rule are thought to be linked to fitness advantages at larger body sizes or an increase in size variance as lineages diversify from a smaller ancestor following a passive trend (15,16). Cope's rule could simply be an evolutionary or temporal manifestation of Bergmann's rule if lineages evolve larger body sizes during periods of climatic cooling (8). ...
Article
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Body size is an important species trait, correlating with life span, fecundity, and other ecological factors. Over Earth’s geological history, climate shifts have occurred, potentially shaping body size evolution in many clades. General rules attempting to summarize body size evolution include Bergmann’s rule, which states that species reach larger sizes in cooler environments and smaller sizes in warmer environments, and Cope’s rule, which poses that lineages tend to increase in size over evolutionary time. Tetraodontiform fishes (including pufferfishes, boxfishes, and ocean sunfishes) provide an extraordinary clade to test these rules in ectotherms owing to their exemplary fossil record and the great disparity in body size observed among extant and fossil species. We examined Bergmann’s and Cope’s rules in this group by combining phylogenomic data (1,103 exon loci from 185 extant species) with 210 anatomical characters coded from both fossil and extant species. We aggregated data layers on paleoclimate and body size from the species examined, and inferred a set of time-calibrated phylogenies using tip-dating approaches for downstream comparative analyses of body size evolution by implementing models that incorporate paleoclimatic information. We found strong support for a temperature-driven model in which increasing body size over time is correlated with decreasing oceanic temperatures. On average, extant tetraodontiforms are two to three times larger than their fossil counterparts, which otherwise evolved during periods of warmer ocean temperatures. These results provide strong support for both Bergmann’s and Cope’s rules, trends that are less studied in marine fishes compared to terrestrial vertebrates and marine invertebrates.
... A pesar de que dentro de los linajes es más común observar una tendencia hacia el aumento del tamaño corporal (Depéret, 1907;McShea, 1994;Carrano, 2006;Zanno y Makovicky, 2012;Benson et al., 2018), los ancestros "fundadores" de "clados exitosos", en términos de número, diversidad y longevidad, suelen ser más "pequeños" que "grandes" (Hanken y Wake, 1993;Turner et al., 2007;Novas et al., 2012;Zanno y Makovicky, 2012;Brusatte et al., 2014;Lee et al., 2014). De hecho, la miniaturización se ha postulado como una característica clave en la evolución de muchos taxones importantes tales como aves, reptiles, lagartos, serpientes, bivalvos, y otros grupos (Carroll, 1969(Carroll, , 1970(Carroll, , 1977(Carroll, , 1980Bolt, 1977;Alberch et al., 1979;Rieppel, 1984Rieppel, , 1988Seilacher, 1984;Turner et al., 2007;Brusatte et al., 2014;Lee et al., 2014). ...
Thesis
This Doctoral Thesis presents an exhaustive review of the Patagonian alvarezsaurids (Dinosauria, Theropoda). It includes a detailed osteological description of specimens of Patagonykus puertai (Holotype, MCF-PVPH-37), cf. Patagonykus puertai (MCF-PVPH-38), Patagonykinae indet. (MCF-PVPH-102), Alvarezsaurus calvoi (Holotype, MUCPv-54), Achillesaurus manazzonei (Holotype, MACN-PV-RN 1116), Bonapartenykus ultimus (Holotype, MPCA 1290), and cf. Bonapartenykus ultimus (MPCN-PV 738). A phylogenetic analysis and a discussion about the taxonomic validity of the recognized species and the taxonomic assignment of the materials MCF-PVPH-38, MCF-PVPH-102 and MPCN-PV 738 are presented. Different evolutionary and paleobiological studies were carried out in order to elucidate functional and behavioral aspects. Alvarezsaurus calvoi (MUCPv-54), Achillesaurus manazzonei (MACN-PV-RN 1116), Patagonykus puertai (MCF-PVPH-37) and Bonapartenykus ultimus (MPCA 1290) are valid species due to the presence of many autapomorphies. In this sense, the hypothesis proposed by P. Makovicky and collaborators that Achillesaurus manazzonei is a junior synonym of Alvarezsaurus calvoi is rejected. Likewise, certain morphological evidence allows hypothesizing that Alvarezsaurus calvoi represents a growth stage earlier than skeletal maturity. Specimen MCF-PVPH-38 is referable as cf. Patagonykus puertai, while MCF-PVPH-102 is considered an indeterminate Patagonykinae. In turn, MPCN-PV 738 is assigned as cf. Bonapartenykus ultimus based on the little overlapping material with the Bonapartenykus ultimus holotype. The results obtained from the mineralogical characterization through the X-ray diffraction method of specimens MPCN-PV 738 and the holotype of Bonapartenykus ultimus (MPCA 1290), allow to suggest that both specimens come from the same geographical area and stratigraphic level. The phylogenetic analysis, which is based upon the matrix of Gianechini and collaborators of 2018 with the inclusion of proper characters, and the database of Xu and collaborators of 2018, recovered the South American members of Alvarezsauria, such as Alnashetri cerropoliciensis (Candeleros Formation; Cenomanian), Patagonykus puertai (Portezuelo Formation, Turonian-Coniacian), Alvarezsaurus calvoi and Achillesaurus manazzonei (Bajo de La Carpa Formation, Coniacian-Santonian), and Bonapartenykus ultimus (Allen Formation, Campanian-Maastrichtian), nesting within the family Alvarezsauridae. In this sense, the forms that come from the Bajo de La Carpa Formation (Coniacian-Santonian) are recovered at the base of the Alvarezsauridae clade, while Alnashetri cerropoliciensis nests as a non-Patagonykinae alvarezsaurid. Regarding the type specimens of Patagonykus puertai and Bonapartenykus ultimus, they are recovered as members of the Patagonykinae subclade, a group that is recovered as a sister taxon of Parvicursorinae, both nested within the Alvarezsauridae. In addition, the topology obtained allows discerning the pattern, rhythm and time of evolution of the highly strange and derived alvarezsaurian skeleton, concluding in a gradual evolution. The Bremer and Bootstrap supports of the nodes (Haplocheirus + Aorun), [Bannykus + (Tugulusaurus + Xiyunykus)], and Patagonykinae, show indices that represent very robust values for these nodes. Likewise, these values suggest that two endemic clades originated early in Asia, while one endemic clade is observed in Patagonia, i.e., Patagonykinae. The analysis of the directional trends of the Alvarezsauria clade, tested by means of a own database on body masses based on the Christiansen and Fariña method, subsequently calibrated with the group's phylogeny using the R software, shows two independent miniaturization events in the alvarezsaurid evolution, namely the former originating from the base of the Alvarezsauridae (sustained by Alvarezsaurus), and the latter within the Parvicursorinae. Analysis of the Alvarezsauria dentition reveals possible dental synapomorphies for the Alvarezsauria clade that should be tested in an integrative phylogenetic analysis. The general characterization of the forelimb and a partial reconstruction of the myology of alvarezsaurs demonstrate different configurations for Patagonykinae and Parvicursorinae. The multivariate analyzes carried out from the databases of Elissamburu and Vizcaíno, plus that of Cau and collaborators, show that the Patagonykinae would have had ranges of movements greater than those observed in Parvicursorinae, although the latter would have had a greater capacity to carry out more strenuous jobs. The morphometric analysis of the hindlimb and the use of the Snively and collaborators equations, show that the configuration of this element in Alvarezsauria is indicative of a highly cursorial lifestyle, as well as possible particular strategies for more efficient locomotion. The topology obtained in the phylogenetic analysis that was carried out in this Doctoral Thesis, allowed clarifying the ontogenetic changes observed in the ontogenetic series of the manual ungueal element II-2 within the clade Alvarezsauridae. In addition, the multivariate analysis carried out from the manual phalanx II-2 allows us to infer that alvarezsaurs could have performed functions such as hook-and-pull and piercing, where the arm would function as a single unit. The anatomy and myology of the alvarezsaurian tail show that the caudal vertebrae of alvarezsaurians exhibit a combination of derived osteological features that suggests functions unique among theropods, such as considerable dorsal and lateral movements, as well as exceptional abilities to support distal loading of their long tail without compromising stability and/or mobility.
... Body length (excluding the telson) shifts from 3-45 mm in the Palaeozoic, to 17-24.5 cm during the Upper Jurassic and Palaeogene, and finally to >1 m in modern forms. The observed body size dynamics are consistent with a 'driven' trend (McShea, 1994), whereby the minimum size limit increases along with the average and upper limit. Driven trends suggest an underlying selection pressure towards larger body sizes, perhaps owing to predation or intraspecific interactions (competition or sexual selection) acting within the bounds of developmental and shape constraints (Bicknell et al., 2019a). ...
Article
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Records of evolutionary stasis over time are central to uncovering large-scale evolutionary modes, whether by long-term gradual change or via enduring stability punctuated by rapid shifts. The key to this discussion is to identify and examine groups with long fossil records that, ideally, extend to the present day. One group often regarded as the quintessential example of stasis is Xiphosurida, the horseshoe crabs. However, when, how and, particularly, why stasis arose in xiphosurids remain fundamental, but complex, questions. Here, we explore the protracted history of fossil and living xiphosurids and demonstrate two levels of evolutionary stability: developmental stasis since at least the Pennsylvanian and shape stasis since the Late Jurassic. Furthermore, shape and diversity are punctuated by two high-disparity episodes during the Carboniferous and Triassic-transitions that coincide with forays into habitation of marginal environments. In an exception to these general patterns, body size increased gradually over this period and, thus, cannot be described under the same, often-touted, static models of evolution. Therefore, we demonstrate that evolutionary stasis can be modular and fixed within the same group at different periods and in different biological traits, while other traits experience altogether different evolutionary modes. This mosaic in the tempo and mode of evolution is not unique to Xiphosurida but likely reflects variable mechanisms acting on biological traits, for example transitions in life modes, niche occupation and major evolutionary radiations.
... The same data show that the major control on diversification is biological interactions leading to diversity equilibrium (Alroy 1996(Alroy , 1998bStucky 1990;Webb 1969) and that major climate shifts have unpredictable and frequently inconsequential effects (Alroy 1996(Alroy , 1998bAlroy et al. 2000;Barnosky 2001;Prothero 1999;Prothero and Heaton 1996). Finally, big-picture research has revealed dramatic trends in the evolution of morphology and ecological strategies, such as the rampantly convergent evolution of dental adaptations for carnivory (Van Valkenburgh 1988) and herbivory (Hunter and Jernvall 1995), the surprisingly decoupled acquisition of cursorial locomotor adaptations in carnivores and ungulates (Janis and Wilhelm 1993;Van Valkenburgh 1985), and Cope's rule of increasing body mass (Alroy 1998a;MacFadden 1986;McShea 1994;Stanley 1973). ...
Article
Body mass distributions of mammalian species are a major focus of macroecological and macroevolutionary studies. However, these distributions may be obscured by taxonomic error, just like any other aspect of biodiversity. The key problem with taxonomy is that many currently used names are synonyms of each other or are biologically indeterminate. This article reassesses body mass patterns in the fossil record of North American mammals using the recently developed flux ratio method for estimating the underlying proportion of invalid names. Current name quality varies very strongly with body mass: small species names are highly unreliable, but names of large species have been evaluated thoroughly. The main reason is that there has been a dramatic fall through historical time in the average size of described species. Hence, there simply has not been enough time yet to reevaluate the names of most small species. This bias only accentuates the previously described bimodal diversity distribution for North American mammals, which suggests the existence of dual body mass optima—so not all evolutionary lineages converge on 100 g. The historical shift in the underlying quality and body mass of newly described species also differentially affects our picture of biodiversity in major taxonomic groups. On the one hand, ungulate and carnivoran names are much more likely to be invalid in the 1st place than are rodent and insectivoran names. On the other hand, most of the invalid names for large mammals already have been identified, but this is not true for the small-mammal groups. Therefore, the most fruitful strategy for future taxonomic research would be to focus on small- and medium-sized mammals.
... In a random walk, the probability of arriving at a larger distance from the origin increases with the number of steps. In same way, the probability of developing more complex organisms increases over time [40,21]. ...
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Machine intelligence can develop either directly from experience or by inheriting experience through evolution. The bulk of current research efforts focus on algorithms which learn directly from experience. I argue that the alternative, evolution, is important to the development of machine intelligence and underinvested in terms of research allocation. The primary aim of this work is to assess where along the spectrum of evolutionary algorithms to invest in research. My first-order suggestion is to diversify research across a broader spectrum of evolutionary approaches. I also define meta-evolutionary algorithms and argue that they may yield an optimal trade-off between the many factors influencing the development of machine intelligence.
... At least two hypotheses could explain variation in patterns of character state saturation among groups and through time, either (1) intrinsic factors such as inherited developmental or functional limitations on the range of possible forms and evolutionary changes or (2) extrinsic factors such as ecological or selective forces. 20,40,41 The occurrence of a rapid, timecorrelated shift as found here (rather than a clade-specific shift) cannot readily be explained by relaxation of intrinsic developmental or functional limitations. We therefore focus on examining extrinsic, ecological explanations for this prominent phase-shift in mammalian phenotypic evolution. ...
Article
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The consensus among evolutionists seems to be (and has been for at least a century) that the morphological complexity of organisms increases in evolution, although almost no empirical evidence for such a trend exists. Most studies of complexity have been theoretical, and the few empirical studies have not, with the exception of certain recent ones, been especially rigorous; reviews are presented of both the theoretical and empirical literature. The paucity of evidence raises the question of what sustains the consensus, and a number of suggestions are offered, including the possibility that certain cultural and/or perceptual biases are at work. In addition, a shift in emphasis from theoretical to empirical inquiry is recommended for the study of complexity, and guidelines for future empirical studies are proposed.
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The morphological order of evolutionary trees has been the traditional argument for the operation of directional causes in macroevolution. We show, in this work, that a similar order can be generated within stochastic systems bounded by minimal biological constraints. Our system generates an evolutionary tree by making random decisions about each lineage in each time interval given preset probabilities for branching, extinction and persistence (Raup, Gould, Schopf, and Simberloff, 1973). Morphology is determined in an independent and equally stochastic manner. Using ten hypothetical characters, the beginning lineage is given an all zero morphology. At each branching point, each character may change by one unit (in a positive or negative direction) according to preset probabilities for positive change, negative change, and no change. Our simulations display most of the ordered features generally associated with uni-directional selection: morphological coherence of monophyletic groups and incomplete filling of "morphological space"; regular "unfolding" of morphology (as seen in strong correspondence between phenetic and cladistic taxonomies); marked evolutionary "trends"; strong correlation among characters; large variation in rates of evolution; and specialization of derived forms. We attribute much of this order to abstract topological properties of the tree itself and urge that the data for inferences about directional causes be sought elsewhere (in functional morphology, for example). We suggest, with caution, that undirected selection may be the rule rather than the exception in nature, if a temporal unit of sufficient duration be used as the yardstick of measurement.
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The ecological context of large-scale evolutionary patterns has been neglected. Several workers have recently reported a bathymetric bias in the evolution of benthic marine communities, such that nearshore assemblages tend to contain advanced taxa and community structures and offshore assemblages contain more archaic features. A clade-by-clade analysis is the most powerful approach for assessing the generality of the pattern and testing hypotheses on underlying mechanisms. We have constructed three detailed time-environment histories from the primary literature, for the crinoid Order Isocrinida (based on 99 early Triassic-Recent occurrences), the bryozoan Order Cheilostomata (67 late Jurassic-late Eocene occurrences), and the bivalve Superfamily Telliinaceae (70 late Triassic-Miocene occurrences). These three time-environment diagrams as well as numerous anectodal reports suggest that the onshore-offshore trends previously reported for communities are actually underlain by individualistic clade histories that only appear to act in concert when viewed on a coarse time scale. -from Authors
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Whether body size will increase or decrease in an evolving population depends on whether mean body size is larger or smaller than the optimum for the population. Cope's Rule, the generalization that most animal groups have evolved toward larger body size, cannot be explained by intrinsic advantages of large size. Rather, it is the tendency of groups to arise at small body size relative to their optima that produces the widely observed pattern of net size increase. The specialized nature of large species of a given body plan, required by problems of similitude, renders these forms unlikely potential ancestors for major new descendent taxa. The adaptive discontinuity that must be crossed for invasion of a new adaptive zone at large body size exists because of the need for descendent taxa to be specialized along new lines. These factors tend to restrict large-scale adaptive breakthroughs to small body sizes. Size changes probably tend to occur sporadically, during speciation events. Size increase is not inherently favored in speciation, but prevails during diversification because origin of a higher taxon at small body size concentrates many early species in the small size range. Nearly all diverse animal orders and classes, and many families and super-families, are composed of species whose body sizes are distributed as positively skewed histograms. The typical pattern of size change during diversification of such a group can be determined from time-series plots for fossil species of diversifying higher taxa. A major taxon normally arises at small body size relative to its potential size range, and a slightly skewed histogram is rapidly formed. The histogram may expand or contract slightly in the small size range as diversification proceeds, but spreads continually farther in a positive direction, to develop a strongly attenuated tail in the large size range. Skewing occurs very rapidly because possible increments of size change with speciation are not constant throughout a taxon's size range, but are a direct function of body size, so that early spreading of the range proceeds more rapidly in a positive direction than in a negative direction. Nearly always an increase in mean size results. Just as taxonomic and morphologic diversification approach limits as a group's potential adaptive zone is filled, the size-frequency plot approaches a limiting distribution. The attenuated right flank of a high-diversity distribution reflects not only well known ecological factors, but also the fact that structural specialization at large relative body size for a given higher taxon gradually limits the range of potential morphologies (and hence diversity). The left flank is often steeper even than that of a Gaussian distribution partly because the onset of factors determining minimum size limits tends to be abrupt. In the Aves and Mammalia, for example, the curve for metabolic rate versus size turns sharply upward in the 4-5 gram range. In both groups many species are only slightly larger than this size, but hardly any are smaller. For most highly diversified poikilothermous metazoan groups, the minimum space required for fundamental organ systems is abruptly limiting. The probabilistic explanation offered for Cope's Rule implies that the rule is more fruitfully viewed as describing evolution from small size rather than toward large size. Although his interpretation was erroneous, Cope himself adopted this viewpoint. Strangely, it has been abandoned by most modern workers who have analyzed Cope's Rule.
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The diversity of body sizes of organisms has traditionally been explained in terms of microevolutionary processes: natural selection owing to differential fitness of individual organisms, or to macroevolutionary processes: species selection owing to the differential proliferation of phylogenetic lineages. Data for terrestrial mammals and birds indicate that even on a logarithmic scale frequency distributions of body mass among species are significantly skewed towards larger sizes. We used simulation models to evaluate the extent to which macro- and microevolutionary processes are sufficient to explain these distributions. Simulations of a purely cladogenetic process with no bias in extinction or speciation rates for different body sizes did not produce skewed log body mass distributions. Simulations that included size-biased extinction rates, especially those that incorporated anagenetic size change within species between speciation and extinction events, regularly produced skewed distributions. We con
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We describe the nonrandom assembly of the North American terrestrial mammalian fauna based on body size and spatial scale. The frequency distribution of body masses among species for the entire continental fauna was highly modal and right skewed, even on a logarithmic scale; the median size of the 465 species was approximately 45 g. In contrast, comparable frequency distributions for 24 small patches of relatively homogeneous habitat were essentially uniform, with approximately equal numbers of species in each logarithmic size class; the median sizes of the 19-37 species ranged from approximately 100 to 2,500 g. Frequency distributions for 21 biomes (large regions of relatively similar vegetation) were intermediate between the continental and local assemblages. This pattern of assembly indicates that species of modal size (20-250 g) tend not to coexist in local habitat patches and they replace each other more frequently from habitat to habitat across the landscape than species of relatively large or sm
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