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1. Clade-level constraints to ecological diversification of Phanerozoic marine animals

   Novack-Gottshall, Philip M (December 2025, Palaeontological Association)

   Why are some animal clades ecologically more diverse than others? Has this pattern held true throughout their history? I address these questions using a standardized 39-trait ecospace (functional trait) database for ca. 50,000 Phanerozoic marine animal genus life habits. This allows quantitative comparisons (using ecological richness and disparity) of how major clades (phyla and classes) have varied throughout geological history. Most clades maintain consistent levels of Phanerozoic ecological diversity. Mollusks and arthropods (and to lesser extent, their classes) are persistently diverse. However, variability is evident for many important clades. At their origins, chordates had low richness and disparity, but diversified significantly during the Devonian radiation of fishes and again after the Permian extinction. Echinoderms achieved peak richness during the Paleozoic, but increased disparity during post-Paleozoic diversification of divergent echinoids and asterozoans. Overall, there is little impact of mass extinctions in resetting clade trajectories at these broad scales. Low correlation with genus richness suggests functional experimentation is largely independent from small-scale diversification. These patterns suggest inherent constraints to ecological innovation across marine taxa, largely set during clade origins, and little impacted by mass extinctions and subsequent turnover within subclades. This research has been supported by NSF grant #2322080. 

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2. Patterns of Phenotypic Evolution Associated with Marine/Freshwater Transitions in Fishes

   https://doi.org/10.1093/icb/icac085  

   de Brito, Victor; Betancur-R, Ricardo; Burns, Michael D; Buser, Thaddaeus J; Conway, Kevin W; Fontenelle, João Pedro; Kolmann, Matthew A; McCraney, W Tyler; Thacker, Christine E; Bloom, Devin D (June 2022, Integrative and Comparative Biology)

   Abstract Evolutionary transitions between marine and freshwater ecosystems have occurred repeatedly throughout the phylogenetic history of fishes. The theory of ecological opportunity predicts that lineages that colonize species-poor regions will have greater potential for phenotypic diversification than lineages invading species-rich regions. Thus, transitions between marine and freshwaters may promote phenotypic diversification in trans-marine/freshwater fish clades. We used phylogenetic comparative methods to analyze body size data in nine major fish clades that have crossed the marine/freshwater boundary. We explored how habitat transitions, ecological opportunity, and community interactions influenced patterns of phenotypic diversity. Our analyses indicated that transitions between marine and freshwater habitats did not drive body size evolution, and there are few differences in body size between marine and freshwater lineages. We found that body size disparity in freshwater lineages is not correlated with the number of independent transitions to freshwaters. We found a positive correlation between body size disparity and overall species richness of a given area, and a negative correlation between body size disparity and diversity of closely related species. Our results indicate that the diversity of incumbent freshwater species does not restrict phenotypic diversification, but the diversity of closely related taxa can limit body size diversification. Ecological opportunity arising from colonization of novel habitats does not seem to have a major effect in the trajectory of body size evolution in trans-marine/freshwater clades. Moreover, competition with closely related taxa in freshwaters has a greater effect than competition with distantly related incumbent species. 

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3. Evolvability and Macroevolution: Overview and Synthesis

   https://doi.org/10.1007/s11692-022-09570-4  

   Jablonski, David (September 2022, Evolutionary Biology)

   Abstract 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. 

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4. Cambrian origin but no early burst in functional disparity for Class Bivalvia

   https://doi.org/10.1098/rsbl.2023.0157  

   Zhou, Sharon; Edie, Stewart M.; Collins, Katie S.; Crouch, Nicholas M.; Jablonski, David (May 2023, Biology Letters)

   Both the Cambrian explosion, more than half a billion years ago, and its Ordovician aftermath some 35 Myr later, are often framed as episodes of widespread ecological opportunity, but not all clades originating during this interval showed prolific rises in morphological or functional disparity. In a direct analysis of functional disparity, instead of the more commonly used proxy of morphological disparity, we find that ecological functions of Class Bivalvia arose concordantly with and even lagged behind taxonomic diversification, rather than the early-burst pattern expected for clades originating in supposedly open ecological landscapes. Unlike several other clades originating in the Cambrian explosion, the bivalves' belated acquisition of key anatomical novelties imposed a macroevolutionary lag, and even when those novelties evolved in the Early Ordovician, functional disparity never surpassed taxonomic diversity. Beyond this early period of animal evolution, the founding and subsequent diversification of new major clades and their functions might be expected to follow the pattern of the early bivalves—one where interactions between highly dynamic environmental and biotic landscapes and evolutionary contingencies need not promote prolific functional innovation. 

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5. Approaches to macroevolution: 2. Sorting of variation, some overarching issues, and general conclusions.

   Jablonski, D. (July 2017, Evolutionary biology)

   Abstract Approaches to macroevolution require integration of its two fundamental components, within a hierarchical framework. Following a companion paper on the origin of variation, I here discuss sorting within an evolutionary hierarchy. Species sorting—sometimes termed species selection in the broad sense, meaning differential origination and extinction owing to intrinsic biological properties—can be split into strict-sense species selection, in which rate differentials are governed by emergent, species-level traits such as geographic range size, and effect macroevolution, in which rates are governed by organism-level traits such as body size; both processes can create hitchhiking effects, indirectly causing the proliferation or decline of other traits. Several methods can operationalize the concept of emergence, so that rigorous separation of these processes is increasingly feasible. A macroevolutionary tradeoff, underlain by the intrinsic traits that influence evolutionary dynamics, causes speciation and extinction rates to covary in many clades, resulting in evolutionary volatility of some clades and more subdued behavior of others; the few clades that break the tradeoff can achieve especially prolific diversification. In addition to intrinsic biological traits at multiple levels, extrinsic events can drive the waxing and waning of clades, and the interaction of traits and events are difficult but important to disentangle. Evolutionary trends can arise in many ways, and at any hierarchical level; descriptive models can be fitted to clade trajectories in phenotypic or functional spaces, but they may not be diagnostic regarding processes, and close attention must be paid to both leading and trailingedges of apparent trends. Biotic interactions can have negative or positive effects on taxonomic diversity within a clade, but cannot be readily extrapolated from the nature of such interactions at the organismic level. The relationships among macroevolutionary currencies through time (taxonomic richness, morphologic disparity, functional variety) are crucial for understanding the nature of evolutionary diversification. A novel approach to diversity-disparity analysis shows that taxonomic diversifications can lag behind, occur in concert with, or precede, increases in disparity. Some overarching issues relating to both the origin and sorting of clades and phenotypes include the macroevolutionary role of mass extinctions, the potential differences between plant and animal macroevolution, whether macroevolutionary processes have changed through geologic time, and the growing human impact on present-day macroevolution. Many challenges remain, but progress is being made on two of the key ones: (a) the integration of variation-generating mechanisms and the multilevel sorting processes that act on that variation, and (b) the integration of paleontological and neontological approaches to historical biology. 

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