Sarah Leventhal

Sarah Leventhal

Postdoctoral fellow, Department of Paleobiology, NMNH. Big fan of modular invertebrates

Conference Presentations

The chaotic evolution of aggregate traits in cheilostome bryozoans

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The evolution of trait variation among populations of animals is difficult to study due to the many overlapping genetic and environmental influences that control phenotypic expression. But, in a group of animals called bryozoans, it is possible to isolate genetic contributions to phenotypic variation, due to the modular nature of bryozoan colonies. Each bryozoan colony represents a snapshot of the phenotypes that correspond to a single genotype, which can be summarized as a phenotypic distribution. We test whether these phenotypic distributions are evolvable across a generation of colonies in two sister species of the extant bryozoan Stylopoma, grown and bred in a common garden breeding experiment. We find that components of phenotypic distributions, specifically the median and median absolute deviation of trait values of colony members, are evolvable between generations of colonies. Furthermore, this evolvability has macroevolutionary importance because it correlates with the morphological distance between these two species. Since these phenotypic distributions are evolvable, and this evolvability corresponds to evolutionary divergence between species, we infer that these distributions can shift and change shape over macroevolutionary timescales. Such changes to phenotypic distributions across many generations of colonies may underpin the emergence of colony-level traits, like division of labor in colonies.

Evolution of Avicularia in Cretaceous Cheilostome Bryozoan Wilbertopora

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Cheilostome bryozoans are a diverse clade of colonial animals that first appeared during the Jurassic period. For the first 100 million years of their existence, colonies of cheilostomes were monomorphic, composed entirely of standard feeding individuals termed autozooids. Divergent body types, termed avicularia, first appeared in colonies in the early Cretaceous period, in the genus Wilbertopora. Over the course of its lifespan, Wilbertopora diversified into a total of 28 species, spanning the early Albian stage at least through the Maastrichtian. As avicularia are derived from autozooids, quantifying the patterns of divergence between autozooids and avicularia size and shape in Wilbertopora will help us to understand the evolutionary processes involved in the evolution of avicularia. In this study, we quantify autozooid and avicularia shape and size to evaluate how the morphological disparity of body types in colonies changes over the course of Wilbertopora diversification. By comparing changes in morphological disparity to changes in diversity we can assess the importance of functional and developmental evolution. If disparity increases more rapidly than diversity, then new species are exploring new ecologies by prioritizing the evolution of form. However, if diversity increases more rapidly than disparity new species are in a mode of ecological exploitation and increasing the frequencies of successful forms.

Colonial division of labor – Wilbertopora and the evolution of avicularia

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Polymorphism, or variation in body type, is a common feature of Cheilostome bryozoans, occurring in several independent evolutionary lineages. Bryozoan polymorphism is often compared to the caste system in ants, where heterochrony allows for different castes in a colony to occupy different morphological spaces. However, ontogeny in bryozoans operates differently than in ants. Individual zooids bud from their mothers, sometimes in several generations at once, and their exoskeletons are constructed separately. In this type of development, there is no ontogenetic progression from juvenile to adult morphology. Therefore, it is not viable to invoke heterochrony as an evolutionary determinant of polymorphism without empirical study. Here we show that the morphological differences between polymorphs and standard feeding zooids in several closely related species of the Cretaceous bryozoan Wilbertopora are perhaps underlain by an entirely different evolutionary mechanism that promoted the differentiation of zooid body type. We propose that the evolution of similarity patterns between zooids in colonies allowed for polymorphism to evolve.

Evolvable Heredity and Morphological Differentiation in Cheilostome Bryozoans, Rocky Mountain Geobiology Symposium 2021

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Polymorphism, or variation in body type, is a common feature of Cheilostome bryozoans, occurring in several independent evolutionary lineages. Bryozoan polymorphism is often compared to the caste system in ants, where heterochrony allows for different castes in a colony to occupy different morphological spaces. However, ontogeny in bryozoans operates differently than in ants. Individual zooids bud from their mothers, sometimes in several generations at once, and their exoskeletons are constructed separately. In this type of development, there is no ontogenetic progression from juvenile to adult morphology. Therefore, it is not viable to invoke heterochrony as an evolutionary determinant of polymorphism without empirical study. Here we show that the morphological differences between polymorphs and autozooids in several closely related species of the Cretaceous bryozoan Wilbertopora cannot be explained by heterochrony alone, and that a different evolutionary mechanism promoted the differentiation of zooid body type. We propose that the evolution of heredity between individuals in colonies allowed for polymorphism to evolve.

Trait Heredity as a predictor of morphological distance between closely related species, American Societey of Naturalists Virtual Asilomar 2021

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Since Dolph Schluter’s landmark paper on structures of trait variation and its relationship to morphological distance between closely related species, it has been accepted that evolution tends to occur along genetic lines of least resistance – that is, in the direction of maximum phenotypic variance in populations. In this small study, I look at the heredity structures in populations of two closely related bryozoans belonging to the genus Stylopoma. Medians of member-level trait values in colonies appear to be heritable between parent and offspring colonies. Interestingly, median trait values that are most heritable correspond to trait values that are the most divergent between the two species. I propose that the degree of heredity of trait values is predictive of morphological distance in closely related taxa, a corollary to work on phenotypic variance and its relationship to evolutionary divergence.

The Colony as an Evolutionary Unit, Geological Society of America Annual Meeting 2020

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Bryozoan colonies grow by asexual propagation of members (“zooids”) within a colony. Many bryozoans produce calcified skeletons that allow for straightforward morphometric analysis of their biology after death. One such genus of bryozoans, Stylopoma­, has a wide range of morphological variation in zooids within a colony. Previous studies on the morphometrics of members within colonies have shown that there are no heritable features passed between individuals. This pattern limits the amount of evolution that occurs within a colony, since phenotypic changes don’t accumulate along specific lineages. However, different Stylopoma species are morphometrically distinct, showing that even though these same attributes cannot evolve within colonies, they can and have evolved across different species. Therefore, the pattern of evolution within colonies must be reconciled with the pattern of evolution between species. To study this evolutionary process, we test the idea that aggregate values of traits present in colony members are heritable between colonies, rather than between individuals. Furthermore, we propose that colonies in Stylopoma, and not individuals, serve as the primary unit of evolutionary change. To test this hypothesis, we studied pairs of parent and offspring colonies raised in a common garden breeding experiment with known maternity, wherein we measured and analyzed 5 different member-level traits. The results suggest that the median value for each of these traits is heritable between generations of colonies, with varying levels of heritability for the interquartile ranges and the median absolute deviations. These findings indicate that aggregate member-level traits serve as heritable features at the colony level of organization in these animals. Therefore, the unit of evolutionary change in Stylopoma appears to be the colony. Our results provide evidence of a novel evolutionary transition present in Stylopoma; inheritance does not follow the individual, but rather a higher hierarchical level: the colony. Additionally, this result suggests that group-level inheritance of aggregate traits is possible, and that further study of modular colonial animals could shed more light on how widespread group-level inheritance may be in the animal kingdom.

The Origin of Phenotypic Plasticity in Stylopoma, North American Paleontological Convention

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Bryozoans colonies grow by asexual propagation of members (“zooids”) within a colony. Previous studies of the cheilostome bryozoan Stylopoma have shown that there is a wide range of morphological variation of zooids within in a colony. This pattern limits the amount of evolution that occurs within a colony, since phenotypic changes don’t accumulate. But different Stylopoma species are morphometrically distinct, showing that even though these same attributes cannot evolve within colonies, they can and have evolved across different species. Therefore, the pattern of evolution within colonies must be screened off from the pattern of evolution between species. To study this evolutionary process, I test the idea that the degree of plasticity of morphology within a colony is evolving rather than individual phenotypes of zooids within in a colony. Using Stylopoma colonies bred in a common garden breeding experiment, I measure the heritability of phenotypic plasticity between colonies.