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1. Diatom abundance in the polar oceans is predicted by genome size

   https://doi.org/10.1371/journal.pbio.3002733  

   Roberts, Wade R; Siepielski, Adam M; Alverson, Andrew J (August 2024, PLOS Biology)

   Tanentzap, Andrew J (Ed.)

   A principal goal in ecology is to identify the determinants of species abundances in nature. Body size has emerged as a fundamental and repeatable predictor of abundance, with smaller organisms occurring in greater numbers than larger ones. A biogeographic component, known as Bergmann’s rule, describes the preponderance, across taxonomic groups, of larger-bodied organisms in colder areas. Although undeniably important, the extent to which body size is the key trait underlying these patterns is unclear. We explored these questions in diatoms, unicellular algae of global importance for their roles in carbon fixation and energy flow through marine food webs. Using a phylogenomic dataset from a single lineage with worldwide distribution, we found that body size (cell volume) was strongly correlated with genome size, which varied by 50-fold across species and was driven by differences in the amount of repetitive DNA. However, directional models identified temperature and genome size, not cell size, as having the greatest influence on maximum population growth rate. A global metabarcoding dataset further identified genome size as a strong predictor of species abundance in the ocean, but only in colder regions at high and low latitudes where diatoms with large genomes dominated, a pattern consistent with Bergmann’s rule. Although species abundances are shaped by myriad interacting abiotic and biotic factors, genome size alone was a remarkably strong predictor of abundance. Taken together, these results highlight the cascading cellular and ecological consequences of macroevolutionary changes in an emergent trait, genome size, one of the most fundamental and irreducible properties of an organism. 

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2. Latitudinal variation in snowshoe hare ( Lepus americanus ) body mass: a test of Bergmann’s rule

   https://doi.org/10.1139/cjz-2019-0184  

   Gigliotti, Laura C.; Berg, Nathan D.; Boonstra, Rudy; Cleveland, Shawn M.; Diefenbach, Duane R.; Gese, Eric M.; Ivan, Jacob S.; Kielland, Knut; Krebs, Charles J.; Kumar, Alexander V.; et al (February 2020, Canadian Journal of Zoology)

   The relationship between body size and latitude has been the focus of dozens of studies across many species. However, results of testing Bergmann’s rule — that organisms in colder climates or at higher latitudes possess larger body sizes — have been inconsistent across studies. We investigated whether snowshoe hares (Lepus americanus Erxleben, 1777) follow Bergmann’s rule by investigating differences in body mass using data from six published studies and from data of 755 individual hares captured from 10 populations across North America covering 26° of north latitude. We also explored alternative hypotheses related to variation in hare body mass, including winter severity, length of growing season, elevation, and snow depth. We found body mass of hares varied throughout their range, but the drivers of body mass differed based on geographic location. In northern populations, females followed Bergmann’s rule, whereas males did not. In northern populations, male mass was related to mean snow depth. In contrast, in southern populations, body mass of both sexes was related to length of the growing season. These differences likely represent variation in the drivers of selection. Specifically, in the north, a large body size is beneficial to conserve heat because of low winter temperatures, whereas in the south, it is likely due to increased food supply associated with longer growing seasons. 

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3. Body size evolution in crocodylians and their extinct relatives

   https://doi.org/10.1002/9780470015902.a0029089  

   Godoy PL, Turner AH (October 2020, Els)

   null (Ed.)

   Crocodylians are currently facing evolutionary decline. This is evinced by the rich fossil record of their extinct relatives, crocodylomorphs, which show not only significantly higher levels of biodiversity in the past but also remarkable morphological disparity and higher ecological diversity. In terms of body size, crocodylians are mostly large animals (>2m), especially when com-pared to other extant reptiles. In contrast, extinct crocodylomorphs exhibited a 10-fold range in body sizes, with early terrestrial forms often quite small.Recent research has shed new light on the tempo and mode of crocodylomorph body size evolution,demonstrating a close relationship with ecology, in which physiological constraints contribute to the larger sizes of marine species. Abiotic environmental factors can also play an important role with in individual subgroups. Crocodylians, for instance,have been experiencing an average size increase during Cenozoic, which seems to be related to along-term process of global cooling. 

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4. Migratory lineages rapidly evolve large body sizes in ray-finned fishes

   https://doi.org/doi.org/10.1098/rspb.2019.2615  

   Burns, M.; Bloom, D.D. (January 2020, Proceedings of the Royal Society of London)

   Migratory animals respond to environmental heterogeneity by predictably moving long distances in their lifetime. Migration has evolved repeatedly in animals, and many adaptations are found across the tree of life that increase migration efficiency. Life-history theory predicts that migratory species should evolve a larger body size than non-migratory species, and some empirical studies have shown this pattern. A recent study analysed the evolution of body size between diadromous and non-diadromous shads, herrings, anchovies and allies, finding that species evolved larger body sizes when adapting to a diadromous lifestyle. It remains unknown whether different fish clades adapt to migration similarly. We used an adaptive landscape framework to explore body size evolution for over 4500 migratory and non-migratory species of ray-finned fishes. By fitting models of macroevolution, we show that migratory species are evolving towards a body size that is larger than non-migratory species. Furthermore, we find that migratory lineages evolve towards their optimal body size more rapidly than non-migratory lineages, indicating body size is a key adaption for migratory fishes. Our results show, for the first time, that the largest vertebrate radiation on the planet exhibited strong evolutionary determinism when adapting to a migratory lifestyle. 

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5. Spotted Hyena skull size variation across geography favors the energetic equivalence rule over Bergmann’s Rule

   https://doi.org/10.1093/jmammal/gyae023  

   Cavalieri, Cybil N.; McElhinny, Teresa L.; Holekamp, Kay E.; Lundrigan, Barbara L.; Wang, ed., Guiming (April 2024, Journal of Mammalogy)

   Abstract Much historic work has focused on establishing geographical and ecological rules that broadly explain patterns in size variation. We examined geographic variation in Spotted Hyena skull size using geometric morphometrics and spatial statistics. We quantified size variation and sexual size dimorphism of the skull, and evaluated the influence of temperature, precipitation, land cover type, and population density on skull size. We found that female spotted hyenas are slightly larger on average than males. Our analysis of regional differences did not indicate geographic variation in sexual size dimorphism. Skull size of Spotted Hyenas varies with geography but does not adhere to Bergmann’s Rule. The smallest individuals of both sexes occur between −5.00° and 10.00° latitude and east of 28.50° longitude, with larger individuals being found elsewhere. Although Spotted Hyena skull size co-varies in some views with such variables as habitat type and climate indicators, skull size in this species most strongly co-varies with population density. The highest population densities are associated with the smallest skull size, possibly reflecting a relationship between high population density and access to resources. These results suggest that geographic variation in Spotted Hyena skull size is better explained by the energetic equivalence rule than Bergmann’s Rule. 

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