Sarnat, E. M. | Search bioRxiv

bioRxiv2019

Macroevolutionary integration of phenotypes within and across ant worker castes

Phenotypic traits are often integrated into evolutionary modules: sets of organismal parts that evolve together. In social insect colonies the concepts of integration and modularity apply to sets of traits both within and among functionally and phenotypically differentiated castes. On macroevolutionary timescales, patterns of integration and modularity within and across castes can be clues to the selective and ecological factors shaping their evolution and diversification. We develop a set of hypotheses describing contrasting patterns of worker integration and apply this framework in a broad (246 species) comparative analysis of major and minor worker evolution in the hyperdiverse ant genus Pheidole. Using geometric morphometrics in a phylogenetic framework, we inferred fast and tightly integrated evolution of mesosoma shape between major and minor workers, but slower and more independent evolution of head shape between the two worker castes. Thus, Pheidole workers are evolving as a mixture of intra- and inter-caste integration and rate heterogeneity. The decoupling of homologous traits across worker castes may represent an important process facilitating the rise of social complexity.

evolutionary biology↗

bioRxiv2018

Evolution of the latitudinal diversity gradient in the hyperdiverse ant genus Pheidole

AimThe latitudinal diversity gradient is the dominant pattern of life on Earth, but a consensus understanding of its origins has remained elusive. The analysis of recently diverged, hyper-rich invertebrate groups provides an opportunity to investigate latitudinal patterns with the statistical power of large trees while minimizing potentially confounding variation in ecology and history. Here, we synthesize global phylogenetic and macroecological data on a hyperdiverse (>1100 species) ant radiation, Pheidole, and evaluate the roles of three general explanations for the latitudinal gradient: variation in diversification rate, tropical conservatism, and ecological regulation.\n\nLocationGlobal.\n\nTime PeriodThe past 35 million years.\n\nMajor taxa studiedThe hyperdiverse ant genus Pheidole Westwood.\n\nMethodsWe assembled geographic data for 1499 species and morphospecies, and inferred a dated phylogeny of Pheidole of 449 species, including 150 species newly sequenced for this study. We tested correlations between diversification rate and latitude with BAMM, HiSSE, GeoSSE, and FiSSE, and examined patterns of diversification as Pheidole spread around the globe.\n\nResultsWe found that Pheidole diversification occurred in series of bursts when new continents were colonized, followed by a slowdown in each region. There was no evidence of systematic variation of net diversification rates with latitude across any of the methods. Additionally, we found latitudinal affinity is moderately conserved with a Neotropical ancestor and phylogenetic inertia alone is sufficient to produce the gradient pattern.\n\nMain ConclusionsOverall our results are consistent with tropical conservatism explaining the diversity gradient, while providing no evidence that diversification rate varies systematically with latitude. There is evidence of ecological regulation on continental scales through the pattern of diversification after colonization. These results shed light on the mechanisms underlying the diversity gradient, while contributing toward a much-needed invertebrate perspective on global biodiversity dynamics.

evolutionary biology↗