The genomic signatures of evolutionary stasis.

Evolutionary stasis characterizes lineages that seldom speciate and show little phenotypic change over long stretches of geological time. Although lineages that appear to exhibit evolutionary stasis are often called living fossils, no single mechanism is thought to be responsible for their slow rates of morphological evolution and low species diversity. Some analyses of molecular evolutionary rates in a handful of living fossil lineages have indicated that these clades exhibit slow rates of genomic change. Here, we investigate mechanisms of evolutionary stasis using a dataset of 1,105 exons for 481 vertebrate species. We demonstrate that two ancient clades of ray-finned fishes classically called living fossils, gars and sturgeons, exhibit the lowest rates of molecular substitution in protein-coding genes among all jawed vertebrates. Comparably low rates of evolution are observed at fourfold degenerate sites in gars and sturgeons, implying a mechanism of stasis decoupled from selection that we speculate is linked to a highly effective DNA repair apparatus. We show that two gar species last sharing common ancestry over 100 million years ago produce morphologically intermediate and fertile hybrids in the wild. This makes gars the oldest naturally hybridizing divergence among eukaryotes and supports a theoretical prediction that slow rates of nucleotide substitution across the genome slow the accumulation of genetic incompatibilities, enabling hybridization across deeply divergent lineages and slowing the rate of speciation over geological timescales. Our results help establish molecular stasis as a barrier to speciation and phenotypic innovation and provide a mechanism to explain the low species diversity in living fossil lineages.

Phylogeography, hybridization, and species discovery in the Etheostoma nigrum complex (Percidae: Etheostoma: Boleosoma).

The history of riverine fish diversification is largely a product of geographic isolation. Physical barriers that reduce or eliminate gene flow between populations facilitate divergence via genetic drift and natural selection, eventually leading to speciation. For freshwater organisms, diversification is often the product of drainage basin rearrangements. In young clades where the history of isolation is the most recent, evolutionary relationships can resemble a tangled web. One especially recalcitrant group of freshwater fishes is the Johnny Darter (Etheostoma nigrum) species complex, where traditional taxonomy and molecular phylogenetics indicate a history of gene flow and conflicting inferences of species diversity. Here we assemble a genomic dataset using double digest restriction site associated DNA (ddRAD) sequencing and use phylogenomic and population genetic approaches to investigate the evolutionary history of the complex of species that includes E. nigrum, E. olmstedi, E. perlongum, and E. susanae. We reveal and validate several evolutionary lineages that we delimit as species, highlighting the need for additional work to formally describe the diversity of the Etheostoma nigrum complex. Our analyses also identify gene flow among recently diverged lineages, including one instance involving E. susanae, a localized and endangered species. Phylogeographic structure within the Etheostoma nigrum species complex coincides with major geologic events, such as parallel divergence in river basins during Pliocene inundation of the Atlantic coastal plain and multiple northward post-glacial colonization routes tracking river basin rearrangements. Our study serves as a nuanced example of how low dispersal rates coupled with geographic isolation among disconnected river systems in eastern North America has produced one of the world's freshwater biodiversity hotspots.

Hidden species diversity in an iconic living fossil vertebrate.

Ancient, species-poor lineages persistently occur across the Tree of life. These lineages are likely to contain unrecognized species diversity masked by the low rates of morphological evolution that characterize living fossils. Halecomorphi is a lineage of ray-finned fishes that diverged from its closest relatives before 200 Ma and is represented by only one living species in eastern North America, the bowfin, Amia calva Linnaeus. Here, we use double digest restriction-site-associated DNA sequencing and morphology to illuminate recent speciation in bowfins. Our results support the delimitation of a second living species of Amia, with the timing of diversification dating to the Plio-Pleistocene. This delimitation expands the species diversity of an ancient lineage that is integral to studies of vertebrate genomics and development, yet is facing growing conservation threats driven by the caviar fishery.

Checklists in Femur Fractures: High Adherence After Implementation of Computer-based Pediatric Femur Guidelines.

INTRODUCTION: The American Academy of Orthopaedic Surgeons (AAOS) created an evidence-based clinical practice guideline for the care of pediatric diaphyseal femur fractures in 2010. Our institution implemented checklists based off these guidelines embedded in a standardized EMR order. The purpose of this study was to describe compliance with checklist completion and to assess safety improvement in a large urban pediatric hospital. METHODS: Retrospective and prospective data were collected from 2 years before and 5 years after checklist implementation. This included the patient safety checklists from August 2011 through August 2016. Patients aged 0 to 18 years with a diaphyseal femur fracture were queried from the EMR and included in this study. Patient charts were reviewed for complications, including nerve injury, pressure sore, leg length discrepancy, loss of reduction, failure of fixation, nonunion, delayed union, and infection. Compliance rates were reported based on the AAOS clinical practice guidelines. RESULTS: A total of 313 patients for the postchecklist period were reviewed in this study. Of 219 patients eligible for inclusion, 198 had checklists completed (group B). This group was compared with 100 patients with diaphyseal femur fractures from the period before implementation of the checklist (group A). We found no statistical difference in the number of patients with complications between groups (12% in both groups, P = 0.988). Postoperative checklists demonstrated that 89.9% of patients (178/198) received age-appropriate treatment consistent with the AAOS guideline recommendations after implementation of the checklist. Before the checklist implementation (group A), 94% (94/100) adhered to the guidelines. CONCLUSION: This study reveals high compliance rates with the AAOS evidence-based clinical practice guideline for the management of pediatric femur fractures. Implementing standardized checklists is possible by embedding them into the EMR. Implementation of checklists did not improve compliance or patient outcomes.