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The number of reference genomes of snakes lags behind several other vertebrate groups (e.g. birds and mammals). However, in the last two years, a concerted effort by researchers from around the world has produced new genomes of snakes representing members from several new families. Here, we present a high-quality, annotated genome of the central ratsnake (Pantherophis alleghaniensis), a member of the most diverse snake lineage, Colubroidea. Pantherophis alleghaniensis is found in the central part of the Nearctic, east of the Mississippi River. This genome was sequenced using 10X Chromium synthetic long reads and polished using Illumina short reads. The final genome assembly had an N50 of 21.82 Mb and an L50 of 22 scaffolds with a maximum scaffold length of 82.078 Mb. The genome is composed of 49.24% repeat elements dominated by long interspersed elements. We annotated this genome using transcriptome assemblies from 14 tissue types and recovered 28,368 predicted proteins. Finally, we estimated admixture proportions between two species of ratsnakes and discovered that this specimen is an admixed individual containing genomes from the western (Pantherophis obsoletus) and central ratsnakes (P. alleghaniensis). We discuss the importance of considering interspecific admixture in downstream approaches for inferring demography and phylogeny.

A fundamental assumption of evolutionary biology is that phylogeny follows a bifurcating process. However, hybrid speciation and introgression are becoming more widely documented in many groups. Hybrid inference studies have been historically limited to small sets of taxa, while exploration of the prevalence and trends of reticulation at deep time scales remains unexplored. We study the evolutionary history of an adaptive radiation of 109 gemsnakes in Madagascar (Pseudoxyrhophiinae) to identify potential instances of introgression. Using several network inference methods, we find 12 reticulation events within the 22-million-year evolutionary history of gemsnakes, producing 28% of the diversity for the group, including one reticulation that resulted in the diversification of an 18 species radiation. These reticulations are found at nodes with high gene tree discordance and occurred among parental lineages distributed along a north-south axis that share similar ecologies. Younger hybrids occupy intermediate contact zones between the parent lineages showing that post-speciation dispersal in this group has not eroded the spatial signatures of introgression. Reticulations accumulated consistently over time, despite drops in overall speciation rates during the Pleistocene. This suggests that while bifurcating speciation rates may decline as the result of species accumulation and environmental change, speciation by hybridization may be more robust to these processes.

Significant advances have been made in species delimitation and numerous methods can test precisely defined models of speciation, though the synthesis of phylogeography and taxonomy is still sometimes incomplete. Emerging consensus treats distinct genealogical clusters in genome-scale data as strong initial evidence of speciation in most cases, a hypothesis that must therefore be falsified under an explicit evolutionary model. We can now test speciation hypotheses linking trait differentiation to specific mechanisms of divergence with increasingly large data sets. Integrative taxonomy can, therefore, reflect an understanding of how each axis of variation relates to underlying speciation processes, with nomenclature for distinct evolutionary lineages. We illustrate this approach here with Seal Salamanders (Desmognathus monticola) and introduce a new unsupervised machine-learning approach for species delimitation. Plethodontid salamanders are renowned for their morphological conservatism despite extensive phylogeographic divergence. We discover 2 geographic genetic clusters, for which demographic and spatial models of ecology and gene flow provide robust support for ecogeographic speciation despite limited phenotypic divergence. These data are integrated under evolutionary mechanisms (e.g., spatially localized gene flow with reduced migration) and reflected in emergent properties expected under models of reinforcement (e.g., ethological isolation and selection against hybrids). Their genetic divergence is prima facie evidence for species-level distinctiveness, supported by speciation models and divergence along axes such as behavior, geography, and climate that suggest an ecological basis with subsequent reinforcement through prezygotic isolation. As data sets grow more comprehensive, species-delimitation models can be tested, rejected, or corroborated as explicit speciation hypotheses, providing for reciprocal illumination of evolutionary processes and integrative taxonomies. [Desmognathus; integrative taxonomy; machine learning; species delimitation.]

Speciation rates vary substantially across the tree of life. These rates should be linked to the rate at which population structure forms if a continuum between micro and macroevolutionary patterns exists. Previous studies examining the link between speciation rates and the degree of population formation in clades have been shown to be either correlated or uncorrelated depending on the group, but no study has yet examined the relationship between speciation rates and population structure in a young group that is constrained spatially to a single‐island system. We examine this correlation in 109 gemsnakes (Pseudoxyrhophiidae) endemic to Madagascar and originating in the early Miocene, which helps control for extinction variation across time and space. We find no relationship between rates of speciation and the formation rates of population structure over space in 33 species of gemsnakes. Rates of speciation show low variation, yet population structure varies widely across species, indicating that speciation rates and population structure are disconnected. We suspect this is largely due to the persistence of some lineages not susceptible to extinction. Importantly, we discuss how delimiting populations versus species may contribute to problems understanding the continuum between shallow and deep evolutionary processes.