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  1. Abstract

    Hybrid zones can be studied by modeling clines of trait variation (e.g., morphology, genetics) over a linear transect. Yet, hybrid zones can also be spatially complex, can shift over time, and can even lead to the formation of hybrid lineages with the right combination of dispersal and vicariance. We reassessed Sibley’s (1950) gradient between Collared Towhee (Pipilo ocai) and Spotted Towhee (Pipilo maculatus) in Central Mexico to test whether it conformed to a typical tension-zone cline model. By comparing historical and modern data, we found that cline centers for genetic and phenotypic traits have not shifted over the course of 70 years. This equilibrium suggests that secondary contact between these species, which originally diverged over 2 million years ago, likely dates to the Pleistocene. Given the amount of mtDNA divergence, parental ends of the cline have very low autosomal nuclear differentiation (FST = 0.12). Dramatic and coincident cline shifts in mtDNA and throat color suggest the possibility of sexual selection as a factor in differential introgression, while a contrasting cline shift in green back color hints at a role for natural selection. Supporting the idea of a continuum between clinal variation and hybrid lineage formation, the towhee gradient can be analyzed as one population under isolation-by-distance, as a two-population cline, and as three lineages experiencing divergence with gene flow. In the middle of the gradient, a hybrid lineage has become partly isolated, likely due to forested habitat shrinking and fragmenting as it moved upslope after the last glacial maximum and a stark environmental transition. This towhee system offers a window into the potential outcomes of hybridization across a dynamic landscape including the creation of novel genomic and phenotypic combinations and incipient hybrid lineages.

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  2. Abstract

    Hybrid zones are natural experiments for the study of avian evolution. Hybrid zones can be dynamic, moving as species adjust to new climates and habitats, with unknown implications for species and speciation. There are relatively few studies that have comparable modern and historic sampling to assess change in hybrid zone location and width over time, and those studies have generally found mixed results, with many hybrid zones showing change over time, but others showing stability. The white‐throated magpie‐jay (Calocitta formosa) and black‐throated magpie‐jay (Calocitta colliei) occur along the western coast of Mexico and Central America. The two species differ markedly in throat color and tail length, and prior observation suggests a narrow hybrid zone in southern Jalisco where individuals have mixed throat color. This study aims to assess the existence and temporal stability of this putative hybrid zone by comparing throat color between georeferenced historical museum specimens and modern photos from iNaturalist with precise locality information. Our results confirm the existence of a narrow hybrid zone in Jalisco, with modern throat scores gradually increasing from the parental ends of the cline toward the cline center in a sigmoidal curve characteristic of hybrid zones. Our temporal comparison suggests that the hybrid zone has not shifted its position between historical (pre‐1973) and modern (post‐2005) time periods—a surprising result given the grand scale of habitat change to the western Mexican lowlands during this time. An anomalous pocket of white‐throated individuals in the northern range of the black‐throated magpie‐jay hints at the possibility of prehistorical long‐distance introduction. Future genomic data will help disentangle the evolutionary history of these lineages and better characterize how secondary contact is affecting both the DNA and the phenotype of these species.

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  3. Abstract

    Genomic data continue to advance our understanding of species limits and biogeographic patterns. However, there is still no consensus regarding appropriate methods of phylogenomic analysis that make the best use of these heterogeneous data sets. In this study, we used thousands of ultraconserved element (UCE) loci from alligator lizards in the genus Gerrhonotus to compare and contrast species trees inferred using multiple contemporary methods and provide a time frame for biological diversification across the Mexican Transition Zone (MTZ). Concatenated maximum likelihood (ML) and Bayesian analyses provided highly congruent results, with differences limited to poorly supported nodes. Similar topologies were inferred from coalescent analyses in Bayesian Phylogenetics and Phylogeography and SVDquartets, albeit with lower support for some nodes. All divergence times fell within the Miocene, linking speciation to local Neogene vicariance and/or global cooling trends following the mid-Miocene Climatic Optimum. We detected a high level of genomic divergence for a morphologically distinct species restricted to the arid mountains of north-eastern Mexico, and erected a new genus to better reflect evolutionary history. In summary, our results further advocate leveraging the strengths and weaknesses of concatenation and coalescent methods, provide evidence for old divergences for alligator lizards, and indicate that the MTZ continues to harbour substantial unrecognized diversity.

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  4. Abstract

    The Great American Biotic Interchange (GABI) was a key biogeographic event in the history of the Americas. The rising of the Panamanian land bridge ended the isolation of South America and ushered in a period of dispersal, mass extinction, and new community assemblages, which sparked competition, adaptation, and speciation. Diversification across many bird groups, and the elevational zonation of others, ties back to events triggered by the GABI. But the exact timing of these events is still being revealed, with recent studies suggesting a much earlier time window for faunal exchange, perhaps as early as 20 million years ago (Mya). Using a time‐calibrated phylogenetic tree, we show that the jay genusCyanolycais emblematic of bird dispersal trends, with an early, pre‐land bridge dispersal from Mesoamerica to South America 6.3–7.3 Mya, followed by a back‐colonization ofC. cucullatato Mesoamerica 2.3–4.8 Mya, likely after the land bridge was complete. AsCyanolycaspecies came into contact in Mesoamerica, they avoided competition due to a prior shift to lower elevation in the ancestor ofC. cucullata. This shift allowedC. cucullatato integrate itself into the Mesoamerican highland avifauna, which our time‐calibrated phylogeny suggests was already populated by higher‐elevation, congeneric dwarf‐jays (C. argentigula,C. pumilo,C. mirabilis, andC. nanus). The outcome of these events and fortuitous elevational zonation was thatC. cucullatacould continue colonizing new highland areas farther north during the Pleistocene. Resultingly, fourC. cucullatalineages became isolated in allopatric, highland regions from Panama to Mexico, diverging in genetics, morphology, plumage, and vocalizations. At least two of these lineages are best described as species (C. mitrataandC. cucullata). Continued study will further document the influence of the GABI and help clarify how dispersal and vicariance shaped modern‐day species assemblages in the Americas.

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  5. Free, publicly-accessible full text available November 22, 2024
  6. Abstract

    Next‐generation sequencing has greatly expanded the utility and value of museum collections by revealing specimens as genomic resources. As the field of museum genomics grows, so does the need for extraction methods that maximize DNA yields. For avian museum specimens, the established method of extracting DNA from toe pads works well for most specimens. However, for some specimens, especially those of birds that are very small or very large, toe pads can be a poor source of DNA. In this study, we apply two DNA extraction methods (phenol–chloroform and silica column) to three different sources of DNA (toe pad, skin punch and bone) from 10 historical avian museum specimens. We show that a modified phenol–chloroform protocol yielded significantly more DNA than a silica column protocol (e.g., Qiagen DNeasy Blood & Tissue Kit) across all tissue types. However, extractions using the silica column protocol contained longer fragments on average than those using the phenol–chloroform protocol, probably as a result of loss of small fragments through the silica column. While toe pads yielded more DNA than skin punches and bone fragments, skin punches proved to be a reliable alternative source of DNA and might be especially appealing when toe pad extractions are impractical. Overall, we found that historical bird museum specimens contain substantial amounts of DNA for genomic studies under most extraction scenarios, but that a phenol–chloroform protocol consistently provides the high quantities of DNA required for most current genomic protocols.

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  7. Molecular studies have uncovered significant diversity in the Mexican Highlands, leading to the description of many new endemic species. DNA approaches to this kind of species discovery have included both mitochondrial DNA (mtDNA) sequencing and multilocus genomic methods. While these marker types have often been pitted against one another, there are benefits to deploying them together, as linked mtDNA data can provide the bridge between uncovering lineages through rigorous multilocus genomic analysis and identifying lineages through comparison to existing mtDNA databases. Here, we apply one class of multilocus genomic marker, ultraconserved elements (UCEs), and linked mtDNA data to a species complex of frogs (Sarcohyla bistincta, Hylidae) found in the Mexican Highlands. We generated data from 1,891 UCEs, which contained 1,742 informative SNPs forS. bistinctaand closely related species and captured mitochondrial genomes for most samples. Genetic analyses based on both whole loci and SNPs agree there are six to seven distinct lineages within what is currently described asS. bistincta. Phylogenies from UCEs and mtDNA mostly agreed in their topologies, and the few differences suggested a more complex evolutionary history of the mtDNA marker. Our study demonstrates that the Mexican Highlands still hold substantial undescribed diversity, making their conservation a particularly urgent goal. The Trans-Mexican Volcanic Range stands out as a significant geographic feature inSarcohylaand may have acted as a dispersal corridor forS. bistinctato spread to the north. Combining multilocus genomic data with linked mtDNA data is a useful approach for identifying potential new species and associating them with already described taxa, which will be especially important in groups with undescribed subadult phenotypes and cryptic species.

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