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Abstract Determining the genetic architecture of traits involved in adaptation and speciation is one of the key components of understanding the evolutionary mechanisms behind biological diversification. Hybrid zones provide a unique opportunity to use genetic admixture to identify traits and loci contributing to partial reproductive barriers between taxa. Many studies have focused on the temporal dynamics of hybrid zones, but geographical variation in hybrid zones that span distinct ecological contexts has received less attention. We address this knowledge gap by analyzing hybridization and introgression between black-capped and Carolina chickadees in two geographically remote transects across their extensive hybrid zone, one located in eastern and one in central North America. Previous studies demonstrated that this hybrid zone is moving northward as a result of climate change but is staying consistently narrow due to selection against hybrids. In addition, the hybrid zone is moving ~5× slower in central North America compared to more eastern regions, reflecting continent-wide variation in the rate of climate change. We use whole genome sequencing of 259 individuals to assess whether variation in the rate of hybrid zone movement is reflected in patterns of hybridization and introgression, and which genes and genomic regions show consistently restricted introgression in distinct ecological contexts. Our results highlight substantial similarities between geographically remote transects and reveal large Z-linked chromosomal rearrangements that generate measurable differences in the degree of gene flow between transects. We further use simulations and analyses of climatic data to examine potential factors contributing to continental-scale nuances in selection pressures. We discuss our findings in the context of speciation mechanisms and the importance of sex chromosome inversions in chickadees and other species. 

Rapid species turnover in tropical mountains has fascinated biologists for centuries. A popular explanation for this heightened beta diversity is that climatic stability at low latitudes promotes the evolution of narrow thermal tolerance ranges, leading to local adaptation, evolutionary divergence and parapatric speciation along elevational gradients. However, an emerging consensus from research spanning phylogenetics, biogeography and behavioural ecology is that this process rarely, if ever, occurs. Instead, closely related species typically occupy a similar elevational niche, while species with divergent elevational niches tend to be more distantly related. These results suggest populations have responded to past environmental change not by adapting and diverging in place, but instead by shifting their distributions to tightly track climate over time. We argue that tropical species are likely to respond similarly to ongoing and future climate warming, an inference supported by evidence from recent range shifts. In the absence of widespread in situ adaptation to new climate regimes by tropical taxa, conservation planning should prioritize protecting large swaths of habitat to facilitate movement. 

Abstract AimIn response to warming, species are shifting their ranges towards higher elevations. These elevational range shifts have been documented in a variety of taxonomic groups across latitude. However, the rate and direction of species range shifts in response to warming vary, potentially as a consequence of variation in species traits across elevation. Specifically, diurnal and seasonal climates are often more variable at higher elevations, which results in high‐elevation species that have broader thermal physiologies relative to low‐elevation species. High‐elevation species that are thermal generalists might not need to move as far to track their thermal niche as low‐elevation thermal specialists. We investigated whether rates of range shifts varied systematically with increasing elevation across taxa and regions. LocationSixteen montane regions world‐wide. Time period1850–2013. TaxonNine hundred and eighty‐seven species of plants and animals. MethodsWe gathered published data on elevational range shifts from 20 transect studies comparing historical and recent distributions and examined how rates of range shifts changed across elevation. Specifically, we performed a meta‐analysis to calculate the pooled effect of elevation on species range shifts. ResultsWe found that rates of range shifts show a negative relationship with elevation such that low‐elevation species have moved upslope farther than high‐elevation species on the same transect. This finding was primarily a result of shifts in the upper range limits. We also found that 28% of species shifted downslope against predictions, but elevation did not show a relationship with downslope range shifts. Main conclusionsIdiosyncratic range shifts will significantly alter montane ecological communities, which are home to some of the greatest biodiversity on Earth. Our results demonstrate that species range shifts vary with elevation and might be a consequence of differences in species traits that also vary along montane gradients. 

Abstract Predictable trait variation across environments suggests shared adaptive responses via repeated genetic evolution, phenotypic plasticity or both. Matching of trait–environment associations at phylogenetic and individual scales implies consistency between these processes. Alternatively, mismatch implies that evolutionary divergence has changed the rules of trait–environment covariation. Here we tested whether species adaptation alters elevational variation in blood traits. We measured blood for 1217 Andean hummingbirds of 77 species across a 4600‐m elevational gradient. Unexpectedly, elevational variation in haemoglobin concentration ([Hb]) was scale independent, suggesting that physics of gas exchange, rather than species differences, determines responses to changing oxygen pressure. However, mechanisms of [Hb] adjustment did show signals of species adaptation: Species at either low or high elevations adjusted cell size, whereas species at mid‐elevations adjusted cell number. This elevational variation in red blood cell number versus size suggests that genetic adaptation to high altitude has changed how these traits respond to shifts in oxygen availability.