Anticipating change: The impact of simulated seasonal heterogeneity on heat tolerances along a latitudinal cline.

An understanding of thermal limits and variation across geographic regions is central to predicting how any population may respond to global change. Latitudinal clines, in particular, have been used to demonstrate that populations can be locally adapted to their own thermal environment and, as a result, not all populations will be equally impacted by an increase in temperature. But how robust are these signals of thermal adaptation to the other ecological challenges that animals commonly face in the wild? Seasonal changes in population density, food availability, or photoperiod are common ecological challenges that could disrupt patterns of thermal tolerance along a cline if each population differentially used these signals to anticipate future temperatures and adjust their thermal tolerances accordingly. In this study, we aimed to test the robustness of a cline in thermal tolerance to simulated signals of seasonal heterogeneity. Experimental animals were derived from clones of the Australian water flea, Daphnia carinata, sampled from nine distinct populations along a latitudinal transect in Eastern Australia. We then factorially combined summer (18 h light, 6 h dark) and winter (6 h light, 18 h dark) photoperiods with high (5 million algal cells individual-1 day-1) and low (1 million algal cells individual-1 day-1) food availabilities, before performing static heat shock assays to measure thermal tolerance. We found that the thermal tolerances of the clonal populations were sensitive to both measures of seasonal change. In general, higher food availability led to an increase in thermal tolerances, with the magnitude of the increase varying by clone. In contrast, a switch in photoperiod led to rank-order changes in thermal tolerances, with heat resistance increasing for some clones, and decreasing for others. Heat resistance, however, still declined with increasing latitude, irrespective of the manipulation of seasonal signals, with clones from northern populations always showing greater thermal resistance, most likely driven by adaptation to winter thermal conditions. While photoperiod and food availability can clearly shape thermal tolerances for specific populations, they are unlikely to overwhelm overarching signals of thermal adaptation, and thus, observed clines in heat resistance will likely have remained robust to these forms of seasonal heterogeneity.

Sediment DNA metabarcoding and morphology provide complementary insight into macrofauna and meiobenthos response to environmental gradients in an Arctic glacial fjord.

Arctic fjords ecosystems are highly dynamic, with organisms exposed to various natural stressors along with productivity clines driven by advection of water masses from shelves. The benthic response to these environmental clines has been extensively studied using traditional, morphology-based approaches mostly focusing on macroinvertebrates. In this study we analyse the effects of glacially mediated disturbance on the biodiversity of benthic macrofauna and meiobenthos (meiofauna and Foraminifera) in a Svalbard fjord by comparing morphology and eDNA metabarcoding. Three genetic markers targeting metazoans (COI), meiofauna (18S V1V2) and Foraminifera (18S 37f) were analyzed. Univariate measures of alpha diversity and multivariate compositional dissimilarities were calculated and tested for similarities in response to environmental gradients using correlation analysis. Our study showed different taxonomic composition of morphological and molecular datasets for both macrofauna and meiobenthos. Some taxonomic groups while abundant in metabarcoding data were almost absent in morphology-based inventory and vice versa. In general, species richness and diversity measures in macrofauna morphological data were higher than in metabarcoding, and similar for the meiofauna. Both methodological approaches showed different patterns of response to the glacially mediated disturbance for the macrofauna and the meiobenthos. Macrofauna showed an evident distinction in taxonomic composition and a dramatic cline in alpha diversity indices between the outer and inner parts of fjord, while the meiobenthos showed a gradual change and more subtle responses to environmental changes along the fjord axis. The two methods can be seen as complementing rather than replacing each other. Morphological approach provides more accurate inventory of larger size species and more reliable quantitative data, while metabarcoding allows identification of inconspicuous taxa that are overlooked in morphology-based studies. As different taxa may show different sensitivities to environmental changes, both methods shall be used to monitor marine biodiversity in Arctic ecosystems and its response to dramatically changing environmental conditions.

Ongoing introgression of a secondary sexual plumage trait in a stable avian hybrid zone.

Hybrid zones are dynamic systems where natural selection, sexual selection, and other evolutionary forces can act on reshuffled combinations of distinct genomes. The movement of hybrid zones, individual traits, or both are of particular interest for understanding the interplay between selective processes. In a hybrid zone involving two lek-breeding birds, secondary sexual plumage traits of Manacus vitellinus, including bright yellow collar and olive belly color, have introgressed asymmetrically ~50 km across the genomic center of the zone into populations more genetically similar to Manacus candei. Males with yellow collars are preferred by females and are more aggressive than parental M. candei, suggesting that sexual selection was responsible for the introgression of male traits. We assessed the spatial and temporal dynamics of this hybrid zone using historical (1989 - 1994) and contemporary (2017 - 2020) transect samples to survey both morphological and genetic variation. Genome-wide SNP data and several male phenotypic traits show that the genomic center of the zone has remained spatially stable, whereas the olive belly color of male M. vitellinus has continued to introgress over this time period. Our data suggest that sexual selection can continue to shape phenotypes dynamically, independent of a stable genomic transition between species.

Discrepancy of flowering time between genetically close sublineages of Aegilops umbellulata Zhuk.

Aegilops umbellulata Zhuk., a wild diploid wheat-related species, has been used as a genetic resource for several important agronomic traits. However, its genetic variations have not been comprehensively studied. We sequenced RNA from 114 accessions of Ae. umbellulata to evaluate DNA polymorphisms and phenotypic variations. Bayesian clustering and phylogenetic analysis based on SNPs detected by RNA sequencing revealed two divergent lineages, UmbL1 and UmbL2. The main differences between them were in the sizes of spikes and spikelets, and culm diameter. UmbL1 is divided into two sublineages, UmbL1e and UmbL1w. These genetic differences corresponded to geographic distributions. UmbL1e, UmbL1w, and UmbL2 are found in Turkey, Iran/Iraq, and Greece, respectively. Although UmbL1e and UmbL1w were genetically similar, flowering time and other morphological traits were more distinct between these sublineages than those between the lineages. This discrepancy can be explained by the latitudinal and longitudinal differences in habitats. Specifically, latitudinal clines of flowering time were clearly observed in Ae. umbellulata, strongly correlated with solar radiation in the winter season. This observation implies that latitudinal differences are a factor in differences in the flowering times of Ae. umbellulata. Differences in flowering time could influence other morphological differences and promote genetic divergence between sublineages.

Hybridization in late stages of speciation: Strong but incomplete genome-wide reproductive isolation and 'large Z-effect' in a moving hybrid zone.

In organisms reproducing sexually, speciation occurs when increasing divergence results in pre- or post-zygotic reproductive isolation between lineages. Studies focusing on reproductive isolation origin in early stages of speciation are common and many rely on genomic scans to infer introgression providing limited information on the genomic architecture of reproductive isolation long-term maintenance. This study analyses a natural hybrid zone between two species in a late stage of speciation. We used ddRADseq genotyping in the contact between Podarcis bocagei and P. carbonelli to examine admixture extent, analyse hybrid zone stability and assess genome-wide variation in selection against introgression. We confirmed strong but incomplete reproductive isolation in a bimodal hybrid zone. New findings revealed population genetic structure within P. carbonelli in the contact zone; geographical and genomic clines analysis suggested strong selection against gene flow, but a relatively small proportion of the loci can introgress, mostly within the narrow contact zone. However, geographical clines revealed that a few introgressed loci show signs of potential positive selection, particularly into P. bocagei. Geographical clines also detected a signal of hybrid zone movement towards P. bocagei distribution. Genomic cline analysis revealed heterogeneous patterns of introgression among loci within the syntopy zone, but the majority maintain a strong association with the genomic background of origin. However, incongruences between both cline approaches were found, potentially driven by confounding effects on genomic clines. Last, an important role of the Z chromosome in reproductive isolation is suggested. Importantly, overall patterns of restricted introgression seem to result from numerous strong intrinsic barriers across the genome.

Temperature adaptation and its impact on the shape of performance curves in Drosophila populations.

Understanding how species adapt to different temperatures is crucial to predict their response to global warming, and thermal performance curves (TPCs) have been employed recurrently to study this topic. Nevertheless, fundamental questions regarding how thermodynamic constraints and evolution interact to shape TPCs in lineages inhabiting different environments remain unanswered. Here, we study Drosophila simulans along a latitudinal gradient spanning 3000 km to test opposing hypotheses based on thermodynamic constrains (hotter-is-better) versus biochemical adaptation (jack-of-all-temperatures) as primary determinants of TPCs variation across populations. We compare thermal responses in metabolic rate and the egg-to-adult survival as descriptors of organismal performance and fitness, respectively, and show that different descriptors of TPCs vary in tandem with mean environmental temperatures, providing strong support to hotter-is-better. Thermodynamic constraints also resulted in a strong negative association between maximum performance and thermal breadth. Lastly, we show that descriptors of TPCs for metabolism and egg-to-adult survival are highly correlated, providing evidence of co-adaptation, and that curves for egg-to-adult survival are systematically narrower and displaced toward lower temperatures. Taken together, our results support the pervasive role of thermodynamics constraining thermal responses in Drosophila populations along a latitudinal gradient, that are only partly compensated by evolutionary adaptation.

Putative adaptive loci show parallel clinal variation in a California-endemic wildflower.

As the global climate crisis continues, predictions concerning how wild populations will respond to changing climate conditions are informed by an understanding of how populations have responded and/or adapted to climate variables in the past. Changes in the local biotic and abiotic environment can drive differences in phenology, physiology, morphology and demography between populations leading to local adaptation, yet the molecular basis of adaptive evolution in wild non-model organisms is poorly understood. We leverage comparisons between two lineages of Calochortus venustus occurring along parallel transects that allow us to identify loci under selection and measure clinal variation in allele frequencies as evidence of population-specific responses to selection along climatic gradients. We identify targets of selection by distinguishing loci that are outliers to population structure and by using genotype-environment associations across transects to detect loci under selection from each of nine climatic variables. Despite gene flow between individuals of different floral phenotypes and between populations, we find evidence of ecological specialization at the molecular level, including genes associated with key plant functions linked to plant adaptation to California's Mediterranean climate. Single-nucleotide polymorphisms (SNPs) present in both transects show similar trends in allelic similarity across latitudes indicating parallel adaptation to northern climates. Comparisons between eastern and western populations across latitudes indicate divergent genetic evolution between transects, suggesting local adaptation to either coastal or inland habitats. Our study is among the first to show repeated allelic variation across climatic clines in a non-model organism.

An Ancestral Balanced Inversion Polymorphism Confers Global Adaptation.

Since the pioneering work of Dobzhansky in the 1930s and 1940s, many chromosomal inversions have been identified, but how they contribute to adaptation remains poorly understood. In Drosophila melanogaster, the widespread inversion polymorphism In(3R)Payne underpins latitudinal clines in fitness traits on multiple continents. Here, we use single-individual whole-genome sequencing, transcriptomics, and published sequencing data to study the population genomics of this inversion on four continents: in its ancestral African range and in derived populations in Europe, North America, and Australia. Our results confirm that this inversion originated in sub-Saharan Africa and subsequently became cosmopolitan; we observe marked monophyletic divergence of inverted and noninverted karyotypes, with some substructure among inverted chromosomes between continents. Despite divergent evolution of this inversion since its out-of-Africa migration, derived non-African populations exhibit similar patterns of long-range linkage disequilibrium between the inversion breakpoints and major peaks of divergence in its center, consistent with balancing selection and suggesting that the inversion harbors alleles that are maintained by selection on several continents. Using RNA-sequencing, we identify overlap between inversion-linked single-nucleotide polymorphisms and loci that are differentially expressed between inverted and noninverted chromosomes. Expression levels are higher for inverted chromosomes at low temperature, suggesting loss of buffering or compensatory plasticity and consistent with higher inversion frequency in warm climates. Our results suggest that this ancestrally tropical balanced polymorphism spread around the world and became latitudinally assorted along similar but independent climatic gradients, always being frequent in subtropical/tropical areas but rare or absent in temperate climates.

The molecular background of the aspartate aminotransferase polymorphism in Littorina snails maintained by strong selection on small spatial scales.

Allozymes present several classical examples of divergent selection, including the variation in the cytosolic aspartate aminotransferase (AAT) in the intertidal snails Littorina saxatilis. AAT is a part of the asparate-malate shuttle, in the interidal molluscs involved in the anaerobic respiration during desiccation. Previous allozyme studies reported the sharp gradient in the frequencies of the AAT100and the AAT120 alleles between the low and high shores in the Northern Europe and the differences in their enzymatic activity, supporting the role of AAT in adaptation to desiccation. However, the populations in the Iberian Peninsula showed the opposite allele cline. Using the mRNA sequencing and the genome pool-seq analyses we characterize DNA sequences of the different AAT alleles, report the amino acid replacements behind the allozyme variation and show that same allozyme alleles in Northern and Southern populations have different protein sequences. Gene phylogeny reveals that the AAT100 and the northern AAT120 alleles represent the old polymorphism, shared among the closely related species of Littorina, while the southern AAT120 allele is more recently derived from AAT100. Further, we show that the Aat gene is expressed at constitutive level in different genotypes and conditions, supporting the role of structural variation in regulation of enzyme activity. Finally, we report the location and the structure of the gene in the L. saxatilis genome and the presence of two additional non-functional gene copies. Altogether, we provide a missing link between the classical allozyme studies and the genome scans and bring together the results produced over decades of the genetic research.

Shell color polymorphism in marine gastropods.

Marine gastropods are characterized by an incredible variation in shell color. In this review, we aim to introduce researchers to previous studies of shell color polymorphism in this group of animals, trying to provide an overview of the topic and highlighting some potential avenues for future research. For this, we tackle the different aspects of shell color polymorphism in marine gastropods: its biochemical and genetic basis, its patterns of spatial and temporal distribution, as well as its potential evolutionary causes. In particular, we put special emphasis on the evolutionary studies that have been conducted so far to reveal the evolutionary mechanisms responsible for the maintenance of shell color polymorphism in this group of animals, as it constitutes the least addressed aspect in existing literature reviews. Several general conclusions can be drawn from our review: First, natural selection is commonly involved in the maintenance of gastropod color polymorphism; second, although the contribution of neutral forces (gene flow-genetic drift equilibrium) to shell color polymorphism maintenance do not seem to be particularly important, it has rarely been studied systematically; third, a relationship between shell color polymorphism and mode of larval development (related to dispersal capability) may exist. As for future studies, we suggest that a combination of both classical laboratory crossing experiments and -Omics approaches may yield interesting results on the molecular basis of color polymorphism. We believe that understanding the various causes of shell color polymorphism in marine gastropods is of great importance not only to understand how biodiversity works, but also for protecting such biodiversity, as knowledge of its evolutionary causes may help implement conservation measures in those species or ecosystems that are threatened.

Environmental Factors Drive Chalcid Body Size Increases with Altitudinal Gradients for Two Hyper-Diverse Taxa.

Body size is the most essential feature that significantly correlates with insects' longevity, fecundity, metabolic rate, and sex ratio. Numerous biogeographical rules have been proposed to illustrate the correlation between the body sizes of different taxa and corresponding geographical or environmental factors. Whether the minute and multifarious chalcids exhibit a similar geographical pattern is still little known. In this research, we analyzed morphological data from 2953 specimens worldwide, including the two most abundant and diverse taxa (Pteromalidae and Eulophidae), which are both composed of field-collected and BOLD system specimens. We examined forewing length as a surrogate of body size and analyzed the average size separately for males and females using two methods (species and assemblage-based method). To verify Bergmann's rule, we included temperature, precipitation, wind speed and solar radiation as explanatory variables in a generalized linear model to analyze the causes of the size variation. We found that there was an increasing trend in the body size of Pteromalidae and Eulophidae with altitude. The optimal Akaike information criterion (AIC) models showed that larger sizes are significantly negatively correlated with temperature and positively correlated with precipitation, and the possible reasons for this variation are discussed and analyzed.

Geographic variability of hybridization between red-breasted and red-naped sapsuckers.

Hybrid zones reveal the strength of reproductive isolation between populations undergoing speciation and are a key tool in evolutionary biology research. Multiple replicate transects across the same hybrid zone offer insight into the dynamics of hybridization in different environments, clarifying the role of extrinsic forces on the speciation process. Red-breasted and red-naped sapsuckers (Sphyrapicus ruber and Sphyrapicus nuchalis) have a long zone of contact over approximately 1,600 km from central British Columbia, Canada to central California, USA. We used Genotyping-by-Sequencing data from three independent sapsucker hybrid zone transects to compare hybridization dynamics between these species under variable geoclimatic conditions. We generated geographic clines of the genomic data to compare hybrid zone widths and used random forests models and linear regression to assess the relationship between climate and sapsucker ancestry along each transect. Our results show variation in the directionality of backcrossing, often indicative of moving hybrid zones. We note variable cline widths among transects, indicating differences in selection maintaining hybrid zone dynamics. Furthermore, random forests models identified different variables in close association with sapsucker ancestry across each transect. These results indicate a lack of repeatability across replicate transects and a strong influence of the local environment on hybrid zone dynamics.

Deep ecomorphological and genetic divergence in Steller's Jays (Cyanocitta stelleri, Aves: Corvidae).

The relationship between ecology and morphology is a cornerstone of evolutionary biology, and quantifying variation across environments can shed light on processes that give rise to biodiversity. Three morphotypes of the Steller's Jay (Cyanocitta stelleri) occupy different ecoregions in western North America, which vary in climate and landcover. These morphotypes (Coastal, Interior, Rocky Mountain) differ in size, plumage coloration, and head pattern. We sampled 1080 Steller's Jays from 68 populations (plus 11 outgroups) to address three main questions using data on morphology, plumage, genetics (mtDNA, microsatellites), and ecological niches: (1) How do phenotypic and genetic traits vary within and among populations, morphotypes, and ecoregions? (2) How do population-level differences in Steller's Jays compare with other sister species pairs of North American birds? (3) What can we infer about the population history of Steller's Jays in relation to past climates, paleoecology, and niche evolution? We found substantial morphological, genetic, and ecological differentiation among morphotypes. The greatest genetic divergence separated Coastal and Interior morphotypes from the Rocky Mountain morphotype, which was associated with warmer, drier, and more open habitats. Microsatellites revealed additional structure between Coastal and Interior groups. The deep mtDNA split between Coastal/Interior and Rocky Mountain lineages of Steller's Jay (ND2 ~ 7.8%) is older than most North American avian sister species and dates to approximately 4.3 mya. Interior and Rocky Mountain morphotypes contact across a narrow zone with steep clines in traits and reduced gene flow. The distribution of the three morphotypes coincides with divergent varieties of ponderosa pine and Douglas fir. Species distribution models support multiple glacial refugia for Steller's Jays. Our integrative dataset combined with extensive geographic sampling provides compelling evidence for recognizing at least two species of Steller's Jay.

Clinal versus disruptive latitudinal variation in fruit traits of a South American mistletoe.

Fruit traits have historically been interpreted as plant adaptations to their seed dispersers. On the other hand, different environmental factors, which vary spatially and temporally, can shape fruit-trait variation. The mistletoe Tristerix corymbosus has a latitudinal distribution along the South American Pacific rim that encompasses two different biomes, the matorral of central Chile and the temperate forest that extends south of the matorral. This mistletoe shows contrasting fruiting phenology (spring vs summer), fruit color (yellow vs green), and seed dispersers (birds vs marsupial) in these two biomes. We characterized geographic variation of morphological and nutritional fruit traits of T. corymbosus to evaluate which macroecological factor, biome or latitude, better explains spatial variation in these variables. For each of 22 populations, we obtained environmental data (temperature, precipitation, and canopy cover), measured fruit and seed morphology traits (size, shape, and weight), and pulp moisture and nutritional content (fiber, protein, fat, carbohydrates, ash, and caloric content). Patterns of variation for each variable were described by fitting and comparing five different simple models varying in slope, intercept or both. Fruit morphology showed a clear biome-related disruptive pattern, seed morphological traits were unrelated to either biome or latitude, whereas nutritional variables showed diverse patterns. Different environmental factors seem to affect fruit development and phenology, determining the observed fruit characteristics, with seed dispersers playing a minor role in shaping these patterns. More generally, the contrasting plant-seed disperser associations we addressed can be interpreted as the outcome of an ecological-fitting rather than of a coevolutionary process.

Clinal variation as a tool to understand climate change.

Clines are observable gradients that reflect continuous change in biological traits of species across geographical ranges. Clinal gradients could vary at geographic scales (latitude and altitude). Since clinal variations represent active genomic responses at the population level they (clines) provide an immense power to address questions related to climatic change. With the fast pace of climate change i.e. warming, populations are also likely to exhibit rapid responses; at both the phenotypic and genotypic levels. We seek to understand how clinal variation could be used to anticipate climatic responses using Drosophila, a pervasively used inter-disciplinary model system owing to its molecular repertoire. The genomic information coupled with the phenotypic variation greatly facilitates our understanding of the Drosophilidae response to climate change. We discuss traits associated with clinal variation at the phenotypic level as well as their underlying genetic regulators. Given prevailing climatic conditions and future projections for climate change, clines could emerge as monitoring tools to track the cross-talk between climatic variables and organisms.

Global biogeography of warning coloration in the butterfly Danaus chrysippus.

Warning coloration provides a textbook example of natural selection, but the frequent observation of polymorphism in aposematic species presents an evolutionary puzzle. We investigated biogeography and polymorphism of warning patterns in the widespread butterfly Danaus chrysippus using records from citizen science (n = 5467), museums (n = 8864) and fieldwork (n = 2586). We find that polymorphism in three traits controlled by known mendelian loci is extensive. Broad allele frequency clines, hundreds of kilometres wide, suggest a balance between long-range dispersal and predation of unfamiliar morphs. Mismatched clines for the white hindwing and forewing tip in East Africa are consistent with a previous finding that the black wingtip allele has spread recently in the region through hitchhiking with a heritable endosymbiont. Light/dark background coloration shows more extensive polymorphism. The darker genotype is more common in cooler regions, possibly reflecting a trade-off between thermoregulation and predator warning. Overall, our findings show how studying local adaptation at the global scale provides a more complete picture of the evolutionary forces involved.

Natural variation at a single gene generates sexual antagonism across fitness components in Drosophila.

Mutations with conflicting fitness effects in males and females accumulate in sexual populations, reducing their adaptive capacity.1,2 Although quantitative genetic studies indicate that sexually antagonistic polymorphisms are common,3-5 their molecular basis and population genetic properties remain poorly understood.6,7 Here, we show in fruit flies how natural variation at a single gene generates sexual antagonism through phenotypic effects on cuticular hydrocarbon (CHC) traits that function as both mate signals and protectors against abiotic stress8 across a latitudinal gradient. Tropical populations of Drosophila serrata have polymorphic CHCs producing sexual antagonism through opposing but sex-limited effects on these two fitness-related functions. We dissected this polymorphism to a single fatty-acyl CoA reductase gene, DsFAR2-B, that is expressed in oenocyte cells where CHCs are synthesized. RNAi-mediated disruption of the DsFAR2-B ortholog in D. melanogaster oenocytes affected CHCs in a similar way to that seen in D. serrata. Population genomic analysis revealed that balancing selection likely operates at the DsFAR2-B locus in the wild. Our study provides insights into the genetic basis of sexual antagonism in nature and connects sexually varying antagonistic selection on phenotypes with balancing selection on genotypes that maintains molecular variation.

Oh, the places you will grow: Intraspecific latitudinal clines in butterfly size suggest a phylogenetic signal.

Within an animal species, the body sizes of individuals at higher latitudes are often different from individuals at lower latitudes. For homeothermic species that maintain a relatively constant body temperature, such as mammals and birds, individuals at higher latitudes tend to be larger. For ectothermic species, such as insects, that do not retain their own body heat and which often do not maintain a relatively constant body temperature, patterns of body size with latitude are highly variable. This has led some authors to contend that patterns in even closely related species cannot be expected to be similar. Indeed, to our knowledge, no studies of invertebrates have found that more closely related species have more similar relationships between body size and latitude. Further, no studies have investigated the potential influence of diet quality on interspecific differences in these clines. We measured wing lengths of specimens (N = 1753) in eight lycaenid butterfly species and one species of the sister family, Riodinidae to determine if more closely related species have similar latitudinal trends. We also estimated the mean nitrogen content of caterpillars' hosts to investigate whether this often-limiting nutrient influences the strength and direction of latitudinal clines in body size. We found that four species are significantly smaller at higher latitudes, an additional species is marginally smaller at higher latitudes (p < .06), and four species had no significant relationship with latitude. We also found a strong phylogenetic signal for latitudinal clines in body size among our species, which indicates that some closely related species may have similar clines. However, the strength and direction of these clines did not depend on the estimated nitrogen content of caterpillars' hosts. Our results indicate that mean nitrogen content of hosts may not be an important driver in latitudinal clines but that phylogenetic relationships among species should be accounted for when exploring other potential drivers of body-size clines in invertebrate species.

Revisiting a classic hybrid zone: Movement of the northern flicker hybrid zone in contemporary times.

Natural hybrid zones have provided important insights into the evolutionary process, and their geographic dynamics over time can help to disentangle the underlying biological processes that maintain them. Here, we leverage replicated sampling of an identical transect across the hybrid zone between yellow-shafted and red-shafted flickers in the Great Plains to assess its stability over ∼60 years (1955-1957 to 2016-2018). We identify a ∼73-km westward shift in the hybrid zone center toward the range of the red-shafted flicker, but find no associated changes in width over our sampling period. In fact, the hybrid zone remains remarkably narrow, suggesting some kind of selective pressure maintains the zone. By comparing to previous work in the same geographic region, it appears likely that the movement in the hybrid zone has occurred in the years since the early 1980s. This recent movement may be related to changes in climate or land management practices that have allowed westward movement of yellow-shafted flickers into the Great Plains.