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1. The cost of travel: How dispersal ability limits local adaptation in host–parasite interactions

   https://doi.org/10.1111/jeb.13754  

   Johnson, Pieter; Calhoun, Dana M.; Moss, Wynne E.; McDevitt‐Galles, Travis; Riepe, Tawni B.; Hallas, Joshua M.; Parchman, Thomas L.; Feldman, Chris R.; Achatz, Tyler J.; Tkach, Vasyl V.; et al (December 2020, Journal of Evolutionary Biology)

   Abstract Classical theory suggests that parasites will exhibit higher fitness in sympatric relative to allopatric host populations (local adaptation). However, evidence for local adaptation in natural host–parasite systems is often equivocal, emphasizing the need for infection experiments conducted over realistic geographic scales and comparisons among species with varied life history traits. Here, we used infection experiments to test how two trematode (flatworm) species (Paralechriorchis syntomenteraandRibeiroia ondatrae) with differing dispersal abilities varied in the strength of local adaptation to their amphibian hosts. Both parasites have complex life cycles involving sequential transmission among aquatic snails, larval amphibians and vertebrate definitive hosts that control dispersal across the landscape. By experimentally pairing 26 host‐by‐parasite population infection combinations from across the western USA with analyses of host and parasite spatial genetic structure, we found that increasing geographic distance—and corresponding increases in host population genetic distance—reduced infection success forP. syntomentera, which is dispersed by snake definitive hosts. For the avian‐dispersedR. ondatrae, in contrast, the geographic distance between the parasite and host populations had no influence on infection success. Differences in local adaptation corresponded to parasite genetic structure; although populations ofP. syntomenteraexhibited ~10% mtDNA sequence divergence, those ofR. ondatraewere nearly identical (<0.5%), even across a 900 km range. Taken together, these results offer empirical evidence that high levels of dispersal can limit opportunities for parasites to adapt to local host populations. 

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2. Footprints of local adaptation span hundreds of linked genes in the Atlantic silverside genome

   https://doi.org/10.1002/evl3.189  

   Wilder, Aryn P.; Palumbi, Stephen R.; Conover, David O.; Therkildsen, Nina Overgaard (October 2020, Evolution Letters)

   Abstract The study of local adaptation in the presence of ongoing gene flow is the study of natural selection in action, revealing the functional genetic diversity most relevant to contemporary pressures. In addition to individual genes, genome-wide architecture can itself evolve to enable adaptation. Distributed across a steep thermal gradient along the east coast of North America, Atlantic silversides (Menidia menidia) exhibit an extraordinary degree of local adaptation in a suite of traits, and the capacity for rapid adaptation from standing genetic variation, but we know little about the patterns of genomic variation across the species range that enable this remarkable adaptability. Here, we use low-coverage, whole-transcriptome sequencing of Atlantic silversides sampled along an environmental cline to show marked signatures of divergent selection across a gradient of neutral differentiation. Atlantic silversides sampled across 1371 km of the southern section of its distribution have very low genome-wide differentiation (median FST = 0.006 across 1.9 million variants), consistent with historical connectivity and observations of recent migrants. Yet almost 14,000 single nucleotide polymorphisms (SNPs) are nearly fixed (FST > 0.95) for alternate alleles. Highly differentiated SNPs cluster into four tight linkage disequilibrium (LD) blocks that span hundreds of genes and several megabases. Variants in these LD blocks are disproportionately nonsynonymous and concentrated in genes enriched for multiple functions related to known adaptations in silversides, including variation in lipid storage, metabolic rate, and spawning behavior. Elevated levels of absolute divergence and demographic modeling suggest selection maintaining divergence across these blocks under gene flow. These findings represent an extreme case of heterogeneity in levels of differentiation across the genome, and highlight how gene flow shapes genomic architecture in continuous populations. Locally adapted alleles may be common features of populations distributed along environmental gradients, and will likely be key to conserving variation to enable future responses to environmental change. 

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3. Incomplete reproductive isolation and low genetic differentiation despite floral divergence across varying geographic scales in Castilleja

   https://doi.org/10.1002/ajb2.1700  

   Wenzell, Katherine_E; McDonnell, Angela_J; Wickett, Norman_J; Fant, Jeremie_B; Skogen, Krissa_A (July 2021, American Journal of Botany)

   PremiseDivergence depends on the strength of selection and frequency of gene flow between taxa, while reproductive isolation relies on mating barriers and geographic distance. Less is known about how these processes interact at early stages of speciation. Here, we compared population‐level differentiation in floral phenotype and genetic sequence variation among recently divergedCastillejato explore patterns of diversification under different scenarios of reproductive isolation. MethodsUsing target enrichment enabled by the Angiosperms353 probe set, we assessed genetic distance among 50 populations of fourCastillejaspecies. We investigated whether patterns of genetic divergence are explained by floral trait variation or geographic distance in two focal groups: the widespreadC. sessilifloraand the more restrictedC. purpureaspecies complex. ResultsWe document thatC. sessilifloraand theC. purpureacomplex are characterized by high diversity in floral color across varying geographic scales. Despite phenotypic divergence, groups were not well supported in phylogenetic analyses, and little genetic differentiation was found across targeted Angiosperms353 loci. Nonetheless, a principal coordinate analysis of single nucleotide polymorphisms revealed differentiation withinC. sessilifloraacross floral morphs and geography and less differentiation among species of theC. purpureacomplex. ConclusionsPatterns of genetic distance inC. sessiliflorasuggest species cohesion maintained over long distances despite variation in floral traits. In theC. purpureacomplex, divergence in floral color across narrow geographic clines may be driven by recent selection on floral color. These contrasting patterns of floral and genetic differentiation reveal that divergence can arise via multiple eco‐evolutionary paths. 

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4. Geographic variation in phenotypic divergence between two hybridizing field cricket species

   https://doi.org/10.3897/jor.32.90713  

   Byerly, Amy R.; Jenck, Clara; Goetz, Alexander R.; Weissman, David B.; Gray, David A.; Ross, Charles L.; Maroja, Luana S.; Larson, Erica L. (September 2023, Journal of Orthoptera Research)

   Patterns of morphological divergence across species’ ranges can provide insight into local adaptation and speciation. In this study, we compared phenotypic divergence among 4,221 crickets from 337 populations of two closely related species of field cricket,Gryllus firmusandG. pennsylvanicus, and their hybrids. We found that these species differ across their geographic range in key morphological traits, such as body size and ovipositor length, and we directly compared phenotype with genotype for a subset of crickets to demonstrate nuclear genetic introgression, phenotypic intermediacy of hybrids, and essentially unidirectional mitochondrial introgression. We discuss how these morphological traits relate to life history differences between the two species. Our comparisons across geographic areas support prior research suggesting that cryptic variation withinG. firmusmay represent different species. Our study highlights how variable morphology can be across wide-ranging species and the importance of studying reproductive barriers in more than one or two transects of a hybrid zone. 

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5. Form–function relationships within species are uncoupled from those across species in swimming and jumping performance in arboreal frogs

   https://doi.org/10.1093/evolut/qpaf058  

   Nouailhetas_Simon, Monique; Wildman, Megan; Moen, Daniel_S; Kaliontzopoulou, ed., Antigoni; Morlon, ed., Hélène (March 2025, Evolution)

   Abstract Variation of form–function relationships within populations is the substrate for adaptation at higher levels. Therefore, assessing similarity in form–function relationships within and between species may help reveal the processes shaping functional diversity. Here, we test such similarity across three levels of anuran phenotypic divergence: within a population, among species in a single family (Hylidae; ~60 myr), and across a much broader sample of all anuran species using a single microhabitat (arboreal; ~120 myr). We expected less interspecific divergence to show higher similarity of form–function relationships with the intraspecific level. We analyzed the relationships between locomotor performance (in both swimming and jumping) and several hindlimb traits across these three evolutionary levels. While we found a positive correlation between swimming and jumping velocity at both intra- and interspecific levels, relationships between performance and body form did not match across levels. We suggest that different strengths of functional constraints or trade-offs may have produced more variation in form–function relationships across species, decoupling them from within-species patterns. We conclude that performance landscapes are likely qualitatively different across the different evolutionary scales, potentially reflecting changes in the relative importance of different behaviors across all arboreal species. 

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