Local adaptation is a common but not ubiquitous feature of species interactions, and understanding the circumstances under which it evolves illuminates the factors that influence adaptive population divergence. Antagonistic species interactions dominate the local adaptation literature relative to mutualistic ones, preventing an overall assessment of adaptation within interspecific interactions. Here, we tested whether the legume Medicago lupulina is adapted to the locally abundant species of mutualistic nitrogen-fixing rhizobial bacteria that vary in frequency across its eastern North American range. We reciprocally inoculated northern and southern M. lupulina genotypes with the northern (Ensifer medicae) or southern bacterium (E. meliloti) in a greenhouse experiment. Despite producing different numbers of root nodules (the structures in which the plants house the bacteria), neither northern nor southern plants produced more seeds, flowered earlier, or were more likely to flower when inoculated with their local rhizobia. We then used a pre-existing dataset to perform a genome scan for loci that showed elevated differentiation between field-collected plants that hosted different bacteria. None of the loci we identified belonged to the well-characterized suite of legume-rhizobia symbiosis genes, suggesting that the rhizobia do not drive genetic divergence between M. lupulina populations. Our results demonstrate that symbiont local adaptation has not evolved in this mutualism despite large-scale geographic variation in the identity of the interacting species. K E Y W O R D Scoevolution, genome scan, legume, mutualism, reciprocal inoculation, rhizobia

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17Local adaptation is a common but not ubiquitous feature of species interactions, and 18 understanding the circumstances under which it evolves illuminates the factors that influence 19 adaptive population divergence. Antagonistic species interactions dominate the local adaptation 20 literature relative to mutualistic ones, preventing an overall assessment of adaptation within 21 interspecific interactions. Here, we tested whether the legume Medicago lupulina is adapted to 22 the locally abundant species of mutualistic nitrogen-fixing bacteria ("rhizobia"), which vary in 23 frequency across its eastern North American range. We reciprocally inoculated northern and 24 southern M. lupulina genotypes with the northern (Ensifer medicae) or southern bacterium (E. 25 meliloti) in a greenhouse experiment. Neither northern nor southern plants produced more seeds, 26 flowered earlier, or were more likely to flower when inoculated with their local rhizobium 27 species, although plants produced more root nodules (the structures that house the bacteria) with 28 their local rhizobia. We used a pre-existing dataset to perform a genome scan for loci that 29showed elevated differentiation between field-collected plants that hosted different bacteria. 30None of the loci we identified belonged to the well-characterized suite of legume-rhizobia 31 symbiosis genes, suggesting that the rhizobia do not drive genetic divergence between M. 32 lupulina populations. Our results demonstrate that symbiont local adaptation is weak in this 33 mutualism despite large-scale geographic variation in the identity of the interacting species.

The fitness of individual species depends on their ability to persist and establish at low densities, just as the diversity of ecological communities depends on the establishment and persistence of low‐density, “invader” species. Theory predicts that abiotic conditions and the competitive make‐up of resident communities jointly shape invader fitness, limiting the phenotypic identity of successful invaders. We use an invasion experiment to ask how competitive traits of 20 introduced plant species alter their absolute fitness in fragments that differ in size, abiotic conditions, and traits of the resident community. We show that abiotic conditions interact with both invader traits and resident functional diversity to determine invader survival. Optimal invader traits depended on the soil characteristics, while greater resident trait diversity lowered invader fitness and had especially strong effects in low‐resource environments. Unlike other abiotic conditions, fragment size had consistent effects irrespective of invader identity, decreasing survival in larger fragments. Synthesis. Our results illustrate how the abiotic environment mediates the effects of resident and invader traits on establishment, creating fitness landscapes that structure local diversity and the functional identities of successful species.

Theory suggests that the evolution of dispersal is balanced by its fitness costs and benefits, yet empirical evidence is sparse due to the difficulties of measuring dispersal and fitness in natural populations. Here, we use spatially explicit data from a multi‐generational capture–mark–recapture study of two populations of Trinidadian guppies (Poecilia reticulata) along with pedigrees to test whether there are fitness benefits correlated with dispersal. Combining these ecological and molecular data sets allows us to directly measure the relationship between movement and reproduction. Individual dispersal was measured as the total distance moved by a fish during its lifetime. We analysed the effects of dispersal propensity and distance on a variety of reproductive metrics. We found that number of mates and number of offspring were positively correlated to dispersal, especially for males. Our results also reveal individual and environmental variation in dispersal, with sex, size, season, and stream acting as determining factors.