Mutualisms, beneficial interactions between species, are expected to be unstable because delivery of benefit likely involves fitness costs and selection should favour partners that deliver less benefit. Yet, mutualisms are common and persistent, even in the largely promiscuous associations between plants and soil microorganisms such as arbuscular mycorrhizal fungi. In two different systems, we demonstrate preferential allocation of photosynthate by host plants to the more beneficial of two AM fungal symbionts. This preferential allocation could allow the persistence of the mutualism if it confers sufficient advantage to the beneficial symbiont that it overcomes the cost of mutualism. We find that the beneficial fungus does increase in biomass when the fungi are spatially separated within the root system. However, in well-mixed fungal communities, non-beneficial fungi proliferate as expected from their reduced cost of mutualism. Our findings suggest that preferential allocation within spatially structured microbial communities can stabilize mutualisms between plants and root symbionts.

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Ecologists often consider how environmental factors limit a species' geographic range. However, recent models suggest that geographic distribution also may be determined by a species' ability to adapt to novel environmental conditions. In this study, we empirically tested whether further evolution would be necessary for northern expansion of the weedy annual cocklebur (Xanthium strumarium) in its native North American range. We transplanted seedlings beyond the northern border and photoperiodically manipulated reproductive timing, a trait important for adaptation to shorter growing seasons at higher latitudes within the range, to determine whether further evolution of this trait would result in a phenotype viable beyond the range. Earlier reproductive induction enabled plants to produce mature seeds beyond the range and to achieve a reproductive output similar to those grown within the range. Therefore, evolution of earlier reproduction in marginal populations would be necessary for northward range expansion. This study is the first to empirically show that evolution in an ecologically important trait would enable a species to survive and reproduce beyond its current range. These results suggest that relatively few traits may limit a species' range and that identifying evolutionary constraints on such traits could be important for predicting geographic distribution.

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