Medicago plants are legume plants naturalized in large parts of the world, with some species, like M. sativa, being used in agricultural settings for both their crop yield and their ability to fix nitrogen for soil regeneration. The rhizobia-Medicago symbiosis is often studied as an example of mutualism: rhizobia reside within the plant root structure, while host plants receive a benefit of nitrogen fixation. The benefit of rhizobia colonization to the plant has exhibited variation among different Medicago genotypes and species. Different species of rhizobia selected for inoculation also vary in compatibility and subsequent benefit on the host plant. This could be the result of adaptation of a host legume to a rhizobia species, or potentially coevolution between the two. The interactions in this mutualism may be the result of the conservation of the symbiosis genes and the corresponding regulatory genes across the Medicago genus. While previous studies have assessed variation in rhizobia response within a species, and some have examined a few species together, often across multiple genera, studies have not been conducted assessing the variation in response across a wide breadth of species within an individual genus. Furthermore, the phylogenetic signal of the mutualism response traits has not been previously determined. This study included nine different Medicago species spread across the genus in order to examine differences in the benefit received by the host plant when inoculated with two Ensifer rhizobia species. Using a previously published Medicago phylogeny, we determined the phylogenetic distance between species and whether species that are close relatives respond similarly to rhizobia colonization, known as the phylogenetic signal of a trait. The phylogenetic analysis did not reveal a trend of closely related species responding similarly to the rhizobia colonization, indicating a significant deviation from the expected phylogenetic signal. The deviation could be indicative of a generalized ability of most Medicago plants to form a mutualism with most species of Ensifer rhizobia, despite the prior adaptation of Medicago plants to an individual Ensifer species. This result highlights the plasticity of this mutualism, and it warrants further investigation into the impact of this plasticity on other symbiotic relationships, such as parasitism in Medicago, or mutualisms in other species.