With respect to number of species, the genus Astragalus is huge. In fact, it is the largest genus of vascular plants, with over two thousand five hundred species. Even though North America contains over five hundred species, it is not the centre of diversity for the genus – that title belongs to southern Asia. This cream milkvetch is representative of many of the plants in the genus: adapted to live in an arid or semi-arid climate.
Why so many species? The diversity of Astragalus is a result of adaptive radiation, i.e., the rapid speciation of a single or few species into dozens or hundreds of species in a very short time. Other examples of adaptive radiation include stickleback fishes (studied by UBC’s honoured evolutionary biologist Dolph Schluter) and the vascular plants known as the Hawaiian silverswords.