Two-way evolution of root form in the riverweed family Podostemaceae, with implications for phenotypic evolution

Whether phenotypic evolution is always adaptive is a major issue in evolutionary biology. Based on the great morphological variation and slight ecological variation, Willis argued that Podostemaceae evolved in the absence of adaptation. Podostemaceae are ecological specialists with roots or holdfasts adhering to macrophyte-free (in Asia), soilless rock surfaces submerged in fast-flowing river currents. In Hydrobryum , the roots are foliose and rarely ribbon-like. The ribbon-like rooted species are almost always sympatric with foliose-rooted species. The mat K phylogeny indicates that ribbon-like roots were derived recurrently from foliose roots. This late evolution is truly opposite to the early evolution, in which the foliose root of most Hydrobryum species was derived from the ribbon-like root of the common ancestor of the Cladopus – Hydrobryum clade. The two-way evolution and subsequent sympatry suggests that root forms do not necessarily affect the microdistribution of the species.