Origin and maintenance of clinal variation in nuphar (Nymphaeaceae)

Natural populations ofNuphar luteum in the coastal plain of North Carolina present a morphological and geographical cline with ssp.macrophyllum representing one extreme and ssp.sagittifolium the other. Plants of intermediate morphology exist in abundance in areas between the two extremes. Data involving 1) analysis of variance within and among clones, 2) analysis of variation in seedling populations cultured under uniform conditions, 3) analysis of seedlings produced through artificial hybridization, and 4) reciprocal transplantation indicate the morphological cline to be genetically controlled with the leaf length-width ratio being the most constant and reliable feature for identification. An analysis of competitive and non-competitive populations within one pond provides evidence that natural selection of specific mature clones occurs under competitive conditions. Likewise, an analysis of the seedling potential of adult populations along the cline indicates that, in nature, a strong selection mechanism is operative during seed germination and/or seedling establishment. Chemical analyses of the aquatic habitats along the cline show several gradients to be correlated with the morphological cline. By extrapolation of the data, the variability evident inNuphar, not only among subspecies but also among populations and clones, is considered to result from adaptation to environmental selection pressures.