LIFE ON THE EDGE - ADAPTATION VERSUS ENVIRONMENTALLY MEDIATED GENE FLOW IN THE SNOW BUTTERCUP, RANUNCULUS-ADONEUS

We used experimental transplant studies to understand how dispersal an d habitat-specific selection interact to influence plant populations o ccupying heterogeneous environments. The snow buttercup (Ranunculus ad oneus) occupies a steep ecological and flowering time gradient caused by persistent snowmelt differences within its snow bed habitat. We tra nsplanted seeds, seedlings, and adults to learn about the potential in teractions between dispersal and selection. We found that adaptive dif ferentiation is not occurring along the snowmelt gradient, despite str iking differences in microhabitat conditions and reproductive phenolog y between early- and late-melting sites. Instead, our results imply th at environmentally based differences in seed quality are contributing to directional gene how from early-melting locations toward late-melti ng locations. Emergence and early survival of seedlings is greater in late-melting sites in some years, but the larger seeds produced by mat ernal plants in early-melting locations consistently have a fitness ad vantage in all parts of the snow bed. Larger seeds survive longer in t he soil and have a second peak of seedling emergence in their third ye ar, but these late-emerging seedlings are successful only if dispersed to less vegetated, late-melting destinations. The longer growing seas on in early melting sites enhances vegetative growth at all life-histo ry stages and increases fecundity of seedling transplants but also lim its the opportunity for establishment from seed. Our demographic analy sis suggests that maternal environmental effects on propagule quality can lead to directional gene flow from benign to marginal sites in pop ulations occupying heterogeneous habitats.