The concept of the ecological niche plays a central role in studies of the origin, maintenance, and distribution of biodiversity.  Recently, attention has focused on the prevalence of ecological similarities among closely related species (phylogenetic niche conservatism) and its importance to community assembly and evolutionary diversification.  However, we know relatively little about the ecological and genetic mechanisms constraining the evolution of the ecological niche within species.  We examine the role of selection and genetic variation in maintaining the ecological niche of Lasthenia fremontii, an annual plant restricted to intermediate depths in California vernal pools. Preliminary results indicate that the within-pool distribution of L. fremontii is a conserved trait, as its position along the depth gradient is similar to that of its close relatives, L. conjugens and L. burkei.  We conducted a field experiment to (a) test for the action of stabilizing selection vs. lack of genetic variation in the within-pool niche of L. fremontii, and (b) examine how competition interacts with these evolutionary mechanisms to influence the distribution of this species.  Using field-collected seeds, we generated 30 paternal half-sib families (3 dams:sire) under greenhouse conditions.  Seeds from these crosses were transplanted into their native vernal pool in plots spanning and exceeding their natural elevation limits.  Neighbor removal treatments were implemented in half of the experimental plots.  Fitness was estimated as the number of seeds produced per seed planted. 

Results/Conclusions

Ecological results indicated that plants in the center of the natural distribution had highest fitness in control plots, while plants at higher elevations produced more seeds in neighbor removal plots.   Phenotypic selection analysis suggested that stabilizing selection for intermediate depth preference occurred in control plots; this pattern was released in neighbor removal plots, where directional selection for occupying higher elevations was detected.  The niche position of each family was estimated as the weighted mean elevation of each plot, with each family’s average fitness in that plot as the weighting factor.  Genotypic selection analysis on niche position detected directional selection for higher pool elevations in neighbor removal plots, but no significant patterns of selection were found in control plots.  Analysis of genetic variation only found significant sire effects in neighbor removal plots, and niche breadth (variance in fitness-weighted elevations) decreased by six-fold in the presence of neighbors.  Together, these results suggest that the presence of competitors reduces the expression of additive genetic variation for niche position along the vernal pool elevation gradient.