SELECTION ON FLORAL MORPHOLOGY AND ENVIRONMENTAL DETERMINANTS OF FECUNDITY IN A HAWK-MOTH-POLLINATED VIOLET

This paper presents the results of a 5-yr field study on the determina nts of individual variation in maternal fecundity (seed production) in the narrowly endemic violet Viola cazorlensis (Violaceae), at a south eastern Spanish locality. Flowers of this species are characterized by a very long, thin spur and broad morphological variability, and are p ollinated by a single species of day-flying hawk moth (Macroglossum st ellatarum; Lepidoptera, Sphingidae). The primary aim of this investiga tion was to answer the question, What are the relative importance, as explanations of individual differences in fecundity, of variability in floral traits and of other fecundity determinants that are of an extr insic nature, such as microhabitat type and interactions with herbivor es? The floral morphology of individual V. cazorlensis plants was char acterized by means of both ''conventional,'' linear measurements of th e size of flower parts (petals, spur, peduncle), and shape analysis of corolla outline (using thin-plate splines relative warps analysis). S patial (among substrate types) and temporal (among years) patterns of variation in flower, fruit, and seed production by V. cazorlensis plan ts are described, with particular emphasis on the comparative effects of floral morphology, herbivory (by mammalian ungulates and two specie s of lepidopteran larvae), and substrate type (rock cliffs, bare rocks at ground level, and sandy soils), on cumulative seed production at t he individual plant level. Cumulative seed production of individual V. cazorlensis plants depended significantly on average floral morpholog y (both size and shape components), thus revealing the existence of ph enotypic selection on the floral morphology of this species at the stu dy population. Among all the floral traits examined, spur length was t he only one for which no significant relationship with fecundity was f ound. Type of substrate largely determined differences between V. cazo rlensis plants in the impact of herbivory (plants growing on the soil exhibited the greatest reproductive losses to herbivores), and it also influenced plant size and flower production per reproductive episode. Plant size, in turn, influenced the supra-annual frequency of floweri ng and the number of flowers produced in each reproductive event. Flow er production and herbivory levels significantly influenced (positivel y and negatively, respectively) fruit number, which was the major dire ct determinant of seed production. Path analysis revealed that the mai n determinants of individual variation in cumulative seed' production over the study period were, in decreasing order of importance (absolut e value of ''effect'' coefficient'' in parentheses), cumulative fruit production (0.946), mean flower production per reproductive event (0.8 68), plant size (0.441), herbivory by ungulates (-0.221), and average score on the first relative warp (0.107), a descriptor of flower shape . After accounting for the effects of substrate type, herbivory, plant size, and flower and fruit production, individual variation in floral morphology (aspects of size and shape) explained a negligable proport ion (2.1%) of total individual variation in cumulative fruit productio n. Phenotypic selection on the floral morphology of V. cazorlensis at the study population, although statistically significant, was therefor e almost inconsequential as a source of individual variation in matern al fitness, its effects being heavily ''diluted'' by the overwhelming influence of other factors. As exemplified by this study, selection on the floral phenotype may often become largely irrelevant in evolution ary terms because other ecological factors are far more important dete rminants of fitness differences among plants. A realistic assessment o f the potential relevance of selection on plant reproductive traits th us requires a quantitative evaluation, in its natural scenario, of the predictable consequences of such selection.