PHENOTYPIC SELECTION IN A NATURAL-POPULATION OF IMPATIENS-PALLIDA NUTT. (BALSAMINACEAE)

In the annual plant Impatiens pallida, individuals exhibit a floral he teromorphism consisting of autogamously selfing, cleistogamous (CL) fl owers and partially outcrossing, chasmogamous (CH) flowers. As part of an investigation into natural selection and mating system evolution i n I. pallida, we measured the magnitude and direction of phenotypic se lection on nine life history characters (two traits measured on three dates, one measured on two dates and one measured once). Three of thes e characters were positively correlated with the ratio of CH/CL flower s produced per plant, which is an important determinant of the mating system. Values for the nine characters and three different measures of fitness (viability, fecundity, lifetime) were estimated for 500 plant s in five locations over a single growing season. Based on lifetime fi tness, linear selection differentials were significant for all nine ch aracters, indicating a selective advantage to tall, leafy, highly bran ched plants that flowered early. However, only two of these characters had a direct effect on fitness. Selection was significant on all nine characters when based on fecundity as well as lifetime fitness; howev er, only three of five characters examined had significant selection b ased on viability fitness. For all fitness components, the frequency o f significant linear and nonlinear selection coefficients was comparab le (23% vs 17% of all cases, respectively), but nonlinear coefficients were generally larger. Finally, the magnitude and direction of direct linear selection was heterogeneous among locations, for all character s and all fitness components. Collectively these results suggest that selection is strong, favouring large size, high allocation to reproduc tion and high CH/CL flower ratios. However, any directional evolutiona ry changes in vegetative or reproductive characters may be constrained by strong non-linear and correlational selection.