NATURAL SELECTION ON GENETICALLY CORRELATED PHENOLOGICAL CHARACTERS IN LYTHRUM SALICARIA L. (LYTHRACEAE)

The seasonal timing of life-history events such as flowering can be critical to plant reproductive success (Rathcke and Lacey 1985). There is considerable evidence for a relationship between the timing of flowering and fecundity (Schemske 1977; Augspurger 1981; Roach 1986; Mazer 1987), and for selection on timing of initial flowering (Schemske 1984; Stewart and Schoen 1987; Campbell 1989, 1991; Kelly 1992a). However, the prevalence of phenotypic (Mazer 1987; Dorn and Mitchell-aids 1991; Kelly 1992a,b) and genetic correlations (Dorn and Mitchell-aids 1991; but see Kelly 1993) between the timing of flowering and other life-history events or other quantitative traits makes it difficult to predict how populations might respond to selection acting on flowering time. Many of the traits that can be correlated with the timing of flowering such as plant size, duration of flowering, and the number of flowers produced, are traits that could be subject to selection themselves. A high degree of intercorrelation among characters can make it difficult to disentangle the effects of direct selection from the effects of selection on correlated traits, especially if correlated traits are not included in the selection analysis. Many of the potential problems associated with the interpretation of selection on correlated characters can be ameliorated if the genetic variance-covariance structure of the traits in the population subject to selection is known. Unfortunately, few studies have simultaneously assayed selection gradients, heritabilities, and genetic correlations (but see Kelly 1992a, 1993; Campbell et al. 1994). Knowledge of heritabilities allows a prediction of whether the population will respond to selection. Moreover, low additive genetic coefficients of variation (Houle 1992) for characters that are subject to strong phenotypic selection can be interpreted as evidence that selection has eroded genetic variation. The existence of genetic correlations can explain the presence of significant additive genetic variance in the face of strong selection if they act as evolutionary constraints. Knowing both the pattern of phenotypic selection and genetic correlations can allow a much more insightful interpretation of the potential for an evolutionary response to indirect selection via selection on a genetically correlated trait (see Campbell et al. 1994). The study of life-history characters such as flowering time provides an ideal system to understand how selection acts on phenotypically and genetically correlated characters. The degree of flowering synchrony among individuals within a plant population could influence reproductive success by enhancing the ability of individual plants to attract pollinators if attraction is density dependent (Augspurger 1980, 1981; Thomson 1980; but see Schemske 1980; Zimmerman 1980; Melampy 1987; Rodriguez-Robles et al. 1992). Plants flowering in synchrony will also have a greater number of potential mates. Thus, stabilizing selection should favor individuals exhibiting flowering phenologies that are similar to the mean phenology of the population (Thomson 1980). The duration of flowering for individual plants could also significantly influence reproductive success. Flowering duration could determine the total number of flowers produced and thus set an upper limit on the number of ovules and pollen grains produced. A protracted flowering period could reduce the uncertainty of pollination (Wilson and Rathcke 1974; Ackerman 1989; Preston 1991) and act as a means of reproductive assurance which could be critical in pollen-limited obligately outcrossing species (Pojar 1974; Bawa 1983). Thus, selection on flowering duration and number of flowers produced should be directional. The purpose of this study was to quantify natural selection on phenological traits in Lythrum salicaria and estimate heritabilities and genetic correlations to determine the genetic potential for a selection response. Specifically, the following questions were addressed: (1) Does stabilizing selection act on initiation of flowering to promote synchronous flowering within a population? (2) Does directional selection act to increase the duration of flowering or flower production? (3) Is there heritable variation for phenological characters or genetic correlations between characters that could act as evolutionary constraints? (4) How do additive genetic coefficients of variation (CV A) and evolvabilities compare for traits subject to differing levels of selection?