Contributions from the field of population biology hold promise for understanding and managing invasiveness; invasive species also offer excellent opportunities to study basic processes in population biology. Life history studies and demographic models may be valuable for examining the introduction of invasive species and identifying life history stages where management will be most effective. Evolutionary processes may be key features in determining whether invasive species establish and spread. Studies of genetic diversity and evolutionary changes should be useful for 0066-4162/01/1215-0305$14.00 305 Annu. Rev. Ecol. Syst. 2001.32:305-332. Downloaded from www.annualreviews.org by NORTH CAROLINA STATE UNIVERSITY on 09/26/12. For personal use only. 306 SAKAI ET AL.understanding the potential for colonization and establishment, geographic patterns of invasion and range expansion, lag times, and the potential for evolutionary responses to novel environments, including management practices. The consequences of biological invasions permit study of basic evolutionary processes, as invaders often evolve rapidly in response to novel abiotic and biotic conditions, and native species evolve in response to the invasion.

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 Campbell , 1991Kelly 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 1992aKelly , 1993Campbell 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 (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 dura...

Here we test whether the potential exists for the independent evolution of allocation to male, female, and attractive functions within a flower. We employed half-sib and parent-offspring regression methods in the tristylous plant Lythrum salicaria to determine whether there is additive genetic variation for characters important to male and female reproductive success and whether genetic correlations could constrain the independent evolution of male and female function. Although significance levels were not consistent among morph types or between populations, there were significant narrow-sense heritabilities for several traits including stamen mass, pistil mass, perianth mass, petal length, and calyx length. Traits that might be under strong stabilizing selection to promote specific pollen transfer, such as stamen and style lengths, had little heritable variation. In the majority of cases in which heritable variation was present, there were positive genetic correlations among floral traits. A strong positive genetic correlation appeared between stamen and pistil mass in the short-styled morph from one of the populations studied. This suggests that selection might not be able to act independently on biomass allocation to male and female flower parts. No evidence of negative genetic correlations appeared that would suggest trade-offs and that could augment a selection response towards sexual specialization. The observed positive correlations could be explained if we consider the "functional architecture" that underlies the covariance structure. If there is more covariance generated by pleiotropic loci controlling overall flower size than at loci controlling male versus female allocation, it could result in the observed positive covariance. At the phenotypic level, we did find significant negative partial correlations between male and female traits when flower size was controlled, but these trade-offs were among rather than within morphs.

The purpose of this study was to assess variation in male and female reproductive success among the three morphs of the tristylous plant, Lythrurn sa/icaria. Fluorescent dyes were used as pollen analogs to determine whether morphs differ in their abilities to donate and receive pollen, and actual and potential seed set was measured with a hand pollination experiment. Dye transfer among morphs was highly asymmetric, with more frequent transfer from the short-styled morph to the longand mid-styled morphs. This suggests that shorts are performing better at pollen donation and longs and mids are performing better at pollen receipt. All flowers on 95 plants were hand pollinated to test whether female reproductive success is more pollen-limited in the short-styled morph than in other morphs. Hand-pollinated short-styled plants had significantly higher total seed mass and more seeds per capsule than short controls, whereas hand pollination failed to increase seed set in long and mid morphs. As predicted, short-styled morphs showed significant pollen limitation, whereas seed set in long-and midstyled morphs was not pollen-limited. Thus, in Lythrurn sa/icaria asymmetrical pollen flow generates morph-specific differences in male and female fitness.Although hermaphroditic plants can potentially contribute genes both through seed production and pollen donation, relatively few studies have measured components of both female and male reproductive success. Determining patterns of gender expression in plants is important for understanding sex allocation patterns, evolutionary constraints on allocation patterns, and natural selection on floral characters via selection on male and female function. The few available data where male fitness has been measured suggest that there can be considerable variation in male performance among individuals (Muller-