GENETIC DRIFT AND FOUNDER EFFECT IN NATIVE VERSUS INTRODUCED POPULATIONS OF AN INVADING PLANT, LYTHRUM-SALICARIA (LYTHRACEAE)

There are few convincing examples of genetic drift at loci under selec tion in natural populations. The plant sexual polymorphism tristyly pr ovides an opportunity to investigate genetic drift because stochastic processes interacting with frequency-dependent selection give rise to a diagnostic pattern of morph-frequency variation. A previous study of 102 Ontario populations of the introduced tristylous wetland herb Lyt hrum salicaria provided evidence for the role of stochastic processes during colonization. However, whether stochastic effects are greater i n these recently introduced populations compared to native Eurasian po pulations remains unclear. The propensity of this species to invade di sturbed habitats suggests that episodes of colonization and periods of small population size must also occur in the native range. A survey o f 102 populations in southwestern France indicated reduced stochastic effects in native populations. Populations exhibited significantly low er morph loss than in Ontario (5% vs. 23%) and significantly higher va lues of morph evenness. The greater incidence of trimorphism in French populations was not associated with larger population sizes; populati ons were significantly smaller than those in Ontario (means: 266 vs. 4 87). Morph evenness was positively correlated with population size amo ng French but not Ontario populations, providing further evidence of n onequilibrium conditions in introduced compared to native populations. The incidence of trimorphism was unexpectedly high in small native po pulations (N less than or equal to 25; 22 of 27 populations trimorphic ). Computer simulations indicated that levels of gene flow on the orde r of rn greater than or equal to 0.05 can account for the maintenance of tristyly in small populations. The high connectivity of populations within the agricultural landscape typical of southwestern France may facilitate levels of gene flow sufficient to maintain trimorphism in s mall populations.