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

Abstract

Abstract.-There are few convincing examples of genetic drift at loci under selection in natural populations. The plant sexual polymorphism tristyly provides 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 Lythrum salicaria provided evidence for the role of stochastic processes during colonization. However, whether stochastic effects are greater in these recently introduced populations compared to native Eurasian populations remains unclear. The propensity of this species to invade disturbed habitats suggests that episodes of colonization and periods of small population size must also occur in the native range. A survey of 102 populations in southwestern France indicated reduced stochastic effects in native populations. Populations exhibited significantly lower morph loss than in Ontario (5% vs. 23%) and significantly higher values of morph evenness. The greater incidence of trimorphism in French populations was not associated with larger population sizes; populations were significantly smaller than those in Ontario (means: 266 vs. 487). Morph evenness was positively correlated with population size among French but not Ontario populations, providing further evidence of nonequilibrium conditions in introduced compared to native populations. The incidence of trimorphism was unexpectedly high in small native populations (N:S 25; 22 of 27 populations trimorphic). Computer simulations indicated that levels of gene flow on the order of m 2: 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 small populations.

Frequency-dependent selection on morph ratios in tristylous Lythrum salicaria (Lythraceae)

“…Lythrum salicaria has tetrasomic inheritance at both style morph loci with no linkage, and double reduction was estimated at 10 per cent and 2.5 per cent for the S and M loci, respectively (Fisher & Mather, 1943;Fisher, 1949;Fyfe, 1953). Models incorporating tetrasomic inheritance (with double reduction) gave results identical to those incorporating diploid inheritance (see also Eckert & Barrett, 1992;Eckert et al, 1996). At the start of each run, populations had equal frequencies of two of the morphs.…”

“…Thirdly, large-scale surveys of morph-ratio variation have demonstrated that genetic drift may strongly influence morph ratios in natural populations of L. salicaria (Eckert & Barrett, 1992;Eckert et a!., 1996). Although drift probably accounts for the substantial variation in morph-ratio change among populations in our sample as well as for the loss of the rare morph in three populations with very low initial frequencies (Fig.…”

How Interactions Between Ecology and Evolution Influence Contemporary Invasion Dynamics

“…This is despite the fact that European populations are smaller on average and experience frequent episodes of dispersal and colonization. Eckert et al (1996) suggest that the difference might reflect higher rates of gene flow between European populations that restores morphs lost during founding. European populations are closer together than North American populations and are often loosely linked by scattered individuals.…”

Evolution, epigenetics and resistance – troublesome weeds

“…Thus, natural selection ultimately contributes for reducing the genetic variation within a population (Gurevitch et al 2009), and not for increasing it (Table 1). It should be noted that natural selection is the main responsible for evolution, but when it is absent, genetic drift plays also an important role (Eckert et al, 1996;Gurevitch et al, 2009). The genetic variation is important to supply a range of individuals to be tested in its aptitude to the environment, but these organisms essentially evolute when the population adapts to a given environment, which is accompanied by a reduction in genetic variation within the population while fixing those genes most favorable to that particular environment.…”