Although the potential genetic risks associated with rare or endangered plants and small populations have been discussed previously, the practical role of population genetics in plant conservation remains unclear. Using theory and the available data, we examine the effects of genetic drift, inbreeding, and gene flow on genetic diversity and fitness in rare plants and small populations.We identify those circumstances that are likely to put these plant species and populations at genetic risk. Warning signs that populations may be vulnerable include changes in factors such as population size, degree of isolation, and fitness. When possible, we suggest potential management strategies.

Accurate estimation of effective population size is important in attempts to conserve small populations of animais or plantl We review the genetic and ecological methods that have been used to estimate effective population size in the past and suggest tha~ while genetic methods may often be appropriate for the estimation of N e and its moni-torin~ ecological methods have the advantage of providing data that can help predict the effect of a changed environment on N e. Estimation of N e is particularly complex in populations with overlappin R generatiomg and we summarize previous empirical estimates of N e that used ecological methods in such population.~ Since it is often difficult to assess what parameters and assumptions have been used in previous calculation~ we suggest a method that provides a good estimate of N~, makes clear what assumptions are in-volvea~ and yet requires a minimum of informatto~ The method is used to analyze data from 14 studte~ In 36% (5) of these studte~ our estimate is in excellent agreement with the original and yet we use significantly less informatiorg in 2196 (3) the originaLestimate is markedly lowe~, in 4396 (6) it is markedly higher. Reasons for the discrepancies are sug-gesteaL Two of the underestimates involve a failure in the original to account for a long maturation tim~ and four of the overestimates involve problems in the original with the correction for overlapping genemtior~ Paper submittedJulF l& 1992; revised manuscript accepted Novemher 11, 1992. Estimacion del tamafio poblacional efectivo de poblaciones protejidas Resumen: La estimaci(m precisa dei tamaf~o poblacional efectivo es importante en los intentos de conservar pequefuss poblaciones de animales y planta~ Nosotros revisamos los gendttcos y ecol6gicos que hart sido utlltzados en el pasado para estimar el tamafJo poblacional efectivo y sugertmos que si bien ~temente los mdtodos gendticos sertan apropiados para la estimaci6n y monitoreo de Ne, los mdtodos ecol6gicos tienen la ventaja de proveer datos que pueden ayudar a predecir el efecto de cambios ambientales sobre el N e. La estimacidn del N e es particularmente com. pleja en poblaciones con generaciones superpuestas y nosotros resumimos estimaciones emptricas de N e anteriores que usaron radtodos ecoldgicos en tales poblacione~ Dado que muchas veces es dificil evaluar que pardmetros y suposi. ciones ban sido usados en cdlculos previo~ nosotros suge. rimos un m~todo que provee una buena estimaci6n de No, pone en claro que supuestos estan tnvolucrados y sin embargo requiem un m[nimo de informacidtt E1 mdtodo es usado para analizar datos de catorce estudio~ En un 36% de los casos (5), nuestro estimador concuerda excelentemente con el original a pesar de usar una cantidad significattva. mente menor de informaci6r4, en un 21% (3) la estimacion original es ~mente meno~, yen un 43% (6) es mar. cadamente mayo~. Se sugier~ ra~ones para estas discrepan-Dos de las subestimaciones involucran una falla en la comtderact~ de un ttonpo de maduraci6n largo en la estimacidn o...

One of the lingering paradoxes in invasion biology is how founder populations of an introduced species are able to overcome the limitations of small size and, in a ''reversal of fortune,'' proliferate in a new habitat. The transition from colonist to invader is especially enigmatic for self-incompatible species, which must find a mate to reproduce. In small populations, the inability to find a mate can result in the Allee effect, a positive relationship between individual fitness and population size or density. Theoretically, the Allee effect should be common in founder populations of selfincompatible colonizing species and may account for the high rate of failed introductions, but little supporting evidence exists. We created a field experiment to test whether the Allee effect affects the maternal fitness of a self-incompatible invasive species, wild radish (Raphanus sativus). We created populations of varying size and relatedness. We measured maternal fitness in terms of both fruit set per flower and seed number per fruit. We found that both population size and the level of genetic relatedness among individuals influence maternal reproductive success. Our results explicitly define an ecological genetic obstacle faced by populations of an exotic species on its way to becoming invasive. Such a mechanistic understanding of the invasions of species that require a mate can and should be exploited for both controlling current outbreaks and reducing their frequency in the future.Allee effect ͉ colonization ͉ invasiveness ͉ Raphanus sativus ͉ self-incompatibility E xotic, invasive species are a modern biological paradox. Founder populations are characteristically small in number of individuals and/or low density. Low density and small population size not only increase the risk of stochastic extinction (1) but also limit opportunities for individuals to mate effectively (2). The latter may result in an Allee effect, a positive relationship between population size and individual fitness (3). Finding compatible mates in a small population presents a problem for plants, which are sessile during the reproductive stage, and is especially challenging for obligately outcrossing species, such as those that are self-incompatible or dioecious. For example, self-incompatibility systems both prohibit self-pollination and significantly reduce successful cross-pollination of individuals with shared S-alleles, (e.g., close relatives) (4). ''Baker's law'' predicts that a self-compatible species will have a much greater chance of establishing in a new location after a long-distance dispersal event than a self-incompatible species because the former can create a sexually reproducing population with a single individual (5, 6). Nonetheless, self-incompatibility is not rare for invasive plants. Founder populations of selfincompatible species that become invasive must experience a ''reversal of fortune.'' That is, individuals within these small populations overcome the limitations of small population size to reproduce and spread successfull...

Engineered genes in transgenic crops may escape into the ambient environment via crop‐weed hybridization However, natural crop‐weed muting rates (gene flow) are largely unknown We measured mating between wild and cultivated radishes in an experiment that simulated natural stands around seed multiplication plots. We used a genetic marker to identify, crop‐weed muting events. Although weeds at the cultivar plot margin (1 m distance) received much more gene flow than distant plants, detectable gene flow occurred at our most distant site (1000 m). For insect‐pollinated outcrossing crops like radish, strategies other than distance must be employed to ensure complete isolation.

We assessed the effects of population size and genetic relatedness on rates of pollen gene flow into experimental populations of the insect-pollinated, self-incompatible plant Raphanus sativus. We created synthetic populations of sizes 2, 5, 10, and 20 with three genetic structures (full siblings, half siblings, and unrelated plants). Following pollination in a natural setting, we conducted a simple paternity exclusion analysis using the allozyme genotypes of progeny to measure apparent gene flow and Monte Carlo simulations to estimate total gene flow. Estimates of apparent pollen gene flow rates ranged from 0 to 100% and were similar in rank to estimates of total gene flow. There were significant effects of population size and relatedness on the rate of apparent gene flow, and there were significant population size by relatedness interactions. Populations of size 2 had higher gene flow rates than larger populations, gene flow being negatively associated with the level of cross-compatibility (as measured by hand pollinations). Gene flow into populations of size 2 was also negatively associated with the distance to the nearest population of size 10 or 20. These results suggest that interactions among demography (population size), genetics (cross-compatibility), and ecology (pollinator behavior) are important influences on pollen gene flow rates into small plant populations.