demonstrate that frequent bottlenecks and low effective population sizes reduce genetic variation, especially the frequencies of rare alleles (Nei et al. 1975, Sirkkomaa 1983). However, strong directional selection during the early stages of population establishment can also reduce genetic variation, particularly at loci that govern fitness. Hence, a population early in its postimmigration history is especially vulnerable because random genetic events erode the little genetic diversity that introduced populations usually contain. Low population size can also result in increased mating among related individuals; increased inbreeding can result in the expression of recessive deleterious alleles in homozygous form and cause reduced fitness or inbreeding depression in the progeny (Barrett and Husband 1990, Barrett and Kohn 1991).
The input of mutational variance, recombination, and gene flow theoretically can counteract those forces. In addition, many species have become invaders despite low genetic diversity. In these cases, extensive areas of the introduced range often comprise a small number of genotypes (Moran and Marshall 1978, Scribailo et al. 1984, Barrett and Shore 1989, Novak and Mack 1993). Low diversity at the population and regional levels is especially evident in plant species that propagate by asexual reproduction, sometimes called selfing.
However, effects of genetic bottlenecks associated with introduction of small populations can also be counterintuitive and complex, as evidenced by the introduction of the Argentine ant (Linepithema humile) into California. In its native Argentina, intraspecific competition limits colony size and population density, and numerous ant species co-exist with the Argentine ants. Suarez et al. (1999) found that populations of the Argentine ant in California exhibited substantially lower heterozygosity than populations in their native range. Genetic changes in the introduced ant populations were associated with altered behavior that reduced nestmate recognition and hence intraspecific competition and resulted in high population densities, competitive displacement of a majority of the native ant species, and adverse effects on ant predators, such as horned lizards (Suarez et al. 2000). Evidence suggests that the imported fire ant (Solenopsis invicta) similarly experienced a pronounced genetic bottleneck when it was introduced into North America (Ross et al. 1993). Altered ecological characteristics and population genetics of the introduced fire ant populations appear to be associated with changes in the social organization of its colonies (Ross et al. 1996). Such changes include multiple queens and zero relatedness between workers and new queens in the introduced populations, compared with few queens and significant relatedness between queens and workers in the native range. Low relatedness has potential advantages for ants and has been associated with rapid colony growth in other ant species (Cole and Wiernasz 1999).