Genetic variability among alien species varies greatly.When introductions involve only a few founder individuals from a limited part of their native range, only a fraction of the genetic variability of the source population is introduced.This "founder effect" may be compounded by genetic drift that leads to the loss of alleles from small founder populations. When a founder population remains small over several to many generations, the loss of genetic variability by genetic drift, termed genetic bottlenecking, may lead to loss of almost all variation in the population (Allendorf 1986). Thus, the size of the founding population and how quickly this population increases are important determinants of the genetic variability of alien populations.
The European starling (Sturnus vulgaris) in North America provides a good example of genetic bottlenecking and at the same time shows that low genetic variability does not always limit the success of an alien species. The North American population, now continent-wide in distribution and enormously abundant, is derived from about 100 individuals released in New York City in 1890 and 1891 (Cabe 1998). Allozyme analysis showed that the North American population lacks about 42% of the alleles that occur at loci showing variability in Europe. Furthermore, starlings from different parts of North America show no significant variation in genetic makeup. Even without a high degree of genetic variabil ity, however, this behaviorally adaptable species has colonized a wide range of habitats in North America.
On the other hand, founder populations often consist of many individuals or result from repeated introductions of individuals from different parts of their native range. In such cases, alien populations may have high genetic diversity. Many of these species have not only proven to be seriously invasive but have also shown genetic adaptation to different environments within their new range.
Kudzu (Pueraria lobata), one of the most troublesome alien plants of the southeastern United States, exemplifies this latter category (Pappert et al. 2000). Allozyme analysis of 14 loci showed that individual populations varied in polymorphism, some having as few as four and some as many as 12 polymorphic loci. Overall, population analyses from 20 localities showed that 13 of the 14 loci examined were polymorphic. The high diversity of kudzu evidently reflects its deliberate introduction on many occasions over about 50 years. Kudzu was repeatedly introduced to the southeastern United States as an ornamental and forage plant, as well as for erosion control. Coupled with sexual reproduction, multiple introductions have made this species one of the most variable alien invaders of North America.
Evolutionary adaptability is one of the key characteristics of many, if not most, invasive alien species. In turn, adaptability reflects especially the degree of additive and epistatic genetic variability, together with the potential for acquiring additional variability through mutation, hybridization, and chromosomal rearrangement. Additive genetic variation represents the component of genetic variation that consists of alleles that have an overall quantitative effect proportional to their number (Lee 2002). In diploid species, for example, instead of being dominant or recessive, some alleles may have an additive effect, so when they are present on both chromosomes of the pair, their effect is greater than when only one member of the chromosome pair contains the allele. In many polyploid species, in which the number of complete sets of chromosomes is greater than two, the potential for accumulation of alleles with additive quantitative effects can be high because a particular gene locus exists on more than two sets of chromosomes. Epistasis refers to the influence of alleles at one locus on the expression of alleles at a different locus.
Additive genetic variation is usually regarded as the material for rapid evolutionary change, such as that seen in many alien species and native species with which they interact. Carroll et al. (2003b), however, found that epistasis is an important component of adaptation by the soapberry bug (Jadera haematoloma) to new alien host plants. We shall examine this case in detail in chapter 13.
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