When populations are introduced to an entirely novel range, a pattern of local adaptation will not instantly be present but introduced populations will initially be distributed rather randomly. For instance, recently introduced Verbascum thapsus
populations seem to show a pattern of local mal-
adaptation in their novel range [5
]. The absence of local adaptation removes a barrier to admixture because there is no selection against dilution of local genomes. Compared with the native range, the benefits of admixture are probably higher in the new range (§1) and now the costs are also much smaller. This shifted balance can permit introduced populations, unlike native populations, to freely benefit from admixture.
If local adaptation masks the negative effects of inbreeding in the native range, then admixture in the novel range can instantly lift this inbreeding cost via heterosis and the sheltering of the genetic load. The shifted balance in the costs and benefits of admixture therefore predicts that the population genetic forces that are involved in attaining high fitness are fundamentally altered during the early invasion process. In native range populations, high fitness may be achieved by maintaining local adaptation and balancing its advantage against the costs of inbreeding, and as long as the net fitness effect is positive it does not pay to admix much. In recent invaders, in contrast, high fitness may be achieved most easily by unconstrained admixture and lifting the inbreeding depression cost. This heterosis benefit may not necessarily be sufficient to explain invasiveness, but it could provide an important fitness boost compared with non-admixed populations and may play a significant role during the establishment phase of recent invaders [19
Testing the effects of local adaptation and inbreeding depression on the cost–benefit balance of admixture, and how this differs between native and introduced populations, requires common garden and reciprocal transplant experiments that assess performance of parental populations and inter-population crosses in both the native and introduced ranges. To our knowledge, such a comprehensive analysis has not yet been done for any species. But recent work in native and introduced Silene latifolia
(the white campion) reveals some interesting differences in the effects of admixture and the levels of inbreeding depression in native versus introduced populations. Silene latifolia
occurs in differentiated populations in its native European range [32
] and was introduced to North America during the late eighteenth or early nineteenth century [10
]. Genetic evidence shows that multiple introductions from different source populations took place and that at least some population admixture occurred in the introduced range [10
]. Introduced populations tend to have higher scores for fitness and performance-related traits than populations from the native range, as consistently demonstrated in common garden experiments that were carried out in both the native range [34
] and the introduced range [35
]. Wolfe et al.
] extended these common garden experiments and evaluated offspring from within- and between-population crosses, both from the native and the novel range. A re-analysis of their data reveals that (i) native populations show evidence of inbreeding depression but introduced populations do not, and (ii) introduced populations have higher biomass than natural native populations but their biomass is similar to that of hybrids between native populations from intermediate crossing distances (). In these common garden experiments, inbreeding depression in native populations is expressed as higher biomass of between-population crosses than within-population crosses. There is a penalty of outbreeding when parental populations are too far apart, which has been documented also in other S. latifolia
studies from the native range [37
]. Intermediate-distance crosses therefore have the highest biomass, which is comparable in magnitude to the biomass levels observed in introduced populations ().
Figure 2. Effect of population crossing distance on offspring biomass in native and introduced ranges of S. latifolia. Crosses were made within and between populations from the species' native European range (a, six populations) and the introduced North American (more ...)
These results are predicted by the hypothesis that introduced populations, after multiple introductions from different source populations, admix freely in their novel range and benefit from heterosis, whereas native range populations remain differentiated and consequently suffer from inbreeding depression. Any of the admixture benefits could be involved (including increased genetic variation and novel genotypes), but the data suggest that heterosis alone could in principle account for the observed superior fitness of invasive populations. However, the results remain open to alternative interpretations, including invasive spread of incidental high-fitness, non-inbred populations from the native range or post-introduction evolution in the absence of admixture [36
]. The data also do not tell us whether isolation-by-adaptation contributed to the observed inbreeding depression of native populations. In fact, this particular species shows good evidence for isolation by distance in its native range [33
]. Also, some adaptation to novel conditions may have evolved already in the introduced populations as well (for instance, in herbivore defences [34
]). Transplantation studies are currently being conducted in the native and introduced ranges that will be able to explicitly examine the extent of local adaptation of Silene
populations in both ranges. Additional detailed studies, such as the wild barley experiment (), are required to unravel the effects of population-level local adaptation on admixture and inbreeding depression in both the introduced and native ranges. The Silene
system may provide a good opportunity for such an extended analysis.