1.2: Balancing selection maintains adaptive diversity that can facilitate invasion
Unlike the example above, pre-existing adaptations that increase invasive spread in the invaded range can sometimes experience a different selective regime in the native range. Lee and Gelembuik (2008) argue that fluctuating selection pressures in the native range (e.g. , through regular disturbance events) can maintain either genetic variation or phenotypic plasticity, which can then be acted on by positive selection in novel environments. This prediction has been tested in the copepod species complex Eurytemora affinis (Stern & Lee, 2020). Several native populations from high-salinity environments have independently invaded freshwater ecosystems in North America. Seasonal fluctuations in salinity in the native environment, combined with several life history traits of E. affinis including overlapping generations, create favourable conditions for balancing selection. In the invaded range, a genome-wide scan showed parallel signatures of directional selection at ion transport genes in replicate populations (Figure 3, upper panel). Furthermore, the loci identified as being under positive selection in the introduced range also showed signatures of long-term balancing selection in the native range (Figure 3, lower panel), consistent with the hypothesis that fluctuating selection had maintained genetic variation that could enable invasive spread in a novel environment. Through the use of genomic data, localised signatures of both directional and balancing selection could be detected without any prior knowledge of the genes or traits underlying invasion success.
[ FIGURE 3 ]