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 ]