Range expansion and time elapsed since the colonization
A common signature of range expansions is a reduced genetic diversity
due to founder effects (Slatkin & Excoffier, 2012). However,
hybridization and introgression with a locally adapted resident species
during the range expansion process can be a counterforce that increases
overall genetic diversity and/or specific alleles (Behm, Ives, &
Boughman, 2010; Mehner et al., 2010; Pfennig, Kelly, & Pierce, 2016;
Rieseberg et al., 2007). Introgressive hybridization during the range
expansion of I. elegans in Spain can be a source of new alleles
to recently expanded populations in the Spanish hybrid region. It
appears that the range expansion has coincided with the hybridization ofI. elegans in Spain, and this has not only prevented the loss of
genetic diversity in both hybrid regions, but also increased diversity
in the north-west hybrid region. Further, consistent with theoretical
expectations, hybridization also resulted in reduced interspecific
differentiation in sympatry, i.e., genetic differentiation betweenI. elegans and I. graellsii was lower in the sympatric
than in the allopatric distribution. Dudaniec et al. (2018) investigated
neutral and adaptive molecular signatures along the northward range
expansion axis of I. elegans in where no other Ischnuraspp. occur. Interestingly, also no decrease in genetic diversity along
this axis was found; and observed heterozygosity was similar between
core and range-expanded populations, even without hybridization
opportunities. Another study investigating the genetic consequences of
northern range expansion in damselflies did, however, report small
genetic diversity losses in recently established populations (Swaegers
et al., 2015). Due to introgression, I. elegans and I.
graellsii populations showed slightly lower overall genetic
differentiation in sympatry (mean=0.691) than in allopatry (mean=0.725),
similar to patterns detected in other taxa (e.g., Anderson & Hubricht,
1938; Fu, Lu, Fu, & Wang, 2020). Demographic processes associated with
range expansions, such as repeated bottlenecks and genetic drift can
also increase the level of differentiation between species (Freedman,
Thomassen, Buermann, & Smith, 2010; Wang, Abbott, Ingvarsson, & Liu,
2014). This may have contributed to the high and significantly observed
pairwise genetic differentiation between I. elegans and I.
graellsii in the north-west hybrid region respect to the north-central
and the allopatric regions. In summary, the higher level of genetic
divergence and genetic differentiation detected in the north-west hybrid
region can be explained by local demographic processes (bottlenecks and
genetic drift) and the time since the colonization.