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.