Intraspecific genomic variation and local adaptation in a young hybrid species

Hybridization increases genetic variation, hence hybrid species may have
greater evolutionary potential once their admixed genomes have stabilized
and incompatibilities have been purged. Yet, little is known about how
such hybrid lineages evolve at the genomic level following their
formation, in particular their adaptive potential. Here we investigate how
the Italian sparrow (Passer italiae), a homoploid hybrid species, has
evolved and locally adapted to its variable environment. Using restriction
site-associated DNA sequencing (RAD-seq) on several populations across the
Italian peninsula, we evaluate how genomic constraints and novel genetic
variation have influenced population divergence and adaptation. We show
that population divergence within this hybrid species has evolved in
response to climatic variation, suggesting ongoing local adaptation. As
found previously in other non-hybrid species, climatic differences appear
to increase population differentiation. We also report strong population
divergence in a gene known to affect beak morphology. Most of the strongly
divergent loci among Italian sparrow populations do not seem to be
differentiated between its parent species, the house and Spanish sparrows.
Unlike in the hybrid, population divergence within each of the parental
taxa has occurred mostly at loci with high allele frequency difference
between the parental species, suggesting that novel combinations of
parental alleles in the hybrid have not necessarily enhanced its
evolutionary potential. Rather, our study suggests that constraints linked
to incompatibilities may have restricted the evolution of this admixed
genome, both during and after hybrid species formation.