Abstract
Anthropogenic changes have altered the historical distributions of many
North American taxa. As environments shift, ecological and evolutionary
processes can combine in complex ways to either stimulate or inhibit
range expansion. Here we examine the role of evolution in a rapid range
expansion whose ecological context has been well-documented, Anna’s
Hummingbird (Calypte anna). Previous work suggests that the
C. anna range expansion is the result of an ecological release
facilitated by human-mediated environmental changes, where access to new
food sources have allowed further filling of the abiotic niche. We
examine the role of gene flow and adaptation during range expansion from
their native California north into Canada and east into New Mexico and
Texas, USA. Using low coverage whole genome sequencing we found high
genetic diversity, low divergence, and little evidence of selection on
the northern and eastern expansion fronts. Additionally, there are few
(if any) limits to gene flow across the native and expanded range. The
lack of selective signals between core and expanded ranges could reflect
i) an absence of novel selection pressure in the extended range
(supporting the ecological release hypothesis), ii) swamping of adaptive
variation due to high gene flow, or iii) limitations of genome scans for
detecting small shifts in allele frequencies across many loci.
Nevertheless, our results provide an example where strong selection is
not apparent during a rapid, contemporary range shift.