Conclusions and implications for previous work
Analyses based on genome-wide markers confirmed the marked genetic
divergence of the Oregon juncos at UCSD, a population that became
established in the early 1980s. Three further, fundamental aspects of
the evolutionary history of the UCSD population have also been
established here based on genomic data: (i) the UCSD junco population is
most closely related to the coastal form pinosus , a group showing
ecological and phenotypic similarities; (ii) environmental factors
played a role in driving rapid divergence at functional loci, some of
which may be involved in adaptations to an anthropogenic habitat; and
(iii) demographic inference supports a scenario of contemporary
population divergence as a result of a founder event by a very small
number of colonizers, consistent with a major role for genetic drift in
the process of differentiation. Together these results suggest that a
combination of drift and directional selection can result in rapid
shifts in both neutral and adaptive loci across the genome, and reveal
the Oregon juncos from UCSD as a rare contemporary case of population
divergence following the colonization of an anthropogenic environment.
Based on the new evidence regarding the origin of the UCSD juncos
presented here, some conclusions from previous studies on this system
regarding rates of evolutionary change and phenotypic plasticity may
have to be reassessed. Specifically, differences in morphological
fitness traits (Rasner et al. 2004), sexual signaling traits (Yeh 2004;
Price et al. 2008), and shifts in a range of behavioral and life-history
traits (Yeh and Price 2004; Price et al. 2008) were thought to be due to
rapid evolutionary change, as were associated differences in hormones
and behavior (Atwell et al. 2012; Atwell et al. 2014; Fudickar et al.
2017). Studies comparing individuals from UCSD to those from the nearby
mountain thurberi population attributed differences to a recent
transition from a mountain climate to a more benign Mediterranean
climate, and from a wild to an anthropogenic urban habitat. These
comparative studies remain relevant to understanding phenotypic
differences between junco populations adapted to different environments,
although assumptions regarding rapid evolution may not be accurate given
the new evidence on phylogeny and co-ancestry.
For example, UCSD juncos’ greater phenotypic similarity in tail white to
their closest relative pinosus , rather than to the mountainthurberi individuals they were previously compared to, suggests
that prior explanations may have overestimated the role of rapid
evolutionary change in plumage variation. Further, prior research
comparing UCSD and the montane thurberi phenotypes could not distinguish
between two potential explanations, adaptation to an anthropogenic
environment or to a mild Mediterranean climate. The fact thatpinosus shares some aspects of a mild Mediterranean climate with
UCSD suggests that selection reported here is associated more strongly
with the anthropogenic environment at UCSD rather than milder climate.
Looking forward, our new findings provide the opportunity to develop new
comparative studies between UCSD and pinosus individuals that
will contribute to effectively addressing the role of urban habitats in
driving rapid evolutionary divergence.