(4) Selection analyses
Starlings encounter a range of precipitation, temperature, and elevation
across their range and redundancy analyses revealed the strongest
signatures of local adaptation, showing that 178 variants are correlated
with environmental differences among populations (F = 1.022, P = 0.002,
Figure 2). Populations living in warmer climates tend to cluster more
closely in the left quadrant and high elevation populations cluster in
the middle right quadrant. However, populations do not cluster based on
geographic distance: for example, starlings from TX and WA cluster
closely due to shared genetic variants, even though the two populations
differ substantially in precipitation and temperature and are
geographically separated. After controlling for population structure,
candidates for selection are equally distributed among elevation,
temperature- and precipitation-associated predictors. Importantly, when
environmental predictors are randomly shuffled, the RDA model is not
significant. Mean annual temperature (BIO1) opposes selective pressure
related to the range of temperatures experienced each year (BIO7),
annual precipitation (BIO12), precipitation in the wettest quarter
(BIO16) and elevation (Figure 2). Genes under selection tend to have
lower allele frequencies, and function in several biological processes
(Table S2).