(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).