Elastic-net multiple regression model testing the association between HA sequence diversity and alternative hypothesized mechanisms.

The associations tested (see Text S1, section S4 for additional analyses) are between subtype-specific average and (I) strength of selection (amino acid subsitution rate), (II) Shannon Index of bird species identified in genetic sequences, (III) geographic population structure (FST), (IV) nucleotide mutation rate, and (VI) environmental durability (Rt). All variables have been standardized (converted to a mean of 0 and a variance of 1) to have directly comparable coefficients.