Exploring the evolutionary consequences of altitudinal expansion in Colias butterflies

Although the extrinsic drivers of migratory behavior in butterflies are relatively well-characterized, their effects on genetic diversity and population structure are still being established. This is especially true of altitudinal migration, typified by a number of butterfly species in western North America. Historically, altitudinal migrants originating from the Central Valley of California have contributed to butterfly abundance and diversity in the Sierra Nevada Mountains during the summer; however, within the last decade this phenomenon has begun to decline as low elevation populations have been negatively affected by development. To better understand the evolutionary consequences of these changes, I analyzed population genomic data from the common, altitudinal migrant, Colias eurytheme (Pieridae). During the summer of 2024, I collected C. eurytheme adults from multiple points in time within the Central Valley and the Sierra Nevada. Analyses thus far suggest little overall population structure in comparing mountain and valley populations, but differences in a small number of loci, which suggest non-random movement out of the valley, are potentially suggestive of genetic variation for migratory behavior. A subset of adults collected at montane sites where overwintering is possible are genetic outliers at a greater number of loci, which raises the possibility of selection maintaining different alleles at those locations. Beyond advancing our understanding of the evolutionary consequences of seasonal, altitudinal migration, this work provides initial insight into one of the mechanisms by which altitudinal migration may preserve genetic diversity and what might be lost as the altitudinal migration fails.