# Evidence of positive selection and a novel phylogeny among five subspecies of song sparrow (Melospiza melodia) in Alaska

## TL;DR

This study explores how five subspecies of song sparrows in Alaska evolved different traits due to local adaptation, revealing genetic evidence of positive selection and a new evolutionary history.

## Contribution

The study identifies seven candidate genes under positive selection and proposes a novel phylogeny for five song sparrow subspecies in Alaska.

## Key findings

- Seven candidate genes related to bill size, circadian rhythm, plumage color, and salt tolerance show signs of positive selection.
- Phylogenetic analysis suggests M. m. maxima is the sister to other subspecies and colonized Alaska earliest.
- Museum specimens revealed significant size differences among subspecies, with M. m. maxima being 1.6 times larger than M. m. rufina.

## Abstract

Local adaptation occurs when populations evolve traits in response to local environmental challenges. Isolated island populations often experience different selection pressures than their mainland counterparts, which enables the study of how phenotypes and genotypes respond to differing selection regimes. We studied a group of five phenotypically differentiated subspecies of song sparrow (Melospiza melodia) in Alaska that demonstrate striking body size, color, and migratory behavioral differences to examine the effects of local adaptation on phenotypes and genotypes. We examined the phenotypic attributes of these populations and used whole-genome data to determine relationships and test candidate loci for evidence of selection. Phenotypic measurements of museum specimens (n = 227) quantified the dramatic size differences among these populations, with westernmost M. m. maxima being ~1.6 times larger than easternmost M. m. rufina. Using ultraconserved elements (UCEs) and McDonald-Kreitman tests, we showed that seven candidate genes associated with bill size, circadian rhythm regulation, plumage color, and salt tolerance exhibited signs of putative positive selection. Phylogenetic analysis of UCEs identified M. m. maxima as sister to the other Alaska M. melodia subspecies. This suggests M. m. maxima colonized earliest, perhaps before the last glacial maximum, and that Alaska was later recolonized by ancestors of the remaining four subspecies.