Abstract
Adaptive radiation unfolds as selection acts on the genetic variation underlying functional
traits. The nature of this variation can be revealed by studying the tips of an
ongoing adaptive radiation. We studied genomic variation at the tips of the Darwin’s
finch radiation; specifically focusing on polymorphism within, and variation among,
three sympatric species of the genus Geospiza. Using restriction site-associated DNA
(RAD-seq), we characterized 32 569 single-nucleotide polymorphisms (SNPs), from
which 11 outlier SNPs for beak and body size were uncovered by a genomewide association
study (GWAS). Principal component analysis revealed that these 11 SNPs
formed four statistically linked groups. Stepwise regression then revealed that the first
PC score, which included 6 of the 11 top SNPs, explained over 80% of the variation in
beak size, suggesting that selection on these traits influences multiple correlated loci.
The two SNPs most strongly associated with beak size were near genes associated with
beak morphology across deeper branches of the radiation: delta-like 1 homologue
(DLK1) and high-mobility group AT-hook 2 (HMGA2). Our results suggest that (i) key
adaptive traits are associated with a small fraction of the genome (11 of 32 569 SNPs),
(ii) SNPs linked to the candidate genes are dispersed throughout the genome (on several
chromosomes), and (iii) micro- and macro-evolutionary variation (roots and tips of
the radiation) involve some shared and some unique genomic regions.
Keywords: adaptive radiation, beak size, Darwin’s finches, genomic regions, RA