PMC

Distribution of allele frequencies indicating a mode-shift. Bars represent the proportion of alleles found in each allele frequency class. Deviation from an L-shaped distribution is indicative of a mode-shift in allele frequency due to a recent genetic bottleneck.

Map of Santa Cruz, Floreana, and Isabela Islands with sampling locations. Sampling sites on Santa Cruz: S1 = highlands; S2 = lowlands, Punta Estrada; S3 = lowlands, Garrapatero; S4 = lowlands, Itabaca. On Floreana and Isabela, one site each in the lowlands (L) and highlands (H) are indicated.

Estimated number of populations from STRUCTURE (a) and GENELAND (b) analyses. (a) Mean ( ± SD) probabilities of the data (LnPr [X|k]) over three replicate STRUCTURE runs plotted as a function of the putative number of clusters (k); (b) Posterior density distribution of the number of clusters estimated from GENELAND analysis in three replicates.

Genetic assignment of P. downsi individuals across three islands using Bayesian clustering analysis. Two genetic clusters are identified: (a) including all individuals from Santa Cruz (n = 62) (bottom left) and Isabela (n = 9) (centre top), and (b) all individuals from Floreana Island (n = 76) (bottom right). Black dots represent independent geographic sampling points (i.e. location of bird nests). Note that two geographic sampling points on Isabela Island were within 5 m of each other and are not distinguishable.