Contrasting patterns of diversification in two sister species of martins (Aves: Hirundinidae): The Sand Martin Riparia riparia and the Pale Martin R. diluta

Seasonal migration patterns and the maintenance of evolutionary diversity in a cryptic bird radiation

“…Genetic differentiation between populations of different subspecies was considerably larger than comparisons within subspecies Here, by using population genomic data, we show that the pale sand martin Riparia diluta contains multiple deep evolutionary lineages despite extremely subtle and gradual morphological variation among them and little genetic differences within lineages over large geographic areas. Differentiation patterns in nuclear SNPs are largely consistent with phylogeographic structure in mtDNA (see also Schweizer et al, 2018) and represent different levels. We discuss how the integrity of the evolutionary lineages could be maintained despite morphological stasis.…”

“…The strong differentiation in genetically distinct but morphologically cryptic lineages within R. diluta is in stark contrast with the lack of any phylogeographic structure in its sister species, the collared sand martin R. riparia, in our study region. Geographically widespread nuclear homogeneity in R. riparia is in agreement with shallow mtDNA diversity over its entire Holarctic breeding range indicating recent demographic expansion (Pavlova et al, 2008;Schweizer et al, 2018). In contrast, extensive genetic structure in R. diluta is more similar to other Central and East Asian bird species complexes and has been related to heterogeneous environments and/or a climate being only mildly affected by Pleistocene climate changes that enabled the persistence of isolated populations in mountainous region at the south-western edge of the Qinghai-Tibetan plateau (e.g., Qu et al, 2014;Liu et al, 2016Liu et al, , 2020.…”

“…Phylogenetic reconstruction based on unique haplotypes was done using maximum likelihood (ML) and Bayesian inference (BI). Barn swallow Hirundo rustica (GeneBank accession number DQ176515) was used as outgroup and one sequence of R. d. indica (Schweizer et al, 2018; GeneBank accession number MG881167) was additionally added. Sequence alignment was done using ClustalW in mega 7 (Kumar et al, 2016).…”

“…The analysis of three morphological traits only, which enabled the inclusion of additional 120 individuals (including R. d. indica) from Schweizer et al (2018), revealed overall less separation between different subspecies of R. diluta (Figure 6b). PC1 (55.19% of variance) separated R. d. fohkienensis and R. d. indica together from R. d. diluta and R. d. tibetana in PC1), while PC2 (30.43% of variance) tended to differentiate subspecies of these two pairs.…”

“…breeds on the Qinghai-Tibetan plateau (Figure 1). While R. d. indica is geographically isolated, the breeding areas of the remaining subspecies are thought to be largely contiguous, but geographic sampling in previous studies was limited, especially in areas of potential secondary contact (Schweizer et al, 2018).…”