Relaxation of selection in the black-faced spoonbill
The elevated level of genetic drift in the black-faced spoonbill might have compromised the efficacy of natural selection (Kimura, 1962). To examine whether the elevated level of genetic drift had lowered selection efficiency in the black-faced spoonbill, we used RELAX(Wertheim, Murrell, Smith, Kosakovsky Pond, & Scheffler, 2015) to detect genes under relaxed selection in the two spoonbills. Of the 5,657 well-aligned homologous genes between the two spoonbill species and another Ciconiformes species, the crested ibis ( Li et al., 2014),RELAX analysis revealed significantly more genes in the black-faced spoonbill (2,094, 37.0%) to be under relaxed selection (relaxation parameter k <1, p < 0.05) than in the royal spoonbill (1,542, 27.2%; χ 2= 123.04, p = 2.2e-16; Figure 4A). Conversely, there was no evidence (χ 2= 3.1671.04, p = 0.075) to support the hypothesis that selection was significantly intensified in the genome of the royal spoonbill (33 and 55 genes withk >1, p < 0.05 for the royal and black-faced spoonbill, respectively; Figure 4A). Therefore, a higher level of genetic drift had probably caused relaxed selection in the black-faced spoonbill.