Genetic drift
Genetic drift in a small population tends to cause the loss of rare
alleles, such as singletons(Motoo Kimura & Ohta, 1969). To evaluate the
relative level of genetic drift in the black-faced spoonbill, we
compared the folded site frequency spectrum of the unlinked autosomal
SNPs between the two spoonbill species. We used the same folded SFS forFastSimcoal2 analysis for the black-faced spoonbill. For the
royal spoonbill, we used the same filtering criteria as those for
evaluating population structure to generate a set of 232,409 unlinked
autosomal SNPs. Because the low frequency of rare alleles rendered by
genetic drift could lead to a higher Tajima’s D value(Tajima,
1989), we also used VCFtools to calculate the Tajima’s Dvalue for all autosomal SNPs of both spoonbill species.