rnal
introgression, such as mtDNA introgression (Haldane, 1922). Thus, we
could suppose that the gene flow between T. naumanni and T.
ruficollis was mainly driven by backcrossing with male hybrids.
Alternatively, nuclear-mitochondrial discordance might also be related
to the sex-biased dispersal pattern of birds. In general, the dispersal
pattern of birds is female biased, and female individuals are more
frequently dispersed to areas where males of the same species do not
exist, which promotes hybridization between different species (Clarke et
al., 1997; Greenwood, 1980; Wang et al., 2014). However, a small number
of birds have opposite dispersal patterns (Dallimer et al., 2002; Li and
Merila, 2010; Williams and Rabenold, 2005). T. naumanni andT. ruficollis might also experience
male-biased
dispersal, and we need to further evaluate this possibility using more
effective methods.
In parallel, increasing global warming might cause breeding areas to
further overlap, which would increase the possibility of interspecific
hybridization and eventually lead to species amalgamation (Rieseberg and
Carney, 1998). In addition, considering the high level of gene flow
between T. naumanni and T. ruficollis , we speculate that
as the introgression between them continues, genetic information will be
exchanged, which might affect their morphological and genetic integrity
and decrease their interspecific differences.