Hybridization among S. corvina lineages
Two out of the three contact zones showed extensive hybridization.
However, contact zones are formed by later-generation hybrids and
backcrossed individuals, with no F1 hybrids, and the contact zones are
wide suggesting that pure parental subspecies are not currently directly
interbreeding. Classic tension zones are usually narrow and are
maintained by low hybrid fitness and dispersion from parental
populations (Barton & Hewitt, 1985). Our results suggest that these
contact zones are hybrid zones (sensu stricto Barton & Hewitt,
1985), a “fairly continuous transitions between distinct forms”, in
which hybrids between S. corvina subspecies likely present
relative fitness comparable to the parental subspecies but seem to be
restricted to the contact zone perhaps due to environmental constraints
and relatively low dispersal. For the third contact zone, which occurs
in the Central Valley of Costa Rica where S. c. corvina andS. c. hoffmanni come into contact, we found no hybrids.
The bimodal distribution of hybrid indices suggests low ancient
hybridization with extensive backcrosses, where the introgressed genomic
variation is highly diluted into the genetic pool of the local
population. Given the extensive hybridization at the other two contact
zones described above, it is very unlikely that postzygotic genetic
incompatibilities (e.g., hybrid inviability), and low hybrid fitness
(e.g., hybrid disadvantage) play an important role in reducing
hybridization in this system. Instead, we hypothesize that premating
reproductive isolation is the main driver maintaining isolation betweenS. c. corvina and S. c. hoffmanni subspecies in Costa
Rica.
Reproductive isolation in secondary contact can be attained by
conspecific recognition resulting in assortative mating. Given that we
found the two genetically distinct populations (S. c. corvina andS. c. hoffmanni ) within the same locality, we predict that
assortative mating due to behavioral isolation will likely be found to
play a major role in reproductive isolation. However, contrary to our
expectations, we found two different outcomes in two contact zones with
similar patterns of plumage divergence (i.e., black vs. pied plumage),
suggesting that factors other than just plumage color may also shape the
different patterns of hybridization. For instance, different patterns of
local adaptation may result in asynchronous breeding phenology (Hau et
al., 2008). For example, S. c. corvina at the Caribbean side of
Costa Rica apparently have longer breeding seasons (almost year-round;
pers. obs.), related with the lack of marked dry and rainy seasons, butS. c. hoffmanni from the Pacific side of Costa Rica appear to
breed only two or three months after the beginning of the marked rainy
season (Skutch, 1954). Additionally, divergence in male song
characteristics is an important source of isolation in tropical birds
(Freeman & Montgomery, 2007), and in this case both subspecies
(S. c. corvina and S. c. hoffmanni ) have different male
songs (unpub. data), which may favor assortative mating.
Interestingly, where subspecies hybridize, plumage traits showed a
sharper transition and reduced cline width compared to the hybrid index
clines (Figure 5; Table 2). The more abrupt change in phenotype than in
hybrid index at the contact zones suggests these plumage traits are
under divergent selection, causing limited introgression. In general,
our results suggest that distinct plumage traits are under selection
(likely as signals for conspecific mate recognition), but that they do
not act as a strong barrier to gene flow by themselves. On the other
hand, the striking intrapopulation variation in morphometric traits
conceals a clear sigmoidal transition among contact zones, confounding
the interpretation of the cline’s centers and widths (Figure 5).
However, the displaced clines of the beak size and tail length across
the corvina-hicksii contact zone (Figure 5B) could result from
directional selection favoring an asymmetrical introgression of some
loci associated with selected phenotypes (Lipshutz et al., 2019; Stein
& Uy, 2006; While et al., 2015).
Our models for demographic inference suggested rapid divergence paired
with recent gene flow between lineages, but not contemporary gene flow
between S. c. corvina and S. c. hoffmanni. This result
suggests that the black S. c. corvina may have evolved in
isolation from the pied subspecies and they subsequently established
secondary contact and gene flow with S. c. hicksii. This is
consistent with the hypothesis of the range of S. c. corvinaexpanding into the distribution of S. c. hicksii, mediated by
periods of population expansions and contractions due to climatic
fluctuations (Stiles, 1996). If the contact is not linked to an
environmental gradient or ecotone, differential introgression may result
in movement of the contact zone (Buggs, 2007). However, the contact zone
that occurs between S. c. corvina and S. c. hicksii in
Central Panama has been stable, at least for more than 100 years (Olson,
1981).