i. deep genetic divergence between cryptic lineages

Many widespread Neotropical birds show substantial phylogeographic breaks across their range. Whether these divergent lineages represent cryptic species is not simply a question of properly accounting for avian species richness, but instead can inform our understanding of the intersection of speciation and community assembly processes. In the lowlands of Panama, I have identified many species of birds where one phylogeographic clade is abruptly replaced by a second clade.

Previously, coauthors and I showed that two abrupt phylogeographic breaks in Ochre-bellied Flycatchers (Mionectes oleagineus) were due to multiple colonizations of Middle America from Amazonia (Miller et al. 2008 Proc Roy Soc).

At left is a map of these three clades in Panama (grey circles represent birds not yet genotyped). My current priority with this species is to fill in the specimen gap between the red and blue clades.



A similar pattern is found in the White-breasted Wood-Wren (Henicorhina leucosticta) in Panama. Below is a map highlighting the contact between two highly-diverged, non-sister, lineages of White-breasted Wood-Wren in the central Panama just east of the Panama Canal.

Although genetic variation among eastern and western birds is huge (8.1%: for comparison, with caveats, humans & chimps vary by just over 9% at the same locus), plumage differences are subtle: wrens from Darien have black caps, birds from central and western Panama have chestnut caps. Interestingly the bird from Cerro Azul with the eastern Panama mtDNA sequence type had a brown, not a black, cap.

The second phase of this project is where things will get interesting. My hunch is that wrens are under selective pressure to avoid interbreeding between lineages. In the Cerro Azul area near the contact zone, Darien-form birds (identified by song) are found at higher elevations than wrens that sing the central Panama song. Ecological segregation along an altitudinal gradient would be one way to minimize interbreeding. Furthermore, Alex Kirschel, a post-doc from UCLA, has data that suggests the two forms may not respond to the other form's song. Over the next year, I'll be comparing and contrasting these ecological and behavioral traits of the two lineages near the contact zone with nuclear DNA data which will be a direct estimate of gene flow across this cryptic contact zone.

ii. genetics of species isolation across shallow divergences

Both Mionectes and Henicorhina are cases of deep mtDNA divergence obscured by cryptic plumage and ecological differences between Neotropical birds. However, other Neotropical birds show the converse, namely a history of rapid speciation with obvious ecological and behavioral barriers to gene flow. An excellent example are Selasphorus hummingbirds from western Panama.

Because the two species overlap geographically (only being separated by altitude and not any vicariant barrier to gene flow), apparently, both plumage and courtship displays (see above) maintain species boundries between these two species that are only shallowly differentiated genetically (1.1% ND2 mtDNA). I'm investigating the genetic "leakiness" of this ecological species boundary.