PRE-PLEISTOCENE DIFFERENTIATION AMONG CHAT-TYRANTS

Most species of chat-tyrants (Ochthoeca, Silvicultrix) are widespread along the length of the Andes, with broadly overlapping ranges, but se gregated in different altitudinal zones or habitats. We used mitochond rial DNA sequences to study their phylogenetic relationships. Our data show considerable divergence and multiple base substitutions at some positions between geographically overlapping species. Such divergence probably predates the period of marked ecoclimatic changes in the Plei stocene, whereas vicariance patterns may reflect events in the lower P leistocene. Only some of the boundaries between allospecies correspond to marked topographic barriers, and we suggest that sharp species-rep lacements indicate secondary contact zones. An evolutionary dynamic sc enario is suggested where divergence of isolated populations is follow ed by establishment of sharp geographical replacements and, with devel opment of full ecological compatibility, establishment of broad overla p along the Andes cordillera. The phylogeny suggests a series of adapt ive shifts from the ancestral habitat of scrub and river margins in tr opical lowlands to bushy highlands and elfin forest, then colonization of edge habitats in the humid montane forest and finally interior mon tane and submontane forest. However, the adaptive redistribution has o bscured the early vicariance events in the group. The results support the monophyly of all chattyrants, but with a distinctive internal stru cture, where Silvicultrix is a monophyletic internal branch within Och thoeca. Support also is given for a close relationship between Tumbezi a salvini and Ochthoeca. We suggest keeping Tumbezia as a separate gen us, merging Silvicultrix with Ochthoeca, and elevating O. frontalis an d spodionota, and O. cinnamomeiventris and thoracica to species rank.