Speciation in Colombian forest birds west of the Andes | Colombian birds

"The main uplift of the Colombian Andes took place at the beginning of the Pleistocene. At that time the present lowlands of southern Central America and along the Pacific and Caribbean coasts of Colombia emerged and were forest covered. The temperature gradient in at least parts of the tropical latitudes during the glacial periods of the Pleistocene was greater than it is today; for this reason the refrigeration affected the higher latitudes and altitudes more (7̊-8̊ C.) than the tropical lowlands (3̊-4̊ C.). The latitudinal extent of the tropical lowlands was not much less during the Pleistocene than it is today: the lowlands of Colombia and great parts of Central America remained in the tropical zone (notwithstanding an extensive glaciation of the Central and South American mountain ranges). The faunas of the tropical lowlands were severely affected by sea-level fluctuations and by alternating wet and dry periods during the Pleistocene, caused by a contraction and expansion of the equatorial rain belt. These climatic changes continued through post-Pleistocene time. During the periods of drought the trans-Andean forest fauna was restricted to rather small humid refuges: (a) on the Caribbean slope of Central America (various refuges in Costa Rica, Nicaragua, Honduras); (b) on the Pacific slope of southwestern Costa Rica and adjacent Panamá ('Chiriquí Refuge'); (c) along the Pacific coast of Colombia ('Chocó Refuge'); and (d) at the foot of the northern slope of the Western and Central Andes of Colombia ('Nechí Refuge'). Strongly marked endemic forms originated in these refuges during periods of geographic isolation resulting from drought, at which time the connection of the trans- and cis-Andean forests was interrupted in the north Colombian lowlands. It is concluded that the uplift of the Colombian Andes was only indirectly responsible for the development of the numerous Central American and west Colombian endemic species. The direct causation for their development was the repeated change of dry and humid periods during the Pleistocene and post-Pleistocene. These climatic changes permitted and interrupted repeatedly the contact of the trans- and cis-Andean populations in the north Colombian lowlands through the expansion and shrinkage of the forests. In this way an increasing number of small founder populations was able to reach the trans-Andean forests. The high concentration of endemic species in the tropical lowland forests of western Colombia and Central America is explained by a gradual accumulation of isolates in the trans-Andean forest refuges. Numerous Chocó elements extended their ranges to Central America, in some cases forming another isolate in the refuges along the Caribbean slope. On the other hand, only a few Central American species advanced into northern Colombia. Zones of allopatric hybridization are developed at the eastern margin of the Caribbean Costa Rica Refuge in western Panamá and at the northern margin of the Chocó Refuge in the Urabá region of northwestern Colombia. These zones are probably of very recent origin. The hybridization of Pteroglossus t. torquatus X P. (torquatus) sanguineus and of Galbula r. ruficauda X G. (ruficauda) melanogenia is described in detail. Double invasions were responsible for the present co-existence of Columbia (Oenoenas) goodsoni and Columbia (O.) subvinacea berlepschi and of Tangara johannae and T. florida in western Colombia. A triple invasion caused the speciation within the black-tailed Trogon melanurus group. The majority of the Amazonian forest birds that reached the trans-Andean lowlands came around the Andes from the north. However, a small group seems to have followed the upper Marañon Valley to cross the Andes in northern Perú or southern Ecuador (or both) during wet climatic periods of the past. A quantitative analysis of the Chocó fauna is included. A comparison of the number of trans- and cis-Andean species of certain families of forest birds shows that at least half of the upper Amazonian fauna reached the trans-Andean forests"--P.51-52.