**5. Gradient hypothesis**

The Atlantic Forest is surrounded by dry forests and forested savannas (Cerrado). Because of the existence of a gradual transition from humid forest to those drier biomes, many organisms associated to humid forests are also found intermingled in the open biomes. Each region, the Atlantic forest and the neighboring regions, present different characteristics, and therefore it is expected to find differential selective regimes that could make organisms to diverge between regions. This hypothesis is known as the ecological gradient hypothesis. Also, there are different types of forests within the Atlantic Forest that could imply differential selective regimes. Even though this scenario is very plausible, few studies addressed the problem of divergence across ecological gradients in this biome. For example, Lara et al. (2005) mentions that the occurence of species of tree rat *Phyllomys* and of spiny rat *Trinomys* is associated with vegetation types and with humidity gradients indicate that evolution across gradients may be important. Bird species show distributed in different zone of humidity and temperature across forest types in the Atlantic forest with, which could suggest and important role of environmental gradients in their evolution. So far only one study addressed the problem of evolution across gradients in the Atlantic forest. Cabanne et al (in press) studied whether the plumage color in *Dendrocolaptes platyrostris* was

Fig. 3. Localization of mainly rivers that influence the distribution of species at Brazilian

The Atlantic Forest is surrounded by dry forests and forested savannas (Cerrado). Because of the existence of a gradual transition from humid forest to those drier biomes, many organisms associated to humid forests are also found intermingled in the open biomes. Each region, the Atlantic forest and the neighboring regions, present different characteristics, and therefore it is expected to find differential selective regimes that could make organisms to diverge between regions. This hypothesis is known as the ecological gradient hypothesis. Also, there are different types of forests within the Atlantic Forest that could imply differential selective regimes. Even though this scenario is very plausible, few studies addressed the problem of divergence across ecological gradients in this biome. For example, Lara et al. (2005) mentions that the occurence of species of tree rat *Phyllomys* and of spiny rat *Trinomys* is associated with vegetation types and with humidity gradients indicate that evolution across gradients may be important. Bird species show distributed in different zone of humidity and temperature across forest types in the Atlantic forest with, which could suggest and important role of environmental gradients in their evolution. So far only one study addressed the problem of evolution across gradients in the Atlantic forest. Cabanne et al (in press) studied whether the plumage color in *Dendrocolaptes platyrostris* was

Atlantic Forest.

**5. Gradient hypothesis** 

related to change by drift in different populations historically isolated, or by selective change in different forest types. They found that the plumage variation was related to different forest types and not to historically isolated lineages, suggesting an important role of selection. *D. platyrostris* at the open vegetation corridor was lighter and less streaked than at the forest habitat, a morph which is suggested to be an adaptation of woodcreepers for habitats with high luminosity levels, as are forests at the open vegetation corridor (Marantz, 1997; Willis, 1992). On the other hand, rainforest individuals are darker and more streaked, what is considered to be an adaptation to live in low luminosity and very humid conditions (Marantz, 1997; Willis, 1992; Zink & Remsen, 1986).


Table 1. The geological time scale and the resume of principal studies of Phylogeography in Atlantic Forest

How Past Vicariant Events Can Explain the Atlantic Forest Biodiversity? 439

This chapter aim to understand the principal evolutionary processes that are related to the

The author thank to Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP), Conselho Nacional de Pesquisa do Governo Brasileiro (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Fundação de Amparo a Pesquisa

Brown, K.S. & Ab'Saber, A.N. (1979). Ice-age forest refuges and evolution in Neotropics:

Cabanne, G.S., D´Horta, F., Meyer, D., Myiaki, C. & Silva, J.C.. (in press) Plumage and

Cabanne, G.S., d'Horta, F.M., Sari, E.H., Santos, F.R. & Miyaki, C.Y. 2008. Nuclear and

Cabanne, G.S., Santos, F. & Miyaki, C.Y. (2007) Phylogeography and demographic history of

Conservation International do Brasil, Fundação SOS Mata Atlântica, Fundação Biodiversitas,

Costa, L.P. (2003) The historical bridge between the Amazon and the Atlantic forest of

Costa, L.P., Leite, Y.L.R., da Fonseca, G.A.B. & da Fonseca, M.T. (2000) Biogeography of

D'Horta, F.M., Cabanne, G.S., Meyer, D. & Miyaki,.C.Y. (2011). The genetic effects of Late

Diniz-Filho, J. A. F., De Campos Telles, M. P., Bonatto, S. L., Eizirik, E., De Freitas, T. R. O.,

correlation of paleoclimatoligical, geomorphological and pedological data with

genetic variation in Dendrocolaptes platyrostris (Aves:Dendrocolaptidae): recent discontinuity between the south American open vegetation corridor and Atlantic

mitochondrial phylogeography of the Atlantic forest endemic *Xiphorhynchus fuscus*  (Aves: Dendrocolaptidae): biogeography and systematics implications. *Molecular* 

Xiphorhynchus fuscus (Passeriformes: Dendrocolaptidae) in the southern Atlantic forest of Brazil and Argentina. *Biological Journal of the Linnean Society,* 91, 73– 84. Carnaval, A.C. & Moritz, C. (2008) Historical climate modeling predicts patterns of current biodiversity in the Brazilian Atlantic forest. *Journal of Biogeography*, 35, 1187–1201. Carnaval, A.C., Hickerson, M.J., Haddad, C.F.B., Rodrigues, M.T., Moritz, C. (2009) Stability

predicts genetic diversity in the Brazilian Atlantic Forest hotspot. Science 323, 785–

Instituto de Pesquisas Ecológicas, Secretaria do Meio Ambiente do Estado de São Paulo, SEMAD/Instituto Estadual de Florestas-MG (2000) Avaliação e acções prioritárias para a conservação da biodiversidade da Mata Atlântica e Campos Sulinos (ed. by Ministério do Meio Ambiente / Secretaria Nacional de

Brazil: a study of molecular phylogeography with small mammals. *Journal of* 

South American forest mammals: endemism and diversity in the Atlantic forest.

Quaternary climatic changes over a tropical latitudinal gradient: diversification of

De Marco, P., Santos, F. R., Sole-Cava, A. & Soares, T. N. (2008) Mapping the

do Estado de Minas Gerais (FAPEMIG) for given funding to develop these studies.

diversification and maintenance of the Atlantic forest biodiversity.

biological endemism. *Paleoclimas,* 5, 1–30.

Forest. *Biological journal of Linnean Society*

*Phylogenetic and Evolution*, 49, 760-773.

Biodiversidade e Florestas). pp. 40. Brasília.

an Atlantic Forest passerine. *Molecular Ecology* xxxx

*Biogeography*, 30, 71–86

*Biotropica*, 32, 872–881.

**7. Acknowledgement** 

**8. References** 

789.

The open vegetation corridor and its network of gallery forests and dry forests are contiguous with the Atlantic and Amazon forests. The results of Cabanne et al. (in press) supported the idea that the two plumages types of *D. platyrostris* may have evolved by divergent selection regimes between habitats. There are several other species that occur in both habitats and might present a similar evolutionary story. Puorto et al. (2001) found one clinal morphological variation to *Bothrops atrox* group, although did not show association with genetic variation, which revealed two clades concordant with division North and South of Forest Atlantic.
