Phylogenies

The empirical paleoenvironmental data discussed above have provided an improved understanding of the response of the vegetation in the Amazon Basin to known global climate change. New phylogenetic studies question some long-held assumptions about the nature and pace of Amazonian speciation and offer some critical insights into when speciation occurred.

Examples of biological insights arising from phylogenies come from a variety of organisms. Heliconius butterflies (Brown, 1987), frogs, and primates (Vanzolini, 1970) were all advanced as exemplars of refugial evolution, and each group has been revisited using modern molecular cladistic techniques.

Heliconius butterflies provide an excellent example of how reliance on modern biogeographic patterns can result in false assumptions about past evolution. H. erato and H. melpomene are co-mimetics, whose close co-evolution has been taken to indicate a similar biogeographic history (Brown, 1987), but a genetic analysis reveals that their patterns of divergence are markedly different (Flanagan et al. 2004). Subpopulations of H. erato show little geographic structure in rapidly evolving alleles, telling us that there is a relatively ancient divergence among a species with fairly high rates of gene flow. H. erato is the model, and the initial mimic, H. melpomene, has discrete sub-populations consistent with a history of local population isolation and expansion. These data—rather than suggesting environmental change as a cause of phenotypic variation—demonstrate the power of positive feedback in Mullerian mimicry when an abundant model (H. erato) is already present (Flanagan et al,. 2004).

Studies of the molecular clock suggest that speciation of birds and Ateles (spider monkeys) was more rapid in the Miocene or Pliocene than the Pleistocene, and that many of the species purported to reflect allopatry in an arid Amazon were already formed prior to the Quaternary (Zink and Slowinski, 1995; Collins and Dubach, 2000; Moritz et al., 2000; Zink et al., 2004). The important messages to emerge from these studies are that different environmental factors serve as barriers to different lifeforms, that common biogeographic patterns do not imply similar evolutionary demographies, that speciation is—and has been—a continuous feature of these systems, and lastly that the scale and ecological complexity of Amazonia can serve to induce species to specialize and perhaps even speciate without a vicariant event.

Simple assumptions about the compositions of Amazonian clades were also challenged by an analysis of small mammals that found an overlap of species— and genetic similarity—between specimens from the central Brazilian dry forests and those from adjacent rainforests (Costa, 2003). In some clades, samples collected from the dry forest were more similar to those from rainforests than other rainforest samples. This study highlighted the integrated, polyphyletic history of clades and that no simple model of vicariance is likely to explain their biogeography.

Phylogenetics has played an important role in demonstrating the temporal pattern of speciation and the development of biogeographic regions within Amazonia. Reviews by Moritz et al. (2000) and Hall and Harvey (2002) reveal that the geographic divergences contain broadly similar themes between groups. Within the Amazon basin, many clades show a basal split that separates northern and western clades

Figure 3.5. Summary phylogenies for a variety of Amazonian animal taxa (after Hall and Harvey, 2002) compared with their biogeographic relationship. The overall combined phylo-geny is reproduced on the map. Levels of the phylogeny are reflected in the weight and dash patterns of lines on both the tree and the map: B = Belem; G = Guiana; Im = Imeri; In = Inambari; N = Napo; P = Para; R = Rondonia.

Figure 3.5. Summary phylogenies for a variety of Amazonian animal taxa (after Hall and Harvey, 2002) compared with their biogeographic relationship. The overall combined phylo-geny is reproduced on the map. Levels of the phylogeny are reflected in the weight and dash patterns of lines on both the tree and the map: B = Belem; G = Guiana; Im = Imeri; In = Inambari; N = Napo; P = Para; R = Rondonia.

from southeastern ones (Figure 3.5), but there is no fine-scale pattern indicating distinct centers of endemism (Bates et al., 1998; Collins and Dubach, 2000; Patton et al., 2000; Hall and Harvey, 2002; Symula et al., 2003). Ideally, the dating of the divisions between clades should be underpinned by multiple dated calibration points derived from the fossil record (Near et al., 2005). However, in Amazonia the fossil record for many taxa is so depauperate that many of these studies rely on previously estimated rates of molecular evolution to convert observed genetic distances to absolute age estimates. As these estimates are consistent with records derived from those in phylogenies supported by fossil data, it is probable that their age estimates are correct within a factor of 2.

In general, for clades that extend into the wider lowland Neotropics (e.g., the Choco), there is a deeper divergence consistent with division of the clade by Miocene Andean orogeny (Hoorn et al., 1995). However, even in Amazonian clades there is increasing evidence that the basal splits took place within the Miocene (Moritz et al., 2000).

Nores (1999) and Grau et al. (2005) have suggested that high sea levels during the Pliocene or Quaternary may have fragmented bird populations and account for their patterns of vicariance. The presence of higher Miocene sea levels has been suggested to be linked to the isolation of the northern and southern massifs within the Guianan highlands and allowed divergence of geographic amphibian clades (Noonan and Gaucher, 2005). Considerable uncertainty exists regarding Miocene and Pliocene sea levels; Miller et al. (2005) suggest that they may have been only 30-60 m higher than present, whereas other estimates place them 110-140m higher (Rasanen et al., 1995; Naish, 1997). The rise of the Andes caused forebasin subsidence in western Amazonia. Although sea-level rise was only c. 30-60 m higher than present, when coupled with tectonic subsidence the effect was a c. 100 m rise in relative sea level. Repeated highstands approached 100 m above modern levels—as suggested by Rasanen et al. (1995) and Nores (1999)—the basal splits between northern and southeastern Amazonian clades, and the Guiana clades separating from other northern clades, are explicable (Figure 3.6). Nevertheless, considerable debate surrounds the extent

Miocene Amazon Seaway

Figure 3.6. Summary diagram showing the relationship between flooding caused by a 100-m marine highstand and known epicontinental seas in Amazonia, and biogeographic patterns. Darkest area is postulated Miocene seaway (after Rasanen et al., 1995). The biogeographic divisions and phylogenies are derived from Figure 3.5: B = Belem; G = Guiana; Im = Imeri; In = Inambari; N = Napo; P = Para; R = Rondonia.

Figure 3.6. Summary diagram showing the relationship between flooding caused by a 100-m marine highstand and known epicontinental seas in Amazonia, and biogeographic patterns. Darkest area is postulated Miocene seaway (after Rasanen et al., 1995). The biogeographic divisions and phylogenies are derived from Figure 3.5: B = Belem; G = Guiana; Im = Imeri; In = Inambari; N = Napo; P = Para; R = Rondonia.

and duration of epicontinental seas within South America during the Miocene (e.g., Hoorn 1993; Hoorn et al., 1995; Rasanen et al., 1995), and before strong conclusions can be drawn regarding their influence on the biogeography of South America, more complete geological research and improved phylogenies are clearly needed.

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