Biology Reference
In-Depth Information
in July 1858, developed a biogeographic theory now called the riverine hy-
pothesis (Wallace 1849, 1876). The theory was based primarily on Wal-
lace's travels in Amazonia, and it posited that the distribution of species
was strongly infl uenced by the pattern of drainage systems, and by former
changes in these patterns. This has proven true for particular groups of
mammals; but as expected, it is neither true for all organisms, nor is it
the only factor operating within an individual group of organisms. Some
tamarin populations, for example, have been identifi ed as having different
mtDNA profi les even though there were no morphological differences be-
tween them (Patton and da Silva 1998, 2005; Patton et al. 1994). Also, the
molecularly defi ned segregates in these studies were not separated on either
side of the obvious barrier (the Juruá River), but rather between the upper
reaches and the mouth of the river: “What is really striking . . . is that all 11
[of the 17 species] are separated at almost the same geographical point on
the river, although there is nothing remarkable about the spot—no bend,
no hill, no valley” (Patton, quoted in Morell 1996, 1497). The separation
among these groups of tamarins was, rather, the Iquitos Arch, one of several
arches created by uplift of the Andes Mountains that subdivided Amazonia
into subbasins. Over 4000 feet of sediment now mostly obscure the arches,
but when they formed, and while they lasted, the distribution of some, but
not all, of the biotic components in Amazonia were fragmented to varying
degrees depending on the dispersal ability of the organisms. Evidence still
remains in the molecular signature of the tamarins, and DNA analysis sug-
gests the divergence dates to 3-1 Ma.
On the subject of the interaction between recent geologic events and
speciation in neotropical ecosystems, there is another intriguing example
from the Galápagos Islands. Isla Isabela has fi ve taxa of the large Galápa-
gos tortoise, one each on its fi ve volcanoes. One taxon,
Geochelone nigra
vanadenburghi
, on Volcano Alecdo, has three to fi ve times less matrilineal
diversity than the others as revealed by mtDNA. All the volcanoes origi-
nated at about the same time (500,000 years ago), but Alecdo was the most
recent to erupt (100,000 years ago), causing a signifi cant reduction in the
biota. The hypothesis is that this decrease resulted in a genetically reduced
parent population from which the modern lineage is derived. According
to Beheregaray and colleagues, these data “emphasize the value of mod-
ern molecular population approaches in obtaining historical demographic
information that cannot be discerned based on contemporary scenarios”
(2003, 75).
In Colombia, the High Plain of Bogotá is located in the eastern cordillera
