Analysis of pollen taken from sediments of Amazon
lakes in Ecuador and Brazil does not support the
refugia model. Pollen is resistant to decomposition,
particularly in anaerobic sediments of lake basins. As
pollen accumulates year after year it forms a vertical
pollen profile, a historical indicator of which plant
species were present in a region. In studies, lake
sediments are cored, and pollen at the bottom of
the core is oldest, pollen at the top youngest. Pollen
profiles from various Amazonian lakes showed that
although the region likely cooled by about 5° C (9° F),
it remained fully forested, not broken up by savanna.
An analysis of Amazonian sedimentary patterns
conducted by Georg Irion suggests that there was
no substantive climatic change or reduction in forest
cover in Amazonia during the Pleistocene. But the
data indicated that sediment patterns were not stable
throughout Amazonia, and that strong oscillations
occurred throughout the Pleistocene in the distribution
of land, water, floodplain forests, and terra firme
(areas of forest and savanna that occur off the riverine
floodplain). Sea level changes are thought to have led
to the alternation of huge Amazonian lakes with strong
valley cutting, geological events that would presumably
have a strong impact on flora and fauna. This view sees
the Amazonian rain forest as subject to vicariance,
though not necessarily reduced to scattered refugia.
Another objection to the refugia model focuses on the
ages of species. The refugia model argues that speciation
has been recent, from about 6 million years ago. Molecular
analysis (unavailable when the refugia model was
proposed) indicates that many bird species in Amazonia
originated well before the Pleistocene. A complex of frog
species of the genus Leptodactylus exhibits high species
richness dating to before the Pleistocene. Much speciation
appears to have occurred in the mid- Tertiary period,
long before the Pleistocene. Studies of plant fossils and
subsequent analyses indicate that the high plant species
richness traces back to 52 million years ago, in the Eocene
epoch of the Tertiary period. That’s a long time ago.
At present the evidence does not strongly support
the original refugia model as the “species pump” in
tropical regions. But what has been learned indicates
that the Neotropics have certainly changed over
geologic time, changes that range from dramatic
mountain uplifts to shifting patterns of the Amazon
and its massive tributaries. These events could very
well have promoted speciation in many groups. And it
certainly looks as if they did.The Great American Faunal
InterchangeThe Panamanian land bridge, now called the Isthmus
of Panama, formed approximately 3 million years ago
because of a combination of uplift of the northern
Andes and a global drop in sea level, of perhaps as
much as 50 m (165 ft), a result of the increasing size
of the polar ice caps. Thus it was just prior to the onset
of the so- called ice age that the continents of North
and South America were no longer isolated by water.
The Panamanian land bridge profoundly altered the
ecology of South America, much more so than it did
that of North America. Consider that the faunas of
North and South America had evolved independently
of each other for at least 40 million years, but their
mingling, once the land connection was made, was
completed within a mere 2 million years.
Once the land bridge formed, various species of South
American animals moved northward, beginning as early
as 2.5 million years ago, literally walking, generation
by generation, to North America. These included two
armadillo species (plate 8- 43), a glyptodont (a kind
of giant version of an armadillo almost the size of a
very compact car), two species of large ground sloths,
a porcupine, a large capybara, and one phorusrhacoid
bird (a large flightless bird with an immense predatory
beak that overall resembled an ostrich from hell).
Glyptodonts looked like gigantic armadillos, covered
with bony armor, but their tail (depending on the species)
sometimes terminated in a mace- like club (plate 8-Plate 8- 43. Ancestors of the Nine- banded Armadillo (Dasypus
novemcinctus) emigrated from the Neotropics across the
Isthmus of Panama land bridge and continue to expand
northward in North America both in the Southeast and the
Midwest. The species remains common and widespread today
throughout the Neotropics. You can easily count the nine
bands in this photo. Photo by John Kricher.chapter 8 evolutionary cornucopia 131